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Before yesterdayMIT News - Quantum computing

Celebrating five years of MIT.nano

There is vast opportunity for nanoscale innovation to transform the world in positive ways — expressed MIT.nano Director Vladimir Bulović as he posed two questions to attendees at the start of the inaugural Nano Summit: “Where are we heading? And what is the next big thing we can develop?”

“The answer to that puts into perspective our main purpose — and that is to change the world,” Bulović, the Fariborz Maseeh Professor of Emerging Technologies, told an audience of more than 325 in-person and 150 virtual participants gathered for an exploration of nano-related research at MIT and a celebration of MIT.nano’s fifth anniversary.

Over a decade ago, MIT embarked on a massive project for the ultra-small — building an advanced facility to support research at the nanoscale. Construction of MIT.nano in the heart of MIT’s campus, a process compared to assembling a ship in a bottle, began in 2015, and the facility launched in October 2018.

Fast forward five years: MIT.nano now contains nearly 170 tools and instruments serving more than 1,200 trained researchers. These individuals come from over 300 principal investigator labs, representing more than 50 MIT departments, labs, and centers. The facility also serves external users from industry, other academic institutions, and over 130 startup and multinational companies.

A cross section of these faculty and researchers joined industry partners and MIT community members to kick off the first Nano Summit, which is expected to become an annual flagship event for MIT.nano and its industry consortium. Held on Oct. 24, the inaugural conference was co-hosted by the MIT Industrial Liaison Program.

Six topical sessions highlighted recent developments in quantum science and engineering, materials, advanced electronics, energy, biology, and immersive data technology. The Nano Summit also featured startup ventures and an art exhibition.

Watch the videos here.

Seeing and manipulating at the nanoscale — and beyond

“We need to develop new ways of building the next generation of materials,” said Frances Ross, the TDK Professor in Materials Science and Engineering (DMSE). “We need to use electron microscopy to help us understand not only what the structure is after it’s built, but how it came to be. I think the next few years in this piece of the nano realm are going to be really amazing.”

Speakers in the session “The Next Materials Revolution,” chaired by MIT.nano co-director for Characterization.nano and associate professor in DMSE James LeBeau, highlighted areas in which cutting-edge microscopy provides insights into the behavior of functional materials at the nanoscale, from anti-ferroelectrics to thin-film photovoltaics and 2D materials. They shared images and videos collected using the instruments in MIT.nano’s characterization suites, which were specifically designed and constructed to minimize mechanical-vibrational and electro-magnetic interference.

Later, in the “Biology and Human Health” session chaired by Boris Magasanik Professor of Biology Thomas Schwartz, biologists echoed the materials scientists, stressing the importance of the ultra-quiet, low-vibration environment in Characterization.nano to obtain high-resolution images of biological structures.

“Why is MIT.nano important for us?” asked Schwartz. “An important element of biology is to understand the structure of biology macromolecules. We want to get to an atomic resolution of these structures. CryoEM (cryo-electron microscopy) is an excellent method for this. In order to enable the resolution revolution, we had to get these instruments to MIT. For that, MIT.nano was fantastic.”

Seychelle Vos, the Robert A. Swanson (1969) Career Development Professor of Life Sciences, shared CryoEM images from her lab’s work, followed by biology Associate Professor Joey Davis who spoke about image processing. When asked about the next stage for CryoEM, Davis said he’s most excited about in-situ tomography, noting that there are new instruments being designed that will improve the current labor-intensive process.

To chart the future of energy, chemistry associate professor Yogi Surendranath is also using MIT.nano to see what is happening at the nanoscale in his research to use renewable electricity to change carbon dioxide into fuel.

“MIT.nano has played an immense role, not only in facilitating our ability to make nanostructures, but also to understand nanostructures through advanced imaging capabilities,” said Surendranath. “I see a lot of the future of MIT.nano around the question of how nanostructures evolve and change under the conditions that are relevant to their function. The tools at MIT.nano can help us sort that out.”

Tech transfer and quantum computing

The “Advanced Electronics” session chaired by Jesús del Alamo, the Donner Professor of Science in the Department of Electrical Engineering and Computer Science (EECS), brought together industry partners and MIT faculty for a panel discussion on the future of semiconductors and microelectronics. “Excellence in innovation is not enough, we also need to be excellent in transferring these to the marketplace,” said del Alamo. On this point, panelists spoke about strengthening the industry-university connection, as well as the importance of collaborative research environments and of access to advanced facilities, such as MIT.nano, for these environments to thrive.

The session came on the heels of a startup exhibit in which eleven START.nano companies presented their technologies in health, energy, climate, and virtual reality, among other topics. START.nano, MIT.nano’s hard-tech accelerator, provides participants use of MIT.nano’s facilities at a discounted rate and access to MIT’s startup ecosystem. The program aims to ease hard-tech startups’ transition from the lab to the marketplace, surviving common “valleys of death” as they move from idea to prototype to scaling up.

When asked about the state of quantum computing in the “Quantum Science and Engineering” session, physics professor Aram Harrow related his response to these startup challenges. “There are quite a few valleys to cross — there are the technical valleys, and then also the commercial valleys.” He spoke about scaling superconducting qubits and qubits made of suspended trapped ions, and the need for more scalable architectures, which we have the ingredients for, he said, but putting everything together is quite challenging.

Throughout the session, William Oliver, professor of physics and the Henry Ellis Warren (1894) Professor of Electrical Engineering and Computer Science, asked the panelists how MIT.nano can address challenges in assembly and scalability in quantum science.

“To harness the power of students to innovate, you really need to allow them to get their hands dirty, try new things, try all their crazy ideas, before this goes into a foundry-level process,” responded Kevin O’Brien, associate professor in EECS. “That’s what my group has been working on at MIT.nano, building these superconducting quantum processors using the state-of-the art fabrication techniques in MIT.nano.”

Connecting the digital to the physical

In his reflections on the semiconductor industry, Douglas Carlson, senior vice president for technology at MACOM, stressed connecting the digital world to real-world application. Later, in the “Immersive Data Technology” session, MIT.nano associate director Brian Anthony explained how, at the MIT.nano Immersion Lab, researchers are doing just that.

“We think about and facilitate work that has the human immersed between hardware, data, and experience,” said Anthony, principal research scientist in mechanical engineering. He spoke about using the capabilities of the Immersion Lab to apply immersive technologies to different areas — health, sports, performance, manufacturing, and education, among others. Speakers in this session gave specific examples in hardware, pediatric health, and opera.

Anthony connected this third pillar of MIT.nano to the fab and characterization facilities, highlighting how the Immersion Lab supports work conducted in other parts of the building. The Immersion Lab’s strength, he said, is taking novel work being developed inside MIT.nano and bringing it up to the human scale to think about applications and uses.

Artworks that are scientifically inspired

The Nano Summit closed with a reception at MIT.nano where guests could explore the facility and gaze through the cleanroom windows, where users were actively conducting research. Attendees were encouraged to visit an exhibition on MIT.nano’s first- and second-floor galleries featuring work by students from the MIT Program in Art, Culture, and Technology (ACT) who were invited to utilize MIT.nano’s tool sets and environments as inspiration for art.

In his closing remarks, Bulović reflected on the community of people who keep MIT.nano running and who are using the tools to advance their research. “Today we are celebrating the facility and all the work that has been done over the last five years to bring it to where it is today. It is there to function not just as a space, but as an essential part of MIT’s mission in research, innovation, and education. I hope that all of us here today take away a deep appreciation and admiration for those who are leading the journey into the nano age.”

© Photo: Jared Charney

MIT faculty and researchers participate in a panel discussion on quantum science and engineering. Left to right: Professor Aram Harrow, Professor Paola Cappellaro, Associate Professor Kevin O'Brien, Research Scientist Jeff Grover, and session chair Professor William Oliver.

Physicists trap electrons in a 3D crystal for the first time

Electrons move through a conducting material like commuters at the height of Manhattan rush hour. The charged particles may jostle and bump against each other, but for the most part they’re unconcerned with other electrons as they hurtle forward, each with their own energy.

But when a material’s electrons are trapped together, they can settle into the exact same energy state and start to behave as one. This collective, zombie-like state is what’s known in physics as an electronic “flat band,” and scientists predict that when electrons are in this state they can start to feel the quantum effects of other electrons and act in coordinated, quantum ways. Then, exotic behavior such as superconductivity and unique forms of magnetism may emerge.

Now, physicists at MIT have successfully trapped electrons in a pure crystal. It is the first time that scientists have achieved an electronic flat band in a three-dimensional material. With some chemical manipulation, the researchers also showed they could transform the crystal into a superconductor — a material that conducts electricity with zero resistance.

The electrons’ trapped state is possible thanks to the crystal’s atomic geometry. The crystal, which the physicists synthesized, has an arrangement of atoms that resembles the woven patterns in “kagome,” the Japanese art of basket-weaving. In this specific geometry, the researchers found that rather than jumping between atoms, electrons were “caged,” and settled into the same band of energy.

Animation of spinning 3D crystal structure that looks like a star made up of latticed cubes and pyramids.

The researchers say that this flat-band state can be realized with virtually any combination of atoms — as long as they are arranged in this kagome-inspired 3D geometry. The results, appearing today in Nature, provide a new way for scientists to explore rare electronic states in three-dimensional materials. These materials might someday be optimized to enable ultraefficient power lines, supercomputing quantum bits, and faster, smarter electronic devices.

“Now that we know we can make a flat band from this geometry, we have a big motivation to study other structures that might have other new physics that could be a platform for new technologies,” says study author Joseph Checkelsky, associate professor of physics.

Checkelsky’s MIT co-authors include graduate students Joshua Wakefield, Mingu Kang, and Paul Neves, and postdoc Dongjin Oh, who are co-lead authors; graduate students Tej Lamichhane and Alan Chen; postdocs Shiang Fang and Frank Zhao; undergraduate Ryan Tigue; associate professor of nuclear science and engineering Mingda Li; and associate professor of physics Riccardo Comin, who collaborated with Checkelsky to direct the study; along with collaborators at multiple other laboratories and institutions.

Setting a 3D trap

In recent years, physicists have successfully trapped electrons and confirmed their electronic flat-band state in two-dimensional materials. But scientists have found that electrons that are trapped in two dimensions can easily escape out the third, making flat-band states difficult to maintain in 2D.

In their new study, Checkelsky, Comin, and their colleagues looked to realize flat bands in 3D materials, such that electrons would be trapped in all three dimensions and any exotic electronic states could be more stably maintained. They had an idea that kagome patterns might play a role.

In previous work, the team observed trapped electrons in a two-dimensional lattice of atoms that resembled some kagome designs. When the atoms were arranged in a pattern of interconnected, corner-sharing triangles, electrons were confined within the hexagonal space between triangles, rather than hopping across the lattice. But, like others, the researchers found that the electrons could escape up and out of the lattice, through the third dimension.

The team wondered: Could a 3D configuration of similar lattices work to box in the electrons? They looked for an answer in databases of material structures and came across a certain geometric configuration of atoms, classified generally as a pyrochlore — a type of mineral with a highly symmetric atomic geometry. The pychlore’s 3D structure of atoms formed a repeating pattern of cubes, the face of each cube resembling a kagome-like lattice. They found that, in theory, this geometry could effectively trap electrons within each cube.

Rocky landings

To test this hypothesis, the researchers synthesized a pyrochlore crystal in the lab.

“It’s not dissimilar to how nature makes crystals,” Checkelsky explains. “We put certain elements together — in this case, calcium and nickel — melt them at very high temperatures, cool them down, and the atoms on their own will arrange into this crystalline, kagome-like configuration.”

They then looked to measure the energy of individual electrons in the crystal, to see if they indeed fell into the same flat band of energy. To do so, researchers typically carry out photoemission experiments, in which they shine a single photon of light onto a sample, that in turn kicks out a single electron. A detector can then precisely measure the energy of that individual electron.

Scientists have used photoemission to confirm flat-band states in various 2D materials. Because of their physically flat, two-dimensional nature, these materials are relatively straightforward to measure using standard laser light. But for 3D materials, the task is more challenging.

“For this experiment, you typically require a very flat surface,” Comin explains. “But if you look at the surface of these 3D materials, they are like the Rocky Mountains, with a very corrugated landscape. Experiments on these materials are very challenging, and that is part of the reason no one has demonstrated that they host trapped electrons.”

The team cleared this hurdle with angle-resolved photoemission spectroscopy (ARPES), an ultrafocused beam of light that is able to target specific locations across an uneven 3D surface and measure the individual electron energies at those locations.

“It’s like landing a helicopter on very small pads, all across this rocky landscape,” Comin says.

With ARPES, the team measured the energies of thousands of electrons across a synthesized crystal sample in about half an hour. They found that, overwhelmingly, the electrons in the crystal exhibited the exact same energy, confirming the 3D material’s flat-band state.

To see whether they could manipulate the coordinated electrons into some exotic electronic state, the researchers synthesized the same crystal geometry, this time with atoms of rhodium and ruthenium instead of nickel. On paper, the researchers calculated that this chemical swap should shift the electrons’ flat band to zero energy — a state that automatically leads to superconductivity.

And indeed, they found that when they synthesized a new crystal, with a slightly different combination of elements, in the same kagome-like 3D geometry, the crystal’s electrons exhibited a flat band, this time at superconducting states.

“This presents a new paradigm to think about how to find new and interesting quantum materials,” Comin says. “We showed that, with this special ingredient of this atomic arrangement that can trap electrons, we always find these flat bands. It’s not just a lucky strike. From this point on, the challenge is to optimize to achieve the promise of flat-band materials, potentially to sustain superconductivity at higher temperatures.”

© Image: Courtesy of the researchers

MIT physicists have trapped electrons in a pure crystal, marking the first achievement of an electronic flat band in a three-dimensional material. The rare electronic state is thanks to a special cubic arrangement of atoms (pictured) that resembles the Japanese art of “kagome.” The results provide a new way for scientists to explore rare electronic states in 3D materials.

MIT receives major National Science Foundation grant for quantum science

The U.S. National Science Foundation’s Physics Frontiers Centers program renewed a grant to the MIT-Harvard Center for Ultracold Atoms (CUA) to fund exploring, understanding, and harnessing mysterious phenomena at the frontiers of physics.

The CUA, which works to enable greater control and programmability of quantum-entangled systems of low-temperature atoms and molecules, will conduct experiments involving quantum gases of atoms and molecules; arrays of exotic atoms in Rydberg states containing a single, highly excited electron; atom-like impurities in semiconductors; and an “unusual” linking of light and matter known as "strong coupling" with the potential for new applications in measurement, sensing and networking.

“At MIT and Harvard, we are all excited to have continued funding for the Center for Ultracold Atoms, which has made a big difference for our community of researchers,” says Professor Wolfgang Ketterle, who added that it is critical to provide adequate funding for new projects and for junior individuals who have joined the CUA.

“CUA is one of the centerpieces of MIT’s strength in quantum science and measurement, and the renewal of the CUA grant is fantastic news,” says physics department head Deepto Chakrabarty.

Ketterle is a member of the CUA group receiving the funding, along with Vladan Vuletic, Martin W. Zwierlein, Paola Cappellaro, Isaac Chuang, Soonwon Choi, Richard Fletcher, and Dirk Englund.
The CUA is one of four U.S. research centers to be backed by a total of $76 million; the three other recipients are the University of Chicago, Caltech, and the University of Colorado at Boulder.

The NSF Physics Frontiers Centers program brings together large teams of researchers for projects that will require years of concentrated effort, a range of scientific and technical expertise, and new types of equipment. NSF now actively supports eight physics centers. 

The centers offer extensive training and mentorship programs for undergraduate and graduate students, as well as postdocs, to nurture future leaders in the field of physics and to strengthen the scientific workforce in the United States. Additionally, the centers also seek to boost middle and high school students’ interest in STEM careers via educational games, videos, workshops, summer schools, and outreach activities.

“Research teams at NSF Physics Frontiers Centers have made breakthrough after breakthrough, such as creating remarkable new states of matter and revealing the first evidence for the gravitational wave background of the universe,” says NSF Director Sethuraman Panchanathan. “While different in their respective areas of focus, NSF's newly funded centers are all bold team efforts to punch through to exciting new vistas of scientific exploration. Achieving transformative opportunities requires us to reach those vistas through new technologies and other advances and have a look around.”

© Photo: Bryce Vickmark

MIT professor of physics Wolfgang Ketterle, along with other members of the Center for Ultracold Atoms group, received a major renewal of its NSF Physics Frontiers Centers program grant for $3.6 million per year for six years to fund research that includes conducting experiments involving quantum gases of atoms and molecules.

From a five-layer graphene sandwich, a rare electronic state emerges

Ordinary pencil lead holds extraordinary properties when shaved down to layers as thin as an atom. A single, atom-thin sheet of graphite, known as graphene, is just a tiny fraction of the width of a human hair. Under a microscope, the material resembles a chicken-wire of carbon atoms linked in a hexagonal lattice.

Despite its waif-like proportions, scientists have found over the years that graphene is exceptionally strong. And when the material is stacked and twisted in specific contortions, it can take on surprising electronic behavior.

Now, MIT physicists have discovered another surprising property in graphene: When stacked in five layers, in a rhombohedral pattern, graphene takes on a very rare, “multiferroic” state, in which the material exhibits both unconventional magnetism and an exotic type of electronic behavior, which the team has coined ferro-valleytricity.

“Graphene is a fascinating material,” says team leader Long Ju, assistant professor of physics at MIT. “Every layer you add gives you essentially a new material. And now this is the first time we see ferro-valleytricity, and unconventional magnetism, in five layers of graphene. But we don’t see this property in one, two, three, or four layers.”

The discovery could help engineers design ultra-low-power, high-capacity data storage devices for classical and quantum computers.

“Having multiferroic properties in one material means that, if it could save energy and time to write a magnetic hard drive, you could also store double the amount of information compared to conventional devices,” Ju says.

His team reports their discovery today in Nature. MIT co-authors include lead author Tonghang Han, plus Zhengguang Lu, Tianyi Han, and Liang Fu; along with Harvard University collaborators Giovanni Scuri, Jiho Sung, Jue Wang, and Hongkun Park; and Kenji Watanabe and Takashi Taniguchi of the National Institute for Materials Science in Japan.

A preference for order

A ferroic material is one that displays some coordinated behavior in its electric, magnetic, or structural properties. A magnet is a common example of a ferroic material: Its electrons can coordinate to spin in the same direction without an external magnetic field. As a result, the magnet points to a preferred direction in space, spontaneously.

Other materials can be ferroic through different means. But only a handful have been found to be multiferroic — a rare state in which multiple properties can coordinate to exhibit multiple preferred states. In conventional multiferroics, it would be as if, in addition to the magnet pointing toward one direction, the electric charge also shifts in a direction that is independent from the magnetic direction.  

Multiferroic materials are of interest for electronics because they could potentially increase the speed and lower the energy cost of hard drives. Magnetic hard drives store data in the form of magnetic domains — essentially, microscopic magnets that are read as either a 1 or a 0, depending on their magnetic orientation. The magnets are switched by an electric current, which consumes a lot of energy and cannot operate quickly. If a storage device could be made with multiferroic materials, the domains could be switched by a faster, much lower-power electric field. Ju and his colleagues were curious about whether multiferroic behavior would emerge in graphene. The material’s extremely thin structure is a unique environment in which researchers have discovered otherwise hidden, quantum interactions. In particular, Ju wondered whether graphene would display multiferroic, coordinated behavior among its electrons when arranged under certain conditions and configurations.

“We are looking for environments where electrons are slowed down — where their interactions with the surrounding lattice of atoms is small, so that their interactions with other electrons can come through,” Ju explains. “That’s when we have some chance of seeing interesting collective behaviors of electrons.”

The team carried out some simple calculations and found that some coordinated behavior among electrons should emerge in a structure of five graphene layers stacked together in a rhombohedral pattern. (Think of five chicken-wire fences, stacked and slightly shifted such that, viewed from the top, the structure would resemble a pattern of diamonds.)

“In five layers, electrons happen to be in a lattice environment where they move very slowly, so they can interact with other electrons effectively,” Ju says. “That’s when electron correlation effects start to dominate, and they can start to coordinate into certain preferred, ferroic orders.”

Magic flakes

The researchers then went into the lab to see whether they could actually observe multiferroic behavior in five-layer graphene. In their experiments, they started with a small block of graphite, from which they carefully exfoliated individual flakes. They used optical techniques to examine each flake, looking specifically for five-layer flakes, arranged naturally in a rhombohedral pattern.

“To some extent, nature does the magic,” said lead author and graduate student Han. “And we can look at all these flakes and tell which has five layers, in this rhombohedral stacking, which is what should give you this slowing-down effect in electrons.”

The team isolated several five-layer flakes and studied them at temperatures just above absolute zero. In such ultracold conditions, all other effects, such as thermally induced disorders within graphene, should be dampened, allowing interactions between electrons, to emerge. The researchers measured electrons’ response to an electric field and a magnetic field, and found that indeed, two ferroic orders, or sets of coordinated behaviors, emerged.

The first ferroic property was an unconventional magnetism: The electrons coordinated their orbital motion, like planets circling in the same direction. (In conventional magnets, electrons coordinate their “spin” — rotating in the same direction, while staying relatively fixed in space.)

The second ferroic property had to do with graphene’s electronic “valley.” In every conductive material, there are certain energy levels that electrons can occupy. A valley represents the lowest energy state that an electron can naturally settle. As it turns out, there are two possible valleys in graphene. Normally, electrons have no preference for either valley and settle equally into both.

But in five-layer graphene, the team found that the electrons began to coordinate, and preferred to settle in one valley over the other. This second coordinated behavior indicated a ferroic property that, combined with the electrons’ unconventional magnetism, gave the structure a rare, multiferroic state.

“We knew something interesting would happen in this structure, but we didn’t know exactly what, until we tested it,” says co-first author Lu, a postdoc in Ju’s group. “It’s the first time we’ve seen a ferro-valleytronics, and also the first time we’ve seen a coexistence of ferro-valleytronics with unconventional ferro-magnet.”

The team showed they could control both ferroic properties using an electric field. They envision that, if engineers can incorporate five-layer graphene or similar multiferroic materials into a memory chip, they could, in principle, use the same, low-power electric field to manipulate the material’s electrons in two ways rather than one, and effectively double the data that could be stored on a chip compared to conventional multiferroics. While that vision is far from practical realization, the team’s results break new ground in the search for better, more efficient electronic, magnetic and valleytronic devices.

This research was done, in part, using the electron-beam lithography facility run by MIT.nano, and is funded, in part, by the National Science Foundation and the Sloan Foundation.

© Image: Sampson Wilcox, RLE

When stacked in five layers in a rhombohedral pattern, graphene takes on a rare “multiferroic” state, in which the material’s electrons (illustrated here as spheres) exhibit two preferred electronic states: an unconventional magnetism (represented as orbits around each electron), and “valley,” or a preference for one of two energy states (depicted in red versus blue). The results could help advance more powerful magnetic memory devices.

Quantum repeaters use defects in diamond to interconnect quantum systems

The popular children's game of telephone is based on a simple premise: The starting player whispers a message into the ear of the next player. That second player then passes along the message to the third person and so on until the message reaches the final recipient, who relays it to the group aloud. Often, what the first person said and the last person heard are laughably different; the information gets garbled along the chain.

Such transmission errors from start to end point are also common in the quantum world. As quantum information bits, or qubits (the analogs of classical bits in traditional digital electronics), make their way over a channel, their quantum states can degrade or be lost entirely. Such decoherence is especially common over longer and longer distances because qubits — whether existing as particles of light (photons), electrons, atoms, or other forms — are inherently fragile, governed by the laws of quantum physics, or the physics of very small objects. At this tiny scale (nanoscale), even slight interactions with their environment can cause qubits to lose their quantum properties and alter the information they store. Like the game of telephone, the original and received messages may not be the same.

"One of the big challenges in quantum networking is how to effectively move these delicate quantum states between multiple quantum systems," says Scott Hamilton, leader of MIT Lincoln Laboratory's Optical and Quantum Communications Technology Group, part of the Communications Systems R&D area. "That's a question we're actively exploring in our group."

As Hamilton explains, today's quantum computing chips contain on the order of 100 qubits. But thousands, if not billions, of qubits are required to make a fully functioning quantum computer, which promises to unlock unprecedented computational power for applications ranging from artificial intelligence and cybersecurity to health care and manufacturing. Interconnecting the chips to make one big computer may provide a viable path forward. On the sensing front, connecting quantum sensors to share quantum information may enable new capabilities and performance gains beyond those of an individual sensor. For example, a shared quantum reference between multiple sensors could be used to more precisely locate radio-frequency emission sources. Space and defense agencies are also interested in interconnecting quantum sensors separated by long ranges for satellite-based position, navigation, and timing systems or atomic clock networks between satellites. For communications, quantum satellites could be used as part of a quantum network architecture connecting local ground-based stations, creating a truly global quantum internet.

However, quantum systems can't be interconnected with existing technology. The communication systems used today to transmit information across a network and connect devices rely on detectors that measure bits and amplifiers that copy bits. These technologies do not work in a quantum network because qubits cannot be measured or copied without destroying the quantum state; qubits exist in a superposition of states between zero and one, as opposed to classical bits, which are in a set state of either zero (off) or one (on). Therefore, researchers have been trying to develop the quantum equivalents of classical amplifiers to overcome transmission and interconnection loss. These equivalents are known as quantum repeaters, and they work similarly in concept to amplifiers, dividing the transmission distance into smaller, more manageable segments to lessen losses.

"Quantum repeaters are a critical technology for quantum networks to successfully send information over lossy links," Hamilton says. "But nobody has made a fully functional quantum repeater yet."

The complexity lies in how quantum repeaters operate. Rather than employing a simple "copy and paste," as classical repeaters do, quantum repeaters work by leveraging a strange quantum phenomenon called entanglement. In quantum entanglement, two particles become strongly connected and correlated across space, no matter the distance between them. If you know the state of one particle in an entangled pair, then you automatically know the state of the other. Entangled qubits can serve as a resource for quantum teleportation, in which quantum information is sent between distant systems without moving actual particles; the information vanishes at one location and reappears at another. Teleportation skips the physical journey along fiber-optic cables and therefore eliminates the associated risk of information loss. Quantum repeaters are what tie everything together: they enable the end-to-end generation of quantum entanglement, and, ultimately, with quantum teleportation, the end-to-end transmission of qubits.

Ben Dixon, a researcher in the Optical and Quantum Communications Technology Group, explains how the process works: "First, you need to generate pairs of specific entangled qubits (called Bell states) and transmit them in different directions across the network link to two separate quantum repeaters, which capture and store these qubits. One of the quantum repeaters then does a two-qubit measurement between the transmitted and stored qubit and an arbitrary qubit that we want to send across the link in order to interconnect the remote quantum systems. The measurement results are communicated to the quantum repeater at the other end of the link; the repeater uses these results to turn the stored Bell state qubit into the arbitrary qubit. Lastly, the repeater can send the arbitrary qubit into the quantum system, thereby linking the two remote quantum systems."

To retain the entangled states, the quantum repeater needs a way to store them — in essence, a memory. In 2020, collaborators at Harvard University demonstrated holding a qubit in a single silicon atom (trapped between two empty spaces left behind by removing two carbon atoms) in diamond. This silicon "vacancy" center in diamond is an attractive quantum memory option. Like other individual electrons, the outermost (valence) electron on the silicon atom can point either up or down, similar to a bar magnet with north and south poles. The direction that the electron points is known as its spin, and the two possible spin states, spin up or spin down, are akin to the ones and zeros used by computers to represent, process, and store information. Moreover, silicon's valence electron can be manipulated with visible light to transfer and store a photonic qubit in the electron spin state. The Harvard researchers did exactly this; they patterned an optical waveguide (a structure that guides light in a desired direction) surrounded by a nanophotonic optical cavity to have a photon strongly interact with the silicon atom and impart its quantum state onto that atom. Collaborators at MIT then showed this basic functionality could work with multiple waveguides; they patterned eight waveguides and successfully generated silicon vacancies inside them all. 

Lincoln Laboratory has since been applying quantum engineering to create a quantum memory module equipped with additional capabilities to operate as a quantum repeater. This engineering effort includes on-site custom diamond growth (with the Quantum Information and Integrated Nanosystems Group); the development of a scalable silicon-nanophotonics interposer (a chip that merges photonic and electronic functionalities) to control the silicon-vacancy qubit; and integration and packaging of the components into a system that can be cooled to the cryogenic temperatures needed for long-term memory storage. The current system has two memory modules, each capable of holding eight optical qubits.

To test the technologies, the team has been leveraging an optical-fiber test bed leased by the laboratory. This test bed features a 50-kilometer-long telecommunications network fiber currently connecting three nodes: Lincoln Laboratory to MIT campus and MIT campus to Harvard. Local industrial partners can also tap into this fiber as part of the Boston-Area Quantum Network (BARQNET).

"Our goal is to take state-of-the-art research done by our academic partners and transform it into something we can bring outside the lab to test over real channels with real loss," Hamilton says. "All of this infrastructure is critical for doing baseline experiments to get entanglement onto a fiber system and move it between various parties."

Using this test bed, the team, in collaboration with MIT and Harvard researchers, became the first in the world to demonstrate a quantum interaction with a nanophotonic quantum memory across a deployed telecommunications fiber. With the quantum repeater located at Harvard, they sent photons encoded with quantum states from the laboratory, across the fiber, and interfaced them with the silicon-vacancy quantum memory that captured and stored the transmitted quantum states. They measured the electron on the silicon atom to determine how well the quantum states were transferred to the silicon atom's spin-up or spin-down position.

"We looked at our test bed performance for the relevant quantum repeater metrics of distance, efficiency (loss error), fidelity, and scalability and found that we achieved best or near-best for all these metrics, compared to other leading efforts around the world," Dixon says. "Our distance is longer than anybody else has shown; our efficiency is decent, and we think we can further improve it by optimizing some of our test bed components; the read-out qubit from memory matches the qubit we sent with 87.5 percent fidelity; and diamond has an inherent lithographic patterning scalability in which you can imagine putting thousands of qubits onto one small chip." 

The Lincoln Laboratory team is now focusing on combining multiple quantum memories at each node and incorporating additional nodes into the quantum network test bed. Such advances will enable the team to explore quantum networking protocols at a system level. They also look forward to materials science investigations that their Harvard and MIT collaborators are pursuing. These investigations may identify other types of atoms in diamond capable of operating at slightly warmer temperatures for more practical operation.

The nanophotonic quantum memory module was recognized with a 2023 R&D 100 Award.

© Photo: Glen Cooper

A packaged prototype quantum repeater module (center), mounted on a gold-plated copper assembly and connected to printed circuit boards (green), features eight optical memories that store qubits in a silicon atom in diamond.

New qubit circuit enables quantum operations with higher accuracy

25 September 2023 at 15:00

In the future, quantum computers may be able to solve problems that are far too complex for today’s most powerful supercomputers. To realize this promise, quantum versions of error correction codes must be able to account for computational errors faster than they occur.

However, today’s quantum computers are not yet robust enough to realize such error correction at commercially relevant scales.

On the way to overcoming this roadblock, MIT researchers demonstrated a novel superconducting qubit architecture that can perform operations between qubits — the building blocks of a quantum computer — with much greater accuracy than scientists have previously been able to achieve.

They utilize a relatively new type of superconducting qubit, known as fluxonium, which can have a lifespan that is much longer than more commonly used superconducting qubits.

Their architecture involves a special coupling element between two fluxonium qubits that enables them to perform logical operations, known as gates, in a highly accurate manner. It suppresses a type of unwanted background interaction that can introduce errors into quantum operations.

This approach enabled two-qubit gates that exceeded 99.9 percent accuracy and single-qubit gates with 99.99 percent accuracy. In addition, the researchers implemented this architecture on a chip using an extensible fabrication process.  

“Building a large-scale quantum computer starts with robust qubits and gates. We showed a highly promising two-qubit system and laid out its many advantages for scaling. Our next step is to increase the number of qubits,” says Leon Ding PhD ’23, who was a physics graduate student in the Engineering Quantum Systems (EQuS) group and is the lead author of a paper on this architecture.

Ding wrote the paper with Max Hays, an EQuS postdoc; Youngkyu Sung PhD ’22; Bharath Kannan PhD ’22, who is now CEO of Atlantic Quantum; Kyle Serniak, a staff scientist and team lead at MIT Lincoln Laboratory; and senior author William D. Oliver, the Henry Ellis Warren professor of electrical engineering and computer science and of physics, director of the Center for Quantum Engineering, leader of EQuS, and associate director of the Research Laboratory of Electronics; as well as others at MIT and MIT Lincoln Laboratory. The research appears today in Physical Review X.

A new take on the fluxonium qubit

In a classical computer, gates are logical operations performed on bits (a series of 1s and 0s) that enable computation. Gates in quantum computing can be thought of in the same way: A single qubit gate is a logical operation on one qubit, while a two-qubit gate is an operation that depends on the states of two connected qubits.

Fidelity measures the accuracy of quantum operations performed on these gates. Gates with the highest possible fidelities are essential because quantum errors accumulate exponentially. With billions of quantum operations occurring in a large-scale system, a seemingly small amount of error can quickly cause the entire system to fail.

In practice, one would use error-correcting codes to achieve such low error rates. However, there is a “fidelity threshold” the operations must surpass to implement these codes. Furthermore, pushing the fidelities far beyond this threshold reduces the overhead needed to implement error correcting codes.

For more than a decade, researchers have primarily used transmon qubits in their efforts to build quantum computers. Another type of superconducting qubit, known as a fluxonium qubit, originated more recently. Fluxonium qubits have been shown to have longer lifespans, or coherence times, than transmon qubits.

Coherence time is a measure of how long a qubit can perform operations or run algorithms before all the information in the qubit is lost.

“The longer a qubit lives, the higher fidelity the operations it tends to promote. These two numbers are tied together. But it has been unclear, even when fluxonium qubits themselves perform quite well, if you can perform good gates on them,” Ding says.

For the first time, Ding and his collaborators found a way to use these longer-lived qubits in an architecture that can support extremely robust, high-fidelity gates. In their architecture, the fluxonium qubits were able to achieve coherence times of more than a millisecond, about 10 times longer than traditional transmon qubits.

“Over the last couple of years, there have been several demonstrations of fluxonium outperforming transmons on the single-qubit level,” says Hays. “Our work shows that this performance boost can be extended to interactions between qubits as well.”

The fluxonium qubits were developed in a close collaboration with MIT Lincoln Laboratory, (MIT-LL), which has expertise in the design and fabrication of extensible superconducting qubit technologies.

“This experiment was exemplary of what we call the ‘one-team model’: the close collaboration between the EQuS group and the superconducting qubit team at MIT-LL,” says Serniak. “It’s worth highlighting here specifically the contribution of fabrication team at MIT-LL — they developed the capability to construct dense arrays of more than 100 Josephson junctions specifically for fluxoniums and other new qubit circuits.”

A stronger connection

Their novel architecture involves a circuit that has two fluxonium qubits on either end, with a tunable transmon coupler in the middle to join them together. This fluxonium-transmon-fluxonium (FTF) architecture enables a stronger coupling than methods that directly connect two fluxonium qubits.

FTF also minimizes unwanted interactions that occur in the background during quantum operations. Typically, stronger couplings between qubits can lead to more of this persistent background noise, known as static ZZ interactions. But the FTF architecture remedies this problem.

The ability to suppress these unwanted interactions and the longer coherence times of fluxonium qubits are two factors that enabled the researchers to demonstrate single-qubit gate fidelity of 99.99 percent and two-qubit gate fidelity of 99.9 percent.

These gate fidelities are well above the threshold needed for certain common error correcting codes, and should enable error detection in larger-scale systems.

“Quantum error correction builds system resilience through redundancy. By adding more qubits, we can improve overall system performance, provided the qubits are individually ‘good enough.’ Think of trying to perform a task with a room full of kindergartners. That’s a lot of chaos, and adding more kindergartners won’t make it better,” Oliver explains. “However, several mature graduate students working together leads to performance that exceeds any one of the individuals — that’s the threshold concept. While there is still much to do to build an extensible quantum computer, it starts with having high-quality quantum operations that are well above threshold.”

Building off these results, Ding, Sung, Kannan, Oliver, and others recently founded a quantum computing startup, Atlantic Quantum. The company seeks to use fluxonium qubits to build a viable quantum computer for commercial and industrial applications.

“These results are immediately applicable and could change the state of the entire field. This shows the community that there is an alternate path forward. We strongly believe that this architecture, or something like this using fluxonium qubits, shows great promise in terms of actually building a useful, fault-tolerant quantum computer,” Kannan says.

While such a computer is still probably 10 years away, this research is an important step in the right direction, he adds. Next, the researchers plan to demonstrate the advantages of the FTF architecture in systems with more than two connected qubits.

“This work pioneers a new architecture for coupling two fluxonium qubits. The achieved gate fidelities are not only the best on record for fluxonium, but also on par with those of transmons, the currently dominating qubit. More importantly, the architecture also offers a high degree of flexibility in parameter selection, a feature essential for scaling up to a multi-qubit fluxonium processor,” says Chunqing Deng, head of the experimental quantum team at the Quantum Laboratory of DAMO Academy, Alibaba’s global research institution, who was not involved with this work. “For those of us who believe that fluxonium is a fundamentally better qubit than transmon, this work is an exciting and affirming milestone. It will galvanize not just the development of fluxonium processors but also more generally that for qubits alternative to transmons.”

This work was funded, in part, by the U.S. Army Research Office, the U.S. Undersecretary of Defense for Research and Engineering, an IBM PhD fellowship, the Korea Foundation for Advance Studies, and the U.S. National Defense Science and Engineering Graduate Fellowship Program.

© Credit: Krantz Nanoart

This artist rendering shows the researchers' superconducting qubit architecture, with the fluxonium qubits in red and the blue, transmon coupler in between them.

Four Lincoln Laboratory technologies win five 2023 R&D 100 awards

Ultrasound that doesn’t require touching patients. A web-based tool that reinvents crew scheduling for the Air Force. Cryptographic hardware that protects sensitive data. And the world’s first practical memory for quantum networking.

These four technologies developed at MIT Lincoln Laboratory, either wholly or with collaborators, received 2023 R&D 100 Awards. The ultrasound technology also received a second award in a special category recognizing market-disrupting products. Bestowed by R&D World magazine, the awards recognize the 100 most significant innovations that have transitioned to use or been made available for sale or license in the past year. The worldwide competition is judged by a panel of science and technology experts and industry professionals.

“Lincoln Laboratory has been very fortunate to receive 86 R&D 100 Awards over the past 14 years. Our rate of unclassified technology transition continues to be very high, and we have a similar high transition rate for our classified programs. The laboratory is truly changing the world through its successful technology development and transition. We congratulate everyone involved,” says Lincoln Laboratory Director Eric Evans.

Medical imaging with noncontact ultrasound

Many people are familiar with the ultrasound process — a sonographer presses a transducer onto a patient’s skin and moves it around, gathering images of tissues and organs. Though a well-established technology, ultrasound suffers from sonographer variability, making it difficult to accurately compare repeat measurements, and is limited by the need to make contact with the skin. For these reasons, magnetic resonance imaging and computerized tomography, despite their high costs and lack of portability, are still the predominant imaging technologies for disease tracking.

The Noncontact Laser Ultrasound (NCLUS) for Medical Imaging overcomes these limitations. The skin-safe laser system acquires ultrasound images without touching a patient. It uses a pulsed laser that emits optical energy, which is converted to ultrasound waves upon hitting tissue. The returning echoes are detected by a laser Doppler vibrometer and are processed to generate images. The system’s laser positioning on the body can be accurately reproduced, thus eliminating variability across repeated scans. This repeatability could enable ultrasound to be used to track disease progression, such as changes in tumor size over time.

Its touchless design also opens up entirely new uses for ultrasound: “NCLUS could image burn or trauma victims, patients with open deep-tissue regions directly during surgery, premature infants requiring intensive medical care, patients with neck and spine injuries, and contagious individuals from standoff distances," says Robert Haupt, NCLUS co-inventor.

With NCLUS, medical staff without sonography training might be able to perform ultrasound imaging outside of a hospital — in a doctor’s office, at home, or in a remote battlefield setting. Because of its game-changing potential in the medical imaging industry, NCLUS also received the R&D 100 Silver Medal in the Special Recognition: Market Disruptor Products category, in addition to the R&D 100 Award.

Both awards are shared with the Massachusetts General Hospital Center for Ultrasound Research and Translation and Sound & Bright LLC.

An optimizer for aircrew scheduling

The U.S. Air Force has intense scheduling needs. Its fleet of C-17s, the cargo aircraft that transports troops and supplies globally, marked 4 million flight hours last year. Until recently, Air Force airmen, such as pilots and loadmasters, would have to schedule each flight’s crew manually, on a whiteboard.  

Puckboard has changed that. The web-based application provides intelligent, training-informed scheduling for the first time since military flight scheduling began about 80 years ago, and is returning valuable time back to airmen to focus on their primary duties.

Puckboard's collaborative tools provide schedulers with assignment recommendations while allowing crew members to volunteer for events that work best for their personal lives. Beyond providing a digital calendaring function, Puckboard applies artificial intelligence techniques that consider metrics such as crew training progression, flight-hour distribution, overqualification avoidance, and assignment fragility to recommend optimal schedules. Today, Puckboard hosts 24,000 users and has scheduled more than 315,000 events across 87 squadrons.

“Puckboard’s impact is a direct reflection of the breadth and depth of skill sets and sincere passion that all the contributors have. From the designers, software engineers, and algorithm experts to the active-duty squadrons and aircrew members, all the way up to senior leadership — everyone is committed to increasing the readiness of the U.S. Air Force through the lens of improving the quality of life of our airmen,” says Michael Snyder, a principal investigator on the project. “Scheduling is a complex topic, made even more difficult under uncertainty, and this effort is a testament to being able to solve any problem with the proper team.”

This R&D 100 Award is shared with MIT, RevaComm, Department of the Air Force – MIT AI Accelerator, Air Force 15th Wing, 60th Air Mobility Wing, 437th Airlift Wing, Headquarters Air Mobility Command, Air Force Research Laboratory, assistant secretary of the Air Force (Installations, Environment, and Energy), and Raytheon-BBN.

A device to secure data on uncrewed platforms

For the U.S. military, the use of uncrewed systems is growing to minimize harm to human operators. Because these systems often transmit sensitive data over the air, their radio components must be certified by the National Security Agency (NSA). For years, this certification process has been an insurmountable hurdle for many small businesses and would-be innovators in radio technology and robotics from which the military could benefit. Now, such developers can use an already-NSA certified security solution, developed by Lincoln Laboratory, that’s ready to drop in and deploy for a wide variety of vehicles and missions.

The Security/Cyber Module (SCM) End Cryptographic Unit (ECU) is a compact device that secures tactical datalinks of uncrewed systems. The module modernizes security by pulling together multiple cybersecurity technologies, most notably a technique called Tactical Key Management that establishes secret keys on the fly for secure communication. The module is the first crypto device designed for a broad swath of uncrewed systems within the Joint Communication Architecture for Unmanned Systems (JCAUS), a recent U.S. Department of Defense effort to modularize uncrewed system radio links and allow reuse of NSA-certified components by standardizing capabilities and interfaces.

Since its delivery, the U.S. Navy has awarded a full-rate production contract to Tomahawk Robotics to supply SCM ECUs for use in their explosive ordnance disposal robots. “While developed primarily for Navy ground robotics, the SCM/ECU’s adherence to JCAUS ensures that it is well-suited to airborne and underwater vehicles alike,” says Ben Nahill, a principal investigator on the program.

The award is shared with the Naval Information Warfare Center Pacific.

A scalable, photonic memory for quantum networking

In quantum information processing, memory receives and stores the state of a quantum bit (qubit), similar to how memory for an ordinary communication system or computer receives and stores information as binary states. Memory makes it possible to reliably send and receive information between separate systems, even across lossy transmission links. Lincoln Laboratory’s quantum memory is the first to combine, in a single module, the three capabilities required for networking together separate quantum systems: a photonic interface, a way to correct for loss errors, and an architecture scalable to tens of memories in a single module. Until now, quantum memory systems have fallen short on one or more of these capabilities. 

“This module eliminates many of the barriers to deploying quantum memories into real-world settings and test beds and to actually using them to develop emerging advanced quantum applications, such as distributed sensing and networked quantum processing,” says Ben Dixon, who leads this work.

A photonic interface allows for qubits to be transferred via particles of light (photons) between the memory and optical-fiber networks. The laboratory’s quantum memory uses silicon-vacancy (SiV) diamond color-centers, which are atom-like structures that can be efficiently manipulated with light, even at the single photon level. This SiV technology can also correct for signal-loss errors resulting from inefficient and lossy network links. Because it makes use of individual atomic color-centers, this technology is compatible with efficient "heralded" protocols, where a signal confirms the successful transmission of a photon across the network and storage of the associated qubit in memory.

The SiV module is also scalable. The SiV memory cells are integrated to a custom-made photonic integrated circuit, a technology that enables sending and receiving signals and can be scaled to hundreds of parallel channels. Combining this integration approach with a unique packaging architecture, laboratory researchers integrated eight quantum memories into a single module. Additional memories can be integrated into this single module, which can be joined with additional modules for further scalability.

In addition to these winning technologies, five other Lincoln Laboratory technologies were named R&D 100 award finalists. A gala celebrating the 2023 award winners will be held on Nov. 16 in San Diego, California.

© Photo: Glen Cooper

One of the R&D 100 honorees: A new quantum memory module, shown in the center of a copper mount, comprises a diamond chiplet fused into a photonic integrated circuit, with optical fibers routed up. This module eliminates many of the barriers to deploying quantum memories into real-world settings.

Canceling noise to improve quantum devices

For years, researchers have tried various ways to coax quantum bits — or qubits, the basic building blocks of quantum computers — to remain in their quantum state for ever-longer times, a key step in creating devices like quantum sensors, gyroscopes, and memories.

A team of physicists from MIT have taken an important step forward in that quest, and to do it, they borrowed a concept from an unlikely source — noise-canceling headphones.

Led by Ju Li, the Battelle Energy Alliance Professor in Nuclear Engineering and professor of materials science and engineering, and Paola Cappellaro, the Ford Professor of Engineering in the Department of Nuclear Science and Engineering and Research Laboratory of Electronics, and a professor of physics, the team described a method to achieve a 20-fold increase in the coherence times for nuclear-spin qubits. The work is described in a paper published in Physical Review Letters. The first author of the study is Guoqing Wang PhD '23, a recent doctoral student in Cappellaro’s lab who is now a postdoc at MIT.

“This is one of the main problems in quantum information,” Li says. “Nuclear spin (ensembles) are very attractive platforms for quantum sensors, gyroscopes, and quantum memory, (but) they have coherence times on the order of 150 microseconds in the presence of electronic spins … and then the information just disappears. What we have shown is that, if we can understand the interactions, or the noise, in these systems, we can actually do much better.”

Extending coherence with an “unbalanced echo”

In much the same way noise-cancelling headphones use specific sound frequencies to filter out surrounding noise, the team developed an approach they dubbed an “unbalanced echo” to extend the system’s coherence time.

By characterizing how a particular source of noise — in this case, heat — affected nuclear quadrupole interactions in the system, the team was able to use that same source of noise to offset the nuclear-electron interactions, extending coherence times from 150 microseconds to as long as 3 milliseconds.

Those improvements, however, may only be the beginning. More advances may be possible, says Wang, first author of the study who came up with the protection protocol, as they explore other possible sources of noise.

“In theory, we could even improve it to hundreds or even thousands of times longer. But in practice there may be other sources of noise in the system, and what we’ve shown is that if we can describe them, we can cancel them.”

The paper will have “significant impact” on future work on quantum devices, says Dmitry Budker, leader of the Matter-Antimatter Section of the Helmholtz Institute Mainz, professor at the Johannes Gutenberg University and at the University of California at Berkeley, who was not involved in the research.

“(This group is) the world leaders in the field of quantum sensing,” he says. “They constantly invent new approaches to stimulate developments in this booming field. In this work, they demonstrate a practical way to stretch nuclear coherence time by an order of magnitude with an ingenious spin-echo technique that should be relatively straightforward to implement in applications.”

Cornell University professor of applied and engineering physics Gregory Fuchs calls the work “innovative and impactful.”  

“This (work) is important because although nuclear spin can in principle have much longer coherence lifetimes than the electron spins native to the NV centers, it has been challenging for anyone to observe long-lived nuclear spin ensembles in diamond NV center experiments,” he says. “What Professor Cappellaro and her students have shown is a rather unexpected strategy for doing that. This approach can be highly impactful for applications of nuclear spin ensembles, such as for rotation sensing (a gyroscope).”

Building a sensor using “10 billion clocks”

The experiments and calculations described in the paper deal with a large ensemble — approximately 10 billion — of atomic-scale impurities in diamond known as nitrogen vacancy centers, or NV centers, each of which exists in a specific quantum spin state for the nitrogen-14 nucleus, as well as a localized electron nearby.

While they have long been identified as an ideal candidate for quantum sensors, gyroscopes, memories and more, the challenge, Wang explains, lay in working out a way to get large ensembles of NV centers to work together.

“If you think of each spin as being like a clock, these 10 billion clocks are all slightly different … and you cannot measure them all individually,” Wang says. “What we see is when you prepare all these clocks, they are initially in sync with each other at the beginning, but after some time, they completely lose their phase. We call this their de-phasing time.

“The goal is to use a billion clocks but achieve the same de-phasing time as a single clock,” he continues. “That allows you to get enhancements from measuring multiple clocks, but at the same time you preserve the phase coherence, so you don’t lose your quantum information as fast.”

The underlying theory of temperature heterogeneity induced de-phasing, which relates to the materials properties, was first outlined in March by a team of researchers that included Li, Cappellaro, Wang, and other MIT graduate students. That paper, published in the Journal of Physical Chemistry Letters, described a theoretical approach for calculating how temperature and strain affect different types of interactions which can lead to decoherence.

The first, known as nuclear quadrupole interaction, occurs because the nitrogen nucleus acts as an imperfect nuclear dipole — essentially a subatomic magnet. Because the nucleus is not perfectly spherical, Wang explains, it deforms, disrupting the dipole, which effectively interacts with itself. Similarly, hyperfine interaction is the result of the nucleus magnetic dipole interacting with the nearby electron magnetic dipole. Both of these two types of interactions can vary spatiotemporally, and when considering an ensemble of nuclear spin qubits, de-phasing can happen since “clocks at different locations can get different phases.”

Based on their earlier paper, the team theorized that, if they could characterize how those interactions were affected by heat, they would be able to offset the effect and extend coherence times for the system.

“Temperature or strain affects both of those interactions,” Wang says. “The theory we described predicted how temperature or strain would affect the quadrupole and hyperfine, and then the unbalanced echo we developed in this work is essentially canceling out the spectral drift due to one physical interaction using another different physical interaction, utilizing their correlation induced by the same noise.”

The key novelty of this work, compared to existing spin echo techniques commonly used in the quantum community, is to use different interaction noises to cancel each other such that the noises to be canceled can be highly selective. “What’s exciting, though, is that we can use this system in other ways,” he continues. “So, we could use this to sense temperature or strain field spatiotemporal heterogeneity. This could be quite good for something like biological systems, where even a very minute temperature shift could have significant effects.”

Additional applications

Those applications, Wang says, barely scratch the surface of the system’s potential applications.

“This system could also be used to examine electrical currents in electric vehicles, and because it can measure strain fields, it could be used for non-destructive structural health evaluation,” Li says. “You could imagine a bridge, if it had these sensors on it, we could understand what type of strain it’s experiencing. In fact, diamond sensors are already used to measure temperature distribution on the surface of materials, because it can be a very sensitive, high spatial resolution sensor.”

Another application, Li says, may be in biology. Researchers have previously demonstrated that the use of quantum sensors to map neuronal activity from electromagnetic fields could offer potential improvements, enabling a better understanding of some biological processes.

The system described in the paper could also represent a significant leap forward for quantum memory.

While there are some existing approaches to extending the coherence time of qubits for use in quantum memory, those processes are complex, and typically involve “flipping” — or reversing the spin — of the NV centers. While that process works to reverse the spectral drift that causes decoherence, it also leads to the loss of whatever information was encoded in the system.

By eliminating the need to reverse the spin, the new system not only extends the coherence time of the qubits, but prevents the loss of data, a key step forward for quantum computing.

Going forward, the team plans to investigate additional sources of noise — like fluctuating electrical field interference — in the system with the goal of counteracting them to further increase coherence time.

“Now that we’ve achieved a 20-fold improvement, we’re looking at how we can improve it even more, because intrinsically, this unbalanced echo can achieve an almost infinite improvement,” Li says. “We are also looking at how we can apply this system to the creation of a quantum gyroscope, because coherence time is just one key parameter to building a gyroscope, and there are other parameters we’re trying to optimize to (understand) the sensitivity we can achieve compared to previous techniques.”

This work was supported in part by the Defense Advanced Research Projects Agency DRINQS program, the National Science Foundation, and the Defense Threat Reduction Agency Interaction of Ionizing Radiation with Matter University Research Alliance. The calculations in this work were performed in part on the Texas Advanced Computing Center and the MIT engaging cluster.

© Photo courtesy of the researchers.

This quantum sensor in the MIT Quantum Engineering Group is based on NV centers in diamond. It was designed and built by the research team.

Wolfgang Ketterle receives Vannevar Bush Faculty Fellowship

MIT professor of physics Wolfgang Ketterle was named one of 10 recipients of the 2023 Class of Vannevar Bush Faculty Fellowships by the U.S. Department of Defense (DoD).

Ketterle will use his award of up to $3 million over five years for his research into quantum science with ultracold atoms on a 50-nanometer scale.

“With the Vannevar Bush faculty fellowship’s generous and flexible funding, I can explore a new direction of research,” says Ketterle, the John D. MacArthur Professor of Physics at MIT. “Using a super-resolution technique, we can put atoms only 50 nm apart and study new quantum science. We will use dysprosium atoms for these studies, since they are the most magnetic atoms in the periodic table. At a 50-nm distance, dysprosium atoms will have strong interactions which are purely magnetic. We want to use this to build a purely magnetic quantum gate.”

As the department's flagship single-investigator award for basic research, the fellowship commemorates Vannevar Bush (1890-1974), the former director of the U.S. Office of Scientific Research and Development. He was nicknamed “The General of Physics” for his role in building up the science and technology enterprise that drove the United States' rapid growth as a military and economic superpower.

Bush received his PhD in 1916 from MIT, where he later served as vice president and dean of engineering. He also founded the defense and electronics company that became Raytheon in 1922.

The fellowship aims to advance “transformative fundamental research within universities, nurturing high-risk ideas in pursuit of breakthrough discoveries, and giving researchers the freedom to explore the frontiers of knowledge in their respective fields.”

The 2023 Class of Vannevar Bush Faculty Fellows will join a group of about 50 current fellows involved in DoD research that includes materials science, cognitive neuroscience, quantum information sciences, and applied mathematics.  

“I am pleased to welcome these exceptional scholars to the DoD family,” says Under Secretary of Defense for Research and Engineering Heidi Shyu. “Their selections for the Vannevar Bush Faculty Fellowship will allow them to truly change the course of science in their fields, and I know they will set the department up for breakthroughs in our future capabilities.”

Formerly known as the National Security Science and Engineering Faculty Fellowship, this award is sponsored by the Basic Research Office within the Office of the Under Secretary of Defense for Research and Engineering, with grants managed by the Office of Naval Research. For the fiscal year 2023 competition, the Basic Research Office received 190 white papers, from which panels of experts invited 31 proposals, for a final selection of 10 fellows.

© Photo courtesy of the Department of Physics.

For his research into quantum science with ultracold atoms on a 50 nanometer scale, professor of physics Wolfgang Ketterle was named one of 10 recipients of the 2023 Class of Vannevar Bush Faculty Fellowships by the U.S. Department of Defense.

Sensing and controlling microscopic spin density in materials

Electronic devices typically use the charge of electrons, but spin — their other degree of freedom — is starting to be exploited. Spin defects make crystalline materials highly useful for quantum-based devices such as ultrasensitive quantum sensors, quantum memory devices, or systems for simulating the physics of quantum effects. Varying the spin density in semiconductors can lead to new properties in a material — something researchers have long wanted to explore — but this density is usually fleeting and elusive, thus hard to measure and control locally.

Now, a team of researchers at MIT and elsewhere has found a way to tune the spin density in diamond, changing it by a factor of two, by applying an external laser or microwave beam. The finding, reported this week in the journal PNAS, could open up many new possibilities for advanced quantum devices, the authors say. The paper is a collaboration between current and former students of professors Paola Cappellaro and Ju Li at MIT, and collaborators at Politecnico of Milano. The first author of the paper, Guoqing Wang PhD ’23, worked on his PhD thesis in Cappellaro’s lab and is now a postdoc at MIT.

A specific type of spin defect known as a nitrogen vacancy (NV) center in diamond is one of the most widely studied systems for its potential use in a wide variety of quantum applications. The spin of NV centers is sensitive to any physical, electrical, or optical disturbance, making them potentially highly sensitive detectors. “Solid-state spin defects are one of the most promising quantum platforms,” Wang says, partly because they can work under ambient, room-temperature conditions. Many other quantum systems require ultracold or other specialized environments.

“The nanoscale sensing capabilities of NV centers makes them promising for probing the dynamics in their spin environment, manifesting rich quantum many body physics yet to be understood”, Wang adds. “A major spin defect in the environment, called P1 center, can usually be 10 to 100 times more populous than the NV center and thus can have stronger interactions, making them ideal for studying many-body physics.”

But to tune their interactions, scientists need to be able to change the spin density, something that had previously seldom been achieved. With this new approach, Wang says, “We can tune the spin density so it provides a potential knob to actually tune such a system. That’s the key novelty of our work.”

Such a tunable system could provide more flexible ways of studying the quantum hydrodynamics, Wang says. More immediately, the new process can be applied to some existing nanoscale quantum-sensing devices as a way to improve their sensitivity.

Li, who holds a joint appointment in MIT’s departments of Nuclear Science and Engineering and Materials Science and Engineering, explains that today’s computers and information processing systems are all based on the control and detection of electrical charges, but some innovative devices are beginning to make use of the property called spin. The semiconductor company Intel, for example, has been experimenting with new kinds of transistors that couple spin and charge, potentially opening a path to devices based on spintronics.

“Traditional CMOS transistors use a lot of energy,” Li says, “but if you use spin, as in this Intel design, then you can reduce the energy consumption by a lot.” The company has also developed solid-state spin qubit devices for quantum computing, and “spin is something people want to control in solids because it’s more energy efficient, and it’s also a carrier of quantum information.”

In the study by Li and his colleagues, the newly achieved level of control over spin density allows each NV center to act like a kind of atomic-scale “radar” that can both sense and control the nearby spins. “We basically use a particular NV defect to sense the surrounding electronic and nuclear spins. This quantum sensor reveals the nearby spin environment and how that’s affected dynamically by the charge flow, which in this case is pumped up by the laser,” Li says.

This system makes it possible to dynamically change the spin concentration by a factor of two, he says. This could ultimately lead to devices where a single point defect or a single atom could be the basic computational unit. “In the long run, a single point defect, and the localized spin and the localized charge on that single point defect, can be a computing logic. It can be a qubit, it can be a memory, it can be a sensor,” he says.

He adds that much work remains to develop this newly found phenomenon. “We’re not exactly there yet,” he says, but what they have demonstrated so far shows that they have “really pushed down the measurement and control of the spin and charge state of point defects to an unprecedented level. So, in the long run, I think this would support using individual defect, or a small number of defects, to become the information processing and sensing devices.”

In this work so far, Wang says, “we find this phenomenon and we demonstrate it,” but further work is needed to fully understand the physical mechanism of what is taking place in these systems. “Our next step is to dig more deeply into the physics, so we would like to know better what’s the underlying physical mechanism” behind the effects they see. In the long term, “with better understanding of these systems, we hope to explore more quantum simulation and sensing ideas, such as simulating interesting quantum hydrodynamics, and even transporting quantum information between different spin defects.”

The findings were made possible, in part, by the team’s development of a new wide-field imaging setup that allows them to measure many different spatial locations within the crystalline material simultaneously, using a fast single-photon detector array, combined with a microscope. “We are able to spatially image the density distribution over different spin species like a fingerprint, and the charge transport dynamics,” although that work is still preliminary, Wang says.

Although their work was done using lab-grown diamond, the principles could be applied to other crystalline solid-state defects, he says. NV centers in diamond have been attractive for research because they can be used at room temperature and they have already been well-studied. But silicon vacancy centers, donors in silicon, rare-earth ions in solids, and other crystal materials may have different properties that could turn out to be useful for particular kinds of applications.

“As information science progresses, eventually people will be able to control the positions and the charge of individual atoms and defects. That’s the long-term vision,” Li says. “If you can have every atom storing different information, it’s a much larger information storage and processing capability” compared to existing systems where even a single bit is stored by a magnetic domain of many atoms. “You can say it’s the ultimate limit of Moore’s Law: eventually going down to one defect or one atom.”

While some applications may require much more research to develop to a practical level, for some kinds of quantum sensing systems, the new insights can be quickly translated into real-world uses, Wang says. “We can immediately improve the quantum sensors’ performance based on our results,” he says.

“Overall, this result is very exciting for the field of solid-state spin defects,” says Chong Zu, an assistant professor of physics at Washington University in St. Louis, who specializes in quantum information but was not involved in this work. “In particular, it introduces a powerful approach of using charge ionization dynamics to continuously tune the local spin defect density, which is important in the context of applications of NV centers for quantum simulation and sensing.”

The research team included Changhao Li, Hao Tang, Boning Li, Francesca Madonini, Faisal Alsallom, and Won Kyu Calvin Sun, all at MIT; Pai Peng at Princeton University; and Federica Villa at the Politecnico de Milano, in Italy. The work was partly supported by the U.S. Defense Advanced Research Projects Agency.

© Image: Courtesy of the researchers

MIT researchers found a way to tune the spin density in diamond by applying an external laser or microwave beam. The finding could open new possibilities for advanced quantum devices. Pictured is a view of the laser equipment used in the researchers’ work.

Researchers grow precise arrays of nanoLEDs

Halide perovskites are a family of materials that have attracted attention for their superior optoelectronic properties and potential applications in devices such as high-performance solar cells, light-emitting diodes, and lasers.

These materials have largely been implemented into thin-film or micron-sized device applications. Precisely integrating these materials at the nanoscale could open up even more remarkable applications, like on-chip light sources, photodetectors, and memristors. However, achieving this integration has remained challenging because this delicate material can be damaged by conventional fabrication and patterning techniques.

To overcome this hurdle, MIT researchers created a technique that allows individual halide perovskite nanocrystals to be grown on-site where needed with precise control over location, to within less than 50 nanometers. (A sheet of paper is 100,000 nanometers thick.) The size of the nanocrystals can also be precisely controlled through this technique, which is important because size affects their characteristics. Since the material is grown locally with the desired features, conventional lithographic patterning steps that could introduce damage are not needed.

The technique is also scalable, versatile, and compatible with conventional fabrication steps, so it can enable the nanocrystals to be integrated into functional nanoscale devices. The researchers used this to fabricate arrays of nanoscale light-emitting diodes (nanoLEDs) — tiny crystals that emit light when electrically activated. Such arrays could have applications in optical communication and computing, lensless microscopes, new types of quantum light sources, and high-density, high-resolution displays for augmented and virtual reality.

“As our work shows, it is critical to develop new engineering frameworks for integration of nanomaterials into functional nanodevices. By moving past the traditional boundaries of nanofabrication, materials engineering, and device design, these techniques can allow us to manipulate matter at the extreme nanoscale dimensions, helping us realize unconventional device platforms important to addressing emerging technological needs,” says Farnaz Niroui, the EE Landsman Career Development Assistant Professor of Electrical Engineering and Computer Science (EECS), a member of the Research Laboratory of Electronics (RLE), and senior author of a new paper describing the work.

Niroui’s co-authors include lead author Patricia Jastrzebska-Perfect, an EECS graduate student; Weikun “Spencer” Zhu, a graduate student in the Department of Chemical Engineering; Mayuran Saravanapavanantham, Sarah Spector, Roberto Brenes, and Peter Satterthwaite, all EECS graduate students; Zheng Li, an RLE postdoc; and Rajeev Ram, professor of electrical engineering. The research is published today in Nature Communications.

Tiny crystals, huge challenges

Integrating halide perovskites into on-chip nanoscale devices is extremely difficult using conventional nanoscale fabrication techniques. In one approach, a thin film of fragile perovskites may be patterned using lithographic processes, which require solvents that may damage the material. In another approach, smaller crystals are first formed in solution and then picked and placed from solution in the desired pattern.

“In both cases there is a lack of control, resolution, and integration capability, which limits how the material can be extended to nanodevices,” Niroui says.

Instead, she and her team developed an approach to “grow” halide perovskite crystals in precise locations directly onto the desired surface where the nanodevice will then be fabricated.

Core to their process is to localize the solution that is used in the nanocrystal growth. To do so, they create a nanoscale template with small wells that contain the chemical process through which crystals grow. They modify the surface of the template and the inside of the wells, controlling a property known as “wettability” so a solution containing perovskite material won’t pool on the template surface and will be confined inside the wells.

“Now, you have these very small and deterministic reactors within which the material can grow,” she says.

And that is exactly what happens. They apply a solution containing halide perovskite growth material to the template and, as the solvent evaporates, the material grows and forms a tiny crystal in each well.

A versatile and tunable technique

The researchers found that the shape of the wells plays a critical role in controlling the nanocrystal positioning. If square wells are used, due to the influence of nanoscale forces, the crystals have an equal chance of being placed in each of the well’s four corners. For some applications, that might be good enough, but for others, it is necessary to have a higher precision in the nanocrystal placement.

By changing the shape of the well, the researchers were able to engineer these nanoscale forces in such a way that a crystal is preferentially placed in the desired location.

As the solvent evaporates inside the well, the nanocrystal experiences a pressure gradient that creates a directional force, with the exact direction being determined using the well’s asymmetric shape.

“This allows us to have very high precision, not only in growth, but also in the placement of these nanocrystals,” Niroui says.

They also found they could control the size of the crystal that forms inside a well. Changing the size of the wells to allow more or less growth solution inside generates larger or smaller crystals.

They demonstrated the effectiveness of their technique by fabricating precise arrays of nanoLEDs. In this approach, each nanocrystal is made into a nanopixel which emits light. These high-density nanoLED arrays could be used for on-chip optical communication and computing, quantum light sources, microscopy, and high-resolution displays for augmented and virtual reality applications.

In the future, the researchers want to explore more potential applications for these tiny light sources. They also want to test the limits of how small these devices can be, and work to effectively incorporate them into quantum systems. Beyond nanoscale light sources, the process also opens up other opportunities for developing halide perovskite-based on-chip nanodevices.

Their technique also provides an easier way for researchers to study materials at the individual nanocrystal level, which they hope will inspire others to conduct additional studies on these and other unique materials.

“Studying nanoscale materials through high-throughput methods often requires that the materials are precisely localized and engineered at that scale,” Jastrzebska-Perfect adds. “By providing that localized control, our technique can improve how researchers investigate and tune the properties of materials for diverse applications.”

“The team has developed a very clever method for deterministic synthesis of individual perovskite nanocrystals on substrates. They can control the exact placement of the nanocrystals in an unprecedented scale, thus enabling a platform for fabrication of highly efficient, nanoscale LEDs based on single nanocrystals,” says Ali Javey, professor of electrical engineering and computer sciences at the University of California at Berkeley, who was not involved with this research. “It is an exciting work as it overcomes a fundamental challenge in the field.”

This work was supported, in part, by the National Science Foundation and the MIT Center for Quantum Engineering. The fabrication and characterization procedures were carried out, in part, using MIT.nano's shared facilities.

© Image: Courtesy of Sampson Wilcox, RLE

A new MIT platform enables researchers to “grow” halide perovskite nanocrystals with precise control over the location and size of each individual crystal, integrating them into nanoscale light-emitting diodes. Pictured is a rendering of a nanocrystal array emitting light.

Superconducting qubit foundry accelerates progress in quantum research

In the past three decades, quantum computing has grown from a theoretical fantasy to a worldwide industry, pushing closer to a technology that could one day solve problems too complex for even the most powerful supercomputers. MIT Lincoln Laboratory is not only at the forefront of research, but is making quantum research accessible to a broader community through its Superconducting Qubits at Lincoln Laboratory (SQUILL) Foundry. 

Quantum bits (qubits) are building blocks of quantum computers, like transistors are to classical computers. There are many ways of making a qubit; one of the most promising is superconducting qubits, which are generated by using circuits made out of superconducting elements. The qubits are fabricated using techniques similar to conventional microelectronics fabrication, such as depositing and etching thin metal films on a substrate. Then, they are operated at near-absolute zero temperatures to form “artificial atoms.”

Realizing the promise of quantum computing requires fundamental research and experimentation using these and other qubits. But superconducting qubits are tricky to build, and a major barrier for scientists wishing to pursue this research is the expensive tooling and specialized processes needed to fabricate the circuits. 

The SQUILL Foundry was stood up to remove this barrier. Sponsored by the Laboratory for Physical Sciences (LPS) Qubit Collaboratory, a National Quantum Initiative-funded center, the program makes Lincoln Laboratory’s cutting-edge fabrication capabilities available at no cost to institutions working on U.S. government-funded research. Researchers can submit quantum circuit designs for fabrication, and the completed circuits are returned to advance scientific inquiry in their home facilities. 

“Democratizing access to robust, reliable qubit fabrication dramatically lowers the barrier to entry in superconducting qubits,” says Mollie Schwartz, the principal investigator of the project and a leader of superconducting qubit research at Lincoln Laboratory. “We want to enable researchers whose core focus is not in materials and fabrication to really focus on driving progress in the areas of superconducting qubit research that they are most interested in, and to allow the community to leverage some of the more advanced capabilities we’ve developed.” 

One foundry program user, Stanford University Professor David Schuster, says that the SQUILL Foundry has allowed his quantum lab to consider experiments they could not have attempted before because of the complexity of nanofabrication required. “It has allowed my younger students to design and measure complex quantum circuits much faster than they could in the past,” Schuster says.

An area of super-specialization 

One of the chief advantages of superconducting qubits is their designability: Their dynamics and interactions are not dictated by nature, like those of a physical atom, but rather are designed by combining capacitors, inductors, and Josephson junctions (a type of superconducting switch) in creative ways to create the energy landscape of interest. Because of this designability, superconducting qubits represent a diverse family of quantum circuits, all of which can look different and behave in unique ways.

Advancing the state of the art in superconducting qubit hardware requires knowledge across a range of disciplines, including materials, fabrication, circuit design and simulation, packaging, cryogenics, low-noise measurement, hardware-software interfacing, and quantum compilation. As understanding of materials and processes has advanced over time, fabricating the highest-quality qubits increasingly relies on millions of dollars of fabrication equipment and countless hours of process development and sustainment.

“It has become increasingly challenging for individual organizations to maintain this full stack of expertise, particularly as circuits become more complex to design, fabricate, and measure,” Schwartz says. “As a result, superconducting qubit hardware research has remained centralized into a relatively small number of laboratories and large universities capable of developing and sustaining this expertise.”  

MIT Lincoln Laboratory is one of these laboratories, with more than 20 years of research and development in superconducting qubits and demonstrations of world-leading qubit performance. The qubits are made on-site at the Microelectronics Laboratory, considered to be one of the U.S. government’s most advanced foundries, and in specialized prototyping facilities. The collective expertise and equipment of this facility have made it possible to stand-up the SQUILL Foundry.  

From qubit design to delivery 

The SQUILL Foundry first ran as a pilot from July 2021 to February 2023. The pilot program was made available to a preselected set of users who had a range of experience in the field, from long-standing leaders to new faculty, and including institutions with sophisticated fabrication facilities and those with limited fabrication or no fabrication resources. 

Users received a design rule guide and samples of physical layout files to provide a starting point for their designs. They also received a high-quality “candle qubit” chip that allows the users to qualify their at-home quantum measurement apparatuses. “It’s a chip that we’ve already done measurements on, and then they can measure the same chip in their systems to make sure that those systems are meeting expectations,” says Cyrus Hirjibehedin, who led user interactions for the project. “Overall, the process has gone very well, and we’ve received overwhelmingly positive feedback from our users.”

Users submitted custom designs to the SQUILL Foundry and received back fabricated devices wire-bonded into cryogenic packages. Users then leveraged these devices for scientific inquiry, resulting in 13 presentations and four scientific papers in preparation or print, with more to come as research proceeds. 

“There is certainly a learning curve when you are used to fabricating devices in-house, but the support and information provided by the foundry to aid users in the process has been phenomenal,” says Professor Machiel Blok, who runs a research group at the University of Rochester. The foundry helped his group overcome a years-long delay, caused by the pandemic, in fabricating quantum processors at their facility.

Professor Kater Murch from Washington University in St. Louis, Missouri, says that participating in the program allowed his team to design, set up, and measure devices far beyond their normal capabilities. “We’re just finalizing initial results from our first projects, but the work we’ve done in the lab to get our devices to work puts us several years ahead of where we would have been without the program,” Murch says.  

In parallel, the laboratory worked to transition its qubit fabrication process from 50-millimeter prototyping wafers (the substrate on which the circuits are fabricated) to 200-mm production-scale wafers. “In addition to giving us more real estate on the wafer, the transition allows us have better process control and to use automated equipment and cleaner resources, with less direct human wafer handling,” says Jeffrey Knecht, the SQUILL Foundry fabrication lead.  

Opening the doors to the research community

Building on the success of the pilot, the SQUILL Foundry is transitioning to a full four-year project, opening the doors to the broader research community in a by-application access model that is available to any sponsor-approved superconducting qubit research project supported by a U.S. government grant.

“Qubit chips from the Lincoln Laboratory foundry effort have already helped nine research groups across the country realize their ideas and accelerate their research,” says Charles Tahan, the director of the LPS Qubit Collaboratory. “LPS is pleased to support the expansion of the Qubits for Computing Foundry program to bring world-class qubits to many more researchers and students.”

The full-scale foundry will roll out more advanced capabilities, including compact arrays of Josephson junctions and flip-chip integration (a compact method of connecting multiple circuits), to enable more advanced qubit designs. Additional capabilities will be developed with user input and made available as the SQUILL Foundry program proceeds, to ensure that the foundry continues to meet the evolving needs of the community.

More than 20 research groups are poised to leverage the foundry as the access model expands, and this number is expected to grow over time. Schwartz says she is especially interested in exploring paths to work with schools that do not have the resources to purchase and maintain fabrication equipment.

“There are incredibly talented and creative students across the country who cannot currently participate in superconducting qubit hardware research, but whose discoveries might change the way we think about this technology. Lowering the barrier to entry will enable some of these institutions to stand up superconducting qubit research groups, which will help expose more students to quantum research and expand our workforce as the industry grows,” Schwartz says.

Those interested in working with the SQUILL Foundry can reach out for more information at

© Photo: Nicole Fandel

This 200-millimeter superconducting qubit wafer was fabricated through the Superconducting Qubits at Lincoln Laboratory (SQUILL) Foundry. The SQUILL Foundry is expanding access to quantum research by fabricating high-quality quantum circuits for U.S. research organizations.

A new mathematical “blueprint” is accelerating fusion device development

Developing commercial fusion energy requires scientists to understand sustained processes that have never before existed on Earth. But with so many unknowns, how do we make sure we’re designing a device that can successfully harness fusion power?

We can fill gaps in our understanding using computational tools like algorithms and data simulations to knit together experimental data and theory, which allows us to optimize fusion device designs before they’re built, saving much time and resources.

Currently, classical supercomputers are used to run simulations of plasma physics and fusion energy scenarios, but to address the many design and operating challenges that still remain, more powerful computers are a necessity, and of great interest to plasma researchers and physicists.

Quantum computers’ exponentially faster computing speeds have offered plasma and fusion scientists the tantalizing possibility of vastly accelerated fusion device development. Quantum computers could reconcile a fusion device’s many design parameters — for example, vessel shape, magnet spacing, and component placement — at a greater level of detail, while also completing the tasks faster. However, upgrading to a quantum computer is no simple task.

In a paper, “Dyson maps and unitary evolution for Maxwell equations in tensor dielectric media,” recently published in Physics Review A, Abhay K. Ram, a research scientist at the MIT Plasma Science and Fusion Center (PSFC), and his co-authors Efstratios Koukoutsis, Kyriakos Hizanidis, and George Vahala present a framework that would facilitate the use of quantum computers to study electromagnetic waves in plasma and its manipulation in magnetic confinement fusion devices.

Quantum computers excel at simulating quantum physics phenomena, but many topics in plasma physics are predicated on the classical physics model. A plasma (which is the “dielectric media” referenced in the paper’s title) consists of many particles — electrons and ions — the collective behaviors of which are effectively described using classic statistical physics. In contrast, quantum effects that influence atomic and subatomic scales are averaged out in classical plasma physics.  

Furthermore, the descriptive limitations of quantum mechanics aren’t suited to plasma. In a fusion device, plasmas are heated and manipulated using electromagnetic waves, which are one of the most important and ubiquitous occurrences in the universe. The behaviors of electromagnetic waves, including how waves are formed and interact with their surroundings, are described by Maxwell’s equations — a foundational component of classical plasma physics, and of general physics as well. The standard form of Maxwell’s equations is not expressed in “quantum terms,” however, so implementing the equations on a quantum computer is like fitting a square peg in a round hole: it doesn’t work.

Consequently, for plasma physicists to take advantage of quantum computing’s power for solving problems, classical physics must be translated into the language of quantum mechanics. The researchers tackled this translational challenge, and in their paper, they reveal that a Dyson map can bridge the translational divide between classical physics and quantum mechanics. Maps are mathematical functions that demonstrate how to take an input from one kind of space and transform it to an output that is meaningful in a different kind of space. In the case of Maxwell’s equations, a Dyson map allows classical electromagnetic waves to be studied in the space utilized by quantum computers. In essence, it reconfigures the square peg so it will fit into the round hole without compromising any physics.

The work also gives a blueprint of a quantum circuit encoded with equations expressed in quantum bits (“qubits”) rather than classical bits so the equations may be used on quantum computers. Most importantly, these blueprints can be coded and tested on classical computers.

For years we have been studying wave phenomena in plasma physics and fusion energy science using classical techniques. Quantum computing and quantum information science is challenging us to step out of our comfort zone, thereby ensuring that I have not ‘become comfortably numb,’” says Ram, quoting a Pink Floyd song.

The paper's Dyson map and circuits have put quantum computing power within reach, fast-tracking an improved understanding of plasmas and electromagnetic waves, and putting us that much closer to the ideal fusion device design.   

© Photo: Julianna Mullen

Abhay Ram, a principal research scientist at the MIT Plasma Science and Fusion Center and a co-author of the recently-published paper "Dyson maps and unitary evolution for Maxwell equations in tensor dielectric media," took to his whiteboard to explain the significance of Dyson maps and quantum computing.

Three Spanish MIT physics postdocs receive Botton Foundation fellowships

Three Spanish MIT postdocs, Luis Antonio Benítez, Carolina Cuesta-Lazaro, and Fernando Romero López, were chosen by the Department of Physics as the first cohort of Mauricio and Carlota Botton Foundation Fellows.

This year’s recipients are provided with a one-year stipend and a research fund to pursue their research interests; they will visit the Botton Foundation in Madrid this summer.

L. Antonio Benítez

A dual citizen of Spain and Colombia, L. Antonio Benítez is an MIT postdoc whose research focuses on the investigation of the electronic properties of novel quantum materials, with a particular emphasis on two-dimensional materials like graphene and transition metal dichalcogenides. His work aims to push the boundaries of our knowledge of these materials and unlock their full potential for future technologies. Benítez received his PhD in physics from the Autonomous University of Barcelona, where he specialized in the spin and electronic properties of these materials, developing a deep understanding of their unique characteristics and behavior.  

Carolina Cuesta-Lazaro

Carolina Cuestra-Lazaro’s main research interests lie on the intersection of cosmology and artificial intelligence. She is interested in developing robust and interpretable machine-learning models for advancement in physics, especially for developing techniques for cosmological inference to understand the accelerated expansion of the universe. She received her PhD in astronomy and astrophysics at the Institute for Computational Cosmology, and now holds a shared position between MIT’s Institute for Artificial Intelligence and Fundamental Interactions and Harvard University’s Institute for Theory and Computation at the Center for Astrophysics. Cuestra-Lazaro hails from Cuenca, where she says “You can find some of the best Manchego cheese.”

Fernando Romero López 

Romero-López completed his PhD in 2021 at the University of Valencia. As a postdoc, his research focuses on understanding the strong interactions among quarks and gluons, described by quantum chromodynamics (QCD). By combining effective field theories with numerical simulations of quantum field theories (lattice QCD) and machine-learning tools, he is seeking a better understanding of the mechanisms of confinement, how protons, neutrons, and other hadrons are formed, the properties of atomic nuclei, and the nature of exotic hadrons that have been detected at the Large Hadron Collider.

The foundation also recently funded scholarships for two PhD physics students at MIT: Oriol Rubies Bigorda, who is researching the physics of interacting quantum particles and their applications in future quantum technologies, and Miguel Calvo Carrera, who is interested in the application of physics to develop renewable energy sources.

Established in 2017, the Mauricio and Carlota Botton Foundation supports scientific research, including the training of young physicists in the most prestigious universities in the world, and to provide support for conferences that bring world experts in the frontier fields of physics to Spain.

© Photos courtesy of the subjects.

Left to right: MIT postdocs Luis Antonio Benítez, Carolina Cuesta-Lazaro, and Fernando Romero López

Life in a hologram

Dan Harlow spends a lot of time thinking in a “boomerang” universe.

The MIT physicist is searching for answers to one of the biggest questions in modern physics: How can our universe abide by two incompatible rulebooks?

The first — the Standard Model of Physics — is the quantum mechanical theory of particles, fields, and forces, and the ways in which they interact to build the universe we live in. The second — Einstein’s theory of general relativity — describes the influence of gravity and how the fundamental force pulls together matter to build the planets, galaxies, and other massive objects.

Both theories do remarkably well in their respective lanes. However, Einstein’s theory breaks down when trying to describe how gravity works at quantum scales, while quantum mechanics makes reality-bending predictions when applied at massive, cosmic dimensions. For over a century, physicists have searched for ways to unite the two theories and get to the truth of what makes our universe tick.

Harlow suspects that any connecting thread may be too delicate to grasp in our existing universe. Instead, he’s looking for answers in a “boomerang” version — an alternate reality that folds back on itself, much like a boomerang's trajectory, rather than stretching and expanding without end as our actual universe does. Quantum gravity in this boomerang universe turns out to be easier to understand, as it can be reformulated in terms of conventional quantum theory (without gravity) using a powerful idea called holographic duality. This makes it far simpler to contemplate, at least from a theory perspective.

In this boomerang environment, Harlow has made some exciting, unexpected revelations. He has shown, for instance, that the equations that describe how gravity behaves in this “toy” universe are the very same equations that control the quantum error-correcting codes that will hopefully soon be used to build real-world quantum computers. That the mathematics describing gravity should have anything to do with protecting information in quantum computers was a surprise in itself. The fact that both phenomena shared the same physics, at least in this alternate universe, suggests a potential connection between Einstein’s theory and quantum mechanics in the real universe.

The discovery, which Harlow made as a postdoc at Princeton University in 2014, sparked fresh lines of inquiry in the study of both quantum gravity and quantum information theory. Since joining MIT and the Center for Theoretical Physics in 2017, Harlow has continued his search for fundamental connections between general relativity and quantum mechanics, and how they may intersect in the contexts of black holes and cosmology.

“One of the things that’s been fun is, even though in physics and more in generally science we’re all studying different systems and experiments, many of the ideas are the same,” says Harlow, an associate professor who received tenure in 2022. “So, I try to have an open mind and keep my ears open, and look for how things may be related.”

“A humanist philosophy”

Born in Cincinnati, Harlow moved as a child with his family to Boston, where he spent several years before the family moved again, putting down roots in Chicago. When he was 10, he took up piano lessons, focusing first on classical music, then rock. In junior high, he played keyboard in various bands before finding his groove in the looser, more improvisational style of jazz.

“I love sitting down and playing with people, and seeing where things will go,” Harlow says.

His love of jazz was partly what drew him to New York City after high school, where he attended Columbia University, which happened to be near some of the best jazz clubs in the city. The university’s core curriculum, which required students to read classic works of literature and philosophy, also appealed.

“You can’t graduate from Columbia without reading “The Iliad,” Harlow says. “That gives you a shared community of things you can talk about. I liked the humanist philosophy that drives the place. Even if I chose to be a physicist, I would still have this broader cultural experience.”

Harlow worked for three years as an undergraduate research assistant in an experimental cosmology lab on campus, where he learned to work in a clean room and run simulations to improve the performance of filters that were designed to pick out subtle signs of radiation left over from the Big Bang.

Harlow particularly appreciated the general approach of the lab’s leader, Amber Miller, who was then a junior faculty member.

“She had this great way she ran her group, where she wasn’t so hung up on publications or getting things done on a short timescale,” Harlow recalls. “She just let us play around.”

Open questions

That mental freedom to explore new ideas would stay with Harlow throughout his career. From Columbia, he went west to Stanford University in 2006. Within the physics department, he found he aligned most naturally with Professor Leonard Susskind, a theoretical physicist and leader in the study of string theory.

“His strong desire to identify the things that aren’t important and set them aside so you can focus on the essence of the problem — that was also the way I try to think,” says Harlow, who ended up choosing Susskind as his advisor. “Lenny said, ‘work on whatever you want, and I’ll talk to you about it.’”

With this open invitation, Harlow kept an ear on conversations within Susskind’s group to get a sense of the big questions in the field. What he heard was a problem that would shape the rest of his research career: the question of how to connect quantum mechanics with general relativity, in the context of cosmology, and scientists’ understanding of the large-scale structure and evolution of the universe.

In search of an answer, Harlow read up on everything he could find on both theories. His reading also bled into quantum information science — primarily, a field that focuses on applying principles of quantum mechanics and information theory to the study and development of quantum computers.  

“Whenever I have a hint that some tool will be important for a problem I’m trying to solve, I learn much more about it than what I think I need,” Harlow says. “More often than not, that investment pays off.”

At the end of his time at Stanford, Harlow decided to “take a hard turn,” pivoting from cosmology to black holes, which he considered to be a simpler system to study for any fundamental threads connecting quantum mechanics and general relativity.

In 2012, he went back east to Princeton for a three-year postdoc, during which he began to explore the quantum behavior of gravitational black holes. To simplify the problem, he did so in a “boomerang” universe — what physicists know as “anti-de Sitter space,” named after the physicist who studied the curvature of the universe. As Harlow read more on quantum information, he noticed, and ultimately confirmed, an unexpected overlap in the physics of gravity around black holes and the quantum error-correcting codes designed to protect information.

“That was a very exploratory, transformative time,” Harlow says. “I’m still exploring a lot of the paths that I started there.”

After a second postdoc at Harvard University, Harlow joined MIT as a junior faculty member in 2017, where he continues to make surprising connections in the study of quantum gravity and quantum information science. At the Institute, and in the field of theoretical physics more broadly, he’s enjoyed a collegial, productive disregard for authority.

“This is a community where I can go up to the most famous theoretical physicist in the world, tell them that they’re wrong, and if I have an argument, they’ll listen to me,” Harlow says. “People are open. There’s this core shared agreement that, what matters is that we find the right answer. It matters less who finds it.”

Among Harlow's accomplishments since coming to MIT are a proof that there are strong restrictions on the possible symmetries of quantum gravity, a deeper understanding of the nature of energy in gravitational systems, and a concrete mathematical framework for understanding the interiors of quantum mechanical black holes.

Beyond research, Harlow is working to bring more diverse voices and perspectives into the field of physics. In addition to mentoring and advocacy work outside of MIT, he is running a program within the physics department that invites students from underrepresented and underprivileged backgrounds to carry out physics research at MIT each summer.

“Unfortunately physics remains rather white and male, and making it more welcoming and accessible to a broader slice of humanity is one of my priorities going forward,” he says.

Looking ahead, Harlow is considering taking a new turn in his research path, perhaps to focus less on black holes in a hologram universe, and more on cosmology, and the quantum structure and evolution of our actual universe.

“I’ve been living in anti-de Sitter space for a long time,” Harlow says. “That’s fine, but I do want to understand the world we live in too. And that should be fun.”

© Photo: Gretchen Ertl

MIT physicist Daniel Harlow seeks to understand how our universe can abide by two incompatible rulebooks, the Standard Model of Physics and Einstein’s theory of general relativity. He is looking for answers in an alternate “boomerang” reality that represents the universe as a hologram of itself.

Fueled by problem-solving

“Every time I try to solve a problem — whether it be physics or computer science — I always try to find an elegant solution,” says MIT senior Thomas Bergamaschi, who spent four years learning how to solve problems while an Undergraduate Research Opportunities Program (UROP) student in the Engineering Quantum Systems (EQUS) laboratory at MIT.

“Of course,” he adds, “there are many times where a problem doesn't have an elegant solution, or finding an elegant solution is much harder than a normal solution, but it is something I always try to do, as it helps me understand at most something. Another compelling reason is that these solutions are usually the simplest to teach other people, which is always appealing to me.”

Now, as the physics and electrical engineering and computer science (EECS) major ponders post-graduation life, he believes he’s ready to tackle challenges in his career as a software engineer at Five Rings, where he had an internship. “There are a lot of hard and interesting problems to be solved there,” he says. “Challenges are something that fuels me.”

STEM family

Born in Brazil, Bergamaschi lived in the United States until he was 6, when his family moved back to a small town in rural Sao Paulo called Vinhedo. His Brazilian father is a software engineer, and his mother, who is from England, studied biology in college and now teaches English. He followed in the footsteps of his older brother, Thiago, who was the first in the family to be drawn to physics. And when his brother entered physics competitions in high school, Thomas did too.

He had high school teachers who encouraged him to study physics beyond the usual curriculum. “One teacher accompanied me on many bus and plane rides to physics competitions and classes,” he recalls. “She was a huge motivator for me to continue studying physics and helped find me new books and problems throughout high school.”  

The younger Bergamaschi went on to win silver medals at the International Physics Olympiad and at the International Young Physicists’ Tournament, and more than a dozen other medals in national and regional Brazilian science competitions in physics, math, and astronomy.

MIT Time

Thiago Bergamaschi '21 joined MIT as a physics and EECS major in 2017, and his brother wasn’t far behind him, entering MIT in 2019.

Bergamaschi ended up spending nearly all four years at MIT as a UROP student in the Engineering Quantum Systems (EQUS) laboratory, under the supervision of PhD student Tim Menke and Professor William Oliver. That’s when he was introduced to quantum computing — his supervisors were constructing a device that had a phenomenon where many qubits could interact simultaneously.

“This type of interaction is very useful for quantum computers, as it gives us a possible way that we can map problems we are interested in onto a quantum computer,” he says. “My project was to try to answer the question of how we can actually measure things, and prove that the constructed device actually had this coupling term we were interested in.”

He proposed and analyzed methods to experimentally detect many-body quantum systems. “These systems are extremely important and interesting as they have many cool applications, and in particular can be used to map computationally hard problems — such as route optimization, Boolean satisfiability, and more — to quantum computers in an easy way.”

This project was supposed to be a warmup project for his UROP. “However, we soon noticed that the problem of accurately measuring these effects was a pretty tricky problem. I ended up working on this problem for around six months — my summer, the fall semester, and the beginning of IAP [Independent Activities Period] — trying to figure out how we can measure these effects.”

He presented his research at the 2021 and 2022 American Physical Society March meetings, and published “Distinguishing multi-spin interactions from lower-order effects” in Physical Review Applied.   

“The experience of presenting my work in a conference and publishing a paper is a huge highlight from my time at MIT and gave me a taste of scientific communication and research, which was invaluable for me,” Bergamaschi says. “Being able to do research with the help of Tim Menke and Professor Oliver was inspiring, and is one of the largest highlights from my time at MIT.”

He also worked with William Isaac Jay, a postdoc at the MIT Center for Theoretical Physics, on lattice quantum field theory. He studies quantum theories at the microscopic level, where strong nuclear interactions are relevant. “This is particularly appealing as we can simulate these theories on a computer — albeit usually a huge supercomputer — and try to make predictions about phenomena involving atoms at a minuscule scale. I UROP'd in this lab over both my junior and senior year, and my project involved implementing techniques from one of these computer simulations, how can we go back to the real world and obtain something that an experiment would measure.”

Brazil blues

Bergamaschi missed Brazil but found community playing soccer with intramural teams Ousadia and Alegria Futebol Clube, and eating churrasco with his friends at Oliveira’s Brazilian-style steakhouse in Somerville, Massachusetts. He also loved going to college with his brother, who graduated in 2021 and is now pursuing his PhD in physics at the University of California at Berkeley.

“One of my favorite memories of MIT is from my sophomore spring, when I managed to take two classes with him just before he graduated,” he recalls. “It was a lot of fun discussing physics problem sets and projects with him.”

What also keeps him in touch with his homeland is working with Brazilian high school students competing in physics tournaments. He is part of an academic committee that creates and grades the physics problems taken by the top 100 Brazilian high school students. Those with top scores go on to the International Physics Olympiad. He says he sees this as a way to pay forward what his high school teacher did for him: to encourage others to study physics.

“These olympiads were one of the main reasons for my interest in physics and me coming to MIT, and I hope that other Brazilian students can have these same opportunities as I had,” he says. “These students are all incredibly talented. A large amount of them end up coming to MIT after they graduate high school, so it’s a very gratifying and incredible experience for me to be able to participate and help in their physics education.”

Post-graduation thoughts

What will he miss most at MIT? “Late-night problem set sessions immediately before a deadline, trying to find a free food event across campus, and getting banana lounge bananas and coffee.”

And what were his biggest lessons? He says that MIT taught him how to work with other people, “handle imposter syndrome,” and most importantly, unravel complicated challenges.

“I think one of my major motivators is my desire to learn new things, whether it be physics or computer science. So, I am a big fan of very difficult problems or projects which require continual work but have large payoffs at the end. I think there are many instances during my time at MIT in which I worked all night for a project, just to get up and hop back on because of the excitement of obtaining a result or solution.”

© Photo: Sandi Miller

“I think there are many instances during my time at MIT in which I worked all night for a project, just to get up and hop back on because of the excitement of obtaining a result or solution,” says senior Thomas Bergamaschi.

Success at the intersection of technology and finance

Citadel founder and CEO Ken Griffin had some free advice for an at-capacity crowd of MIT students at the Wong Auditorium during a campus visit in April. “If you find yourself in a career where you’re not learning,” he told them, “it’s time to change jobs. In this world, if you’re not learning, you can find yourself irrelevant in the blink of an eye.”

During a conversation with Bryan Landman ’11, senior quantitative research lead for Citadel’s Global Quantitative Strategies business, Griffin reflected on his career and offered predictions for the impact of technology on the finance sector. Citadel, which he launched in 1990, is now one of the world’s leading investment firms. Griffin also serves as non-executive chair of Citadel Securities, a market maker that is known as a key player in the modernization of markets and market structures.

“We are excited to hear Ken share his perspective on how technology continues to shape the future of finance, including the emerging trends of quantum computing and AI,” said David Schmittlein, the John C Head III Dean and professor of marketing at MIT Sloan School of Management, who kicked off the program. The presentation was jointly sponsored by MIT Sloan, the MIT Schwarzman College of Computing, the School of Engineering, MIT Career Advising and Professional Development, and Citadel Securities Campus Recruiting.

The future, in Griffin’s view, “is all about the application of engineering, software, and mathematics to markets. Successful entrepreneurs are those who have the tools to solve the unsolved problems of that moment in time.” He launched Citadel only one year after graduating from college. “History so far has been kind to the vision I had back in the late ’80s,” he said.

Griffin realized very early in his career “that you could use a personal computer and quantitative finance to price traded securities in a way that was much more advanced than you saw on your typical equity trading desk on Wall Street.” Both businesses, he told the audience, are ultimately driven by research. “That’s where we formulate the ideas, and trading is how we monetize that research.”

It’s also why Citadel and Citadel Securities employ several hundred software engineers. “We have a huge investment today in using modern technology to power our decision-making and trading,” said Griffin.

One example of Citadel’s application of technology and science is the firm’s hiring of a meteorological team to expand the weather analytics expertise within its commodities business. While power supply is relatively easy to map and analyze, predicting demand is much more difficult. Citadel’s weather team feeds forecast data obtained from supercomputers to its traders. “Wind and solar are huge commodities,” Griffin explained, noting that the days with highest demand in the power market are cloudy, cold days with no wind. When you can forecast those days better than the market as a whole, that’s where you can identify opportunities, he added.

Pros and cons of machine learning

Asking about the impact of new technology on their sector, Landman noted that both Citadel and Citadel Securities are already leveraging machine learning. “In the market-making business,” Griffin said, “you see a real application for machine learning because you have so much data to parametrize the models with. But when you get into longer time horizon problems, machine learning starts to break down.”

Griffin noted that the data obtained through machine learning is most helpful for investments with short time horizons, such as in its quantitative strategies business. “In our fundamental equities business,” he said, “machine learning is not as helpful as you would want because the underlying systems are not stationary.”

Griffin was emphatic that “there has been a moment in time where being a really good statistician or really understanding machine-learning models was sufficient to make money. That won’t be the case for much longer.” One of the guiding principles at Citadel, he and Landman agreed, was that machine learning and other methodologies should not be used blindly. Each analyst has to cite the underlying economic theory driving their argument on investment decisions. “If you understand the problem in a different way than people who are just using the statistical models,” he said, “you have a real chance for a competitive advantage.”

ChatGPT and a seismic shift

Asked if ChatGPT will change history, Griffin predicted that the rise of capabilities in large language models will transform a substantial number of white collar jobs. “With open AI for most routine commercial legal documents, ChatGPT will do a better job writing a lease than a young lawyer. This is the first time we are seeing traditionally white-collar jobs at risk due to technology, and that’s a sea change.”

Griffin urged MIT students to work with the smartest people they can find, as he did: “The magic of Citadel has been a testament to the idea that by surrounding yourself with bright, ambitious people, you can accomplish something special. I went to great lengths to hire the brightest people I could find and gave them responsibility and trust early in their careers.”

Even more critical to success is the willingness to advocate for oneself, Griffin said, using Gerald Beeson, Citadel’s chief operating officer, as an example. Beeson, who started as an intern at the firm, “consistently sought more responsibility and had the foresight to train his own successors.” Urging students to take ownership of their careers, Griffin advised: “Make it clear that you’re willing to take on more responsibility, and think about what the roadblocks will be.”

When microphones were handed to the audience, students inquired what changes Griffin would like to see in the hedge fund industry, how Citadel assesses the risk and reward of potential projects, and whether hedge funds should give back to the open source community. Asked about the role that Citadel — and its CEO — should play in “the wider society,” Griffin spoke enthusiastically of his belief in participatory democracy. “We need better people on both sides of the aisle,” he said. “I encourage all my colleagues to be politically active. It’s unfortunate when firms shut down political dialogue; we actually embrace it.”

Closing on an optimistic note, Griffin urged the students in the audience to go after success, declaring, “The world is always awash in challenge and its shortcomings, but no matter what anybody says, you live at the greatest moment in the history of the planet. Make the most of it.”

© Photo courtesy of MIT Career Advising and Professional Development.

Bryan Landman ’11 (left) and Ken Griffin discussed how technology will continue to transform trading and investing, during an event at the Wong Auditorium.

MIT physicists predict exotic new phenomena and give “recipe” for realizing them

In work that could lead to important new physics with potentially heady applications in computer science and more, MIT scientists have shown that two previously separate fields in condensed matter physics can be combined to yield new, exotic phenomena.

The work is theoretical, but the researchers are excited about collaborating with experimentalists to realize the predicted phenomena. The team includes the conditions necessary to achieve that ultimate goal in a paper published in the Feb. 24 issue of Science Advances.

“This work started out as a theoretical speculation, and ended better than we could have hoped,” says Liang Fu, a professor in MIT’s Department of Physics, an affiliate of the MIT Materials Research Laboratory, and leader of the work. His colleagues are Nisarga Paul, an MIT graduate student in physics, and Yang Zhang, a former MIT postdoc who is now a professor at the University of Tennessee.

2D materials

The current work was guided by recent advances in 2D materials, or those consisting of only one or a few layers of atoms. “The whole world of two-dimensional materials is very interesting because you can stack them and twist them, and sort of play Legos with them to get all sorts of cool sandwich structures with unusual properties,” says Paul, who is first author of the paper.

Those sandwich structures, in turn, are called moiré materials. MIT professor of physics Pablo Jarillo-Herrero and his colleagues have been leaders in the field with moiré graphene, which is composed of two or more sheets of atomically thin graphene placed on top of each other and rotated at a slight angle.

Separately, other scientists have advanced the field of 2D magnets.

What might happen if the two fields — 2D magnets and moiré materials — are combined? That is the focus of the current work.

Specifically, the team predicts that a structure made of two layers of a 2D magnet topped by a layer of a 2D semiconductor material will generate a phenomenon called a flat band, in which the electrons in the semiconductor stand still. “That was the theoretically challenging part because it’s not a very straightforward thing to ask of an electron. They want to move around. And it takes a lot of fine-tuning to get them to stand still,” says Paul.

Getting electrons to be still, however, allows them “to really talk to each other. And that’s when all the really interesting things in our field [condensed matter physics] happen,” Paul continues.

How does it work?

Key to the research is an exotic particle called a skyrmion that involves a property of electrons called spin (another, more familiar property of electrons is their charge). The spin can be thought of as an elementary magnet, in which the electrons in an atom are like little needles orienting in a certain way. In the magnets on your refrigerator, the spins all point in the same direction.

In a skyrmion, the spins form knot-like whirls that are distributed across the surface of a material. Importantly, skyrmions are topological objects, or those whose properties do not change even when they are subjected to large deformations. (In 2016 the Nobel Prize in Physics was awarded to the three scientists who discovered topological phases of matter.) The implication is that future applications of skyrmions would be very robust, or difficult to disrupt, perhaps leading to a better form of computer memory storage.

The MIT team predicts that skyrmions in the 2D magnet layer will “imprint” themselves on the electrons in the semiconductor layer, endowing them with skyrmion-like properties themselves. These properties also stop the movement of the semiconductor’s electrons, resulting in the flat band.

Toward a recipe

In the Science Advances paper, the physicists also define the best conditions for creating a magnet-semiconductor structure with a flat band.

Yang Zhang used a method called density functional theory to predict what materials would allow the strongest interactions between the electrons in the semiconductor and the skyrmions in the magnet. “For something interesting to happen, you need the electrons in one layer to really feel the skyrmions in the other layer,” says Paul. “This is quantified by a parameter called the proximity exchange, or J. So Yang was looking for a combination of materials with a large J.”

He found that the best combination involves a layer of molybdenum disulfide (the semiconductor) over layers of chromium tribromide (the magnet). Says Paul, “Typical combinations in these two families of materials will have a J of about one or two millielectronvolts. Yang found that this specific combination has a J of around seven millielectronvolts. That’s huge.”

The team further identified a certain “magic” level of magnetization that is also key to realizing a strong flat band.

"Engineering flat electronic bands through moiré superlattices has emerged as a powerful technique for exploring [a variety of unusual] effects," says Xiaodong Xu of the University of Washington, who was not involved in the work. The team "present[s] an innovative method for creating topologically flat bands by combining 2D semiconductors with 2D magnetic moirés. The appeal of this approach lies in the fact that [the team’s predictions] make experimental implementation feasible. This will undoubtedly inspire numerous experimental teams."

Adds Inti Sodemann of the Max Planck Institute, who was also not involved in the research: "The authors have demonstrated the possibility to engineer in these [structures] very flat topological Chern bands. These flat bands have a great potential for the realization of exotic states that could be potential platforms for building topological quantum computers."

This work was funded, in part, by the Air Force Office of Scientific Research.

© Image: Nisarga Paul

Artist’s rendition of skyrmions, exotic particles represented by the whirls of red, yellow, and white arrows.

It’s a weird, weird quantum world

In 1994, as Professor Peter Shor PhD ’85 tells it, internal seminars at AT&T Bell Labs were lively affairs. The audience of physicists was an active and inquisitive bunch, often pelting speakers with questions throughout their talks. Shor, who worked at Bell Labs at the time, remembers several occasions when a speaker couldn’t get past their third slide, as they attempted to address a rapid line of questioning before their time was up.

That year, when Shor took his turn to present an algorithm he had recently worked out, the physicists paid keen attention to Shor’s entire talk — and then some.

“Mine went pretty well,” he told an MIT audience yesterday.

In that 1994 seminar talk, Shor presented a proof that showed how a quantum system could be applied to solve a particular problem more quickly than a classical computer. That problem, known as the discrete logarithm problem, was known to be unsolvable by classical means. As such, discrete logarithms had been used as the basis for a handful of security systems at the time.

Shor’s work was the first to show that a quantum computer could solve a real, practical problem. His talk set the seminar abuzz, and the news spread, then became conflated. Four days after his initial talk, physicists across the country were assuming Shor had solved a related, though much thornier problem: prime factorization — the challenge of finding a very large number’s two prime factors. Though some security systems employ discrete logarithms, most encryption schemes today are based on prime factorization and the assumption that it is impossible to crack.

 “It was like the children’s game of ‘telephone,’ where the rumor spread that I had figured out factoring,” Shor says. “And in the four days since [the talk], I had!”

By tweaking his original problem, Shor happened to find a similar quantum solution for prime factorization. His solution, known today as Shor’s algorithm, showed how a quantum computer could factorize very large numbers. Quantum computing, once thought of as a thought experiment, suddenly had in Shor’s algorithm an instruction manual for a very real, and potentially disruptive application. His work simultaneously ignited multiple new lines of research in quantum computing, information science, and cryptography.

The rest is history, the highlights of which Shor recounted to a standing-room-only audience in MIT’s Huntington Hall, Room 10-250. Shor, who is the Morss Professor of Applied Mathematics at MIT, spoke as this year’s recipient of the James R. Killian, Jr. Faculty Achievement Award, which is the highest honor the Institute faculty can bestow upon one of its members each academic year.

In introducing Shor’s talk, Lily Tsai, chair of the faculty, quoted the award citation:

“Without exception, the faculty who nominated him all commented on his vision, genius, and technical mastery, and commended him for the brilliance of his work,” Tsai said. “Professor Shor’s work demonstrates that quantum computers have the potential to open up new avenues of human thought and endeavor.”

A quantum history

During the one-hour lecture, Shor took the audience through a brief history of quantum computing, peppering the talk with personal recollections of his own role. The story, he said, begins in the 1930s with the discovery of quantum mechanics — the physical behavior of matter at the smallest, subatomic scales — and the question that soon followed: Why was quantum so strange?

Physicists grappled with the new description of the physical world, which was so different from the “classical” Newtonian mechanics that had been understood for centuries. Shor says that the physicist Erwin Schrödinger attempted to “illustrate the absurdity” of the new theory with his now-famous thought experiment involving a cat in a box: How can it embody both states — dead and alive? The exercise challenged the idea of superposition, a key property of quantum mechanics that predicts a quantum bit such as an atom should hold more than one state simultaneously.

Spookier still was the prediction of entanglement, which posed that two atoms could be inextricably linked. Any change to one should then affect the other, no matter the distance separating them.

“Nobody considered using this strangeness for information storage, until Wiesner,” Shor said.

Wiesner was Stephen Wiesner, who in the late 1960s was a graduate student at Columbia University who was later credited with formulating some of the basic principles of quantum information theory. Wiesner’s key contribution was a paper that was initially spurned. He had proposed a way to create “quantum money,” or currency that was resistant to forgery, by harnessing a strange property in which quantum states cannot be perfectly duplicated — a prediction known as the “no-cloning” theorem.

As Shor remembers it, Wiesner wrote out his idea on a typewriter, sent it off for consideration by his peers, and was roundly rejected. It wasn’t until another physicist, Charles Bennett, found the paper, “pulled it out of a drawer, and got it published,” solidifying Wiesner’s role in quantum computing’s history. Bennett went further, realizing that the basic idea of quantum money could be applied to develop a scheme of quantum key distribution, in which the security of a piece of information, such as a private key passed between parties, is protected by another weird quantum property.

Bennett worked out the idea with Gilles Brassard in 1984. The BB84 algorithm was the first protocol for a crypto system that relied entirely on the weird phenomena of quantum physics. Sometime in the 1980s, Bennett came around to Bell Labs to present BB84. It was Shor’s first time hearing of quantum computing, and he was hooked.

Shor initially tried to work out an answer to a question Bennett posed to the audience: How can the protocol be proven mathematically to indeed be secure? The problem, however, was too thorny, and Shor abandoned the question, though not the subject. He followed the efforts of his colleagues in the growing field of quantum information science, eventually landing on a paper by physicist Daniel Simon, who proposed something truly weird: that a system of quantum computing bits could solve a particular problem exponentially faster than a classical computer.

The problem itself, as Simon posed it, was an esoteric one, and his paper, like Wiesner’s, was initially rejected. But Shor saw something in its structure — specifically, that the problem related to the much more concrete problems of discrete logarithms and factoring. He worked from Simon’s starting point to see whether a quantum system could solve discrete logarithms more quickly than a classical system. His first attempts were a draw. The quantum algorithm solved a problem just as fast as its classical counterpart. But there were hints that it could do better.

“There’s still hope in trying,” Shor remembers thinking.

When he did work it out, he presented his algorithm for a quantum discrete log algorithm in the 1994 symposium at Bell Labs. In the four days since his talk, he managed to also work out his eponymous prime factorization algorithm.

The reception was overwhelming but also skeptical, as physicists assumed that a practical quantum computer would instantly crumble at the barest hint of noise, resulting in a cascade of errors in its factoring computation.

“I worried about this problem,” Shor said.

So, he again went to work, looking for a way to correct errors in a quantum system without disturbing the state of the computing quantum bits. He found an answer through concatenation, which broadly refers to a series of interconnected events. In his case, Shor found a way to link qubits, and store the information of one logical, or computing qubit among nine highly entangled, physical qubits. In this way, any error in the logical qubit can be measured and fixed within the physical qubits, without having to measure (and therefore destroy) the qubit involved in the actual computation.

Shor’s new algorithm was the first quantum error correcting code that proved a quantum computer could be tolerant to faults, and therefore a very real possibility.

“The world of quantum mechanics is not the world of your intuition,” Shor said in closing his remarks. “Quantum mechanics is the way the world really is.”

Quantum’s future

Following his talk, Shor took several questions from the audience, including one that drives a huge effort in quantum information science today: When will we see a real, practical quantum computer?

To factor a large number, Shor estimates that a quantum system would require at least 1,000 qubits. To factor the very large numbers that underpin today’s internet and security systems would require millions of qubits.

“That’s going to take a whole bunch of years,” Shor said. “We may never make a quantum computer, ever… but if someone has a great idea, maybe we could see one 10 years from now.”

In the meantime, he noted that, as work in quantum computing has ballooned in recent years, so has work toward post-quantum cryptography and efforts to develop alternative crypto systems  that are secure against quantum-based code cracking. Shor compares these efforts to the scramble leading up to “Y2K,” and the prospect of a digital catastrophe at the turn of the last century.

“You probably should have started years ago,” Shor said. “If you wait until the last minute, when it’s clear quantum computers will be built, you will probably be too late.”

Shor received his PhD from MIT in 1985, and went on to complete a postdoc at the Mathematical Sciences Research Institute at Berkeley, California. He then spent several years at AT&T Bell Labs, and then at AT&T Shannon Labs, before returning to MIT as a tenured faculty member in 2003.

Shor’s contributions have been recognized by numerous awards, most recently with the 2023 Breakthrough Prize in Fundamental Physics, which he shared with Bennett, Brassard, and physicist David Deutsch. His other accolades include the MacArthur Fellowship, the Nevanlinna Prize (now the IMU Abacus Medal), the Dirac Medal, the King Faisal International Prize in Science, and the BBVA Foundation Frontiers of Knowledge Award. Shor is a member of the National Academy of Sciences and the American Academy of Arts and Sciences. He is also a fellow of the American Mathematical Society and the Association for Computing Machinery.

© Photo: Jake Belcher

Peter Shor, the Morss Professor of Applied Mathematics, is this year’s recipient of the James R. Killian, Jr. Faculty Achievement Award, which is the highest honor the Institute faculty can bestow upon one of its members each academic year.