Topological quantum frequency comb is a burgeoning topic that combines topological phases and quantum systems, which inspires many intriguing sparks in topological quantum optics. Producing quantum frequency combs in valley photonic crystal topological resonators can introduce the robustness to quantum states in integrated photonic devices.

Abstract

Recent advances in manipulating topological phases in quantum systems have promised integrated quantum devices with conspicuous functionalities, such as robustness against fabrication defects. At the same time, the introduction of quantum frequency combs enables extreme expansion of quantum resources. Here, it theoretically propose the generation of high-dimensional entangled quantum frequency combs via four-wave mixing processes in the valley-Hall topological resonators. Specifically, it demonstrates two irregular photonic crystal resonators supporting the whispering-gallery resonator modes that lead to coherent quantum frequency combs at telecommunication wavelengths. By using the Schmidt decomposition, It shows that the quantum frequency combs are frequency entangled, and it also concludes that the effective dimensions of quantum frequency combs in these two resonators are at least seven and six, respectively. Moreover, these quantum frequency combs inherit the topological protection of valley kink states, showing robustness against defects in the resonators. The topological quantum frequency combs have shown intriguing potentiality in the generation and control of topologically protected high-dimensional quantum states in integrated photonic crystal platforms.

Introducing a groundbreaking achievement in the field of quantum optics and communication, the research unveils a high-performance telecom-wavelength biphoton source from a hot ⁸⁷Rb atomic vapor cell. With its remarkable advantages of compatibility with existing telecom networks, seamless long-distance communication, exceptional efficiency, and minimal noise, the work paves the way for the realization of optical-fiber-based quantum communications and networks.

Abstract

Telecom-band quantum light sources are critical to the development of long-distance quantum communication technologies. A high-performance telecom-wavelength biphoton source from a hot ⁸⁷Rb atomic vapor cell is reported. Time-correlated biphotons are generated from the cascade-type 5S_1/2–5P_3/2–4D_5/2 transition of ⁸⁷Rb via a spontaneous four-wave mixing process. The maximum value gSI(2)(τ)$g_{{\mathrm{SI}}}^{( 2 )}( \tau )$ of biphoton cross-correlation to be 44(3) is achieved, under the condition of a high optical depth of 112(3), including two-photon absorption, with a spectral width of approximately 300 MHz. The coincidence count rate of biphoton is estimated to be of the order of 38 000 cps mW⁻¹. It is believed that the telecom-wavelength biphoton source from an atomic vapor cell can be applied in long-distance quantum networks and practical quantum repeaters based on atom–photon interactions.

A capacitively-coupled coplanar waveguide microwave resonator is fabricated and characterized, revealing an unconventional reduction of loss with decreasing temperature below 50 mK at low photon numbers. This anomalous behavior is attributed to the response bandwidth of a single two-level system (TLS) dropping below the TLS-resonance detuning at low temperatures, reducing the intrinsic loss of the resonator.

Abstract

Superconducting resonators are widely used in many applications such as qubit readout for quantum computing, and kinetic inductance detectors. These resonators are susceptible to numerous loss and noise mechanisms, especially the dissipation due to two-level systems (TLS) which become the dominant source of loss in the few-photon and low temperature regime. In this study, capacitively-coupled aluminum half-wavelength coplanar waveguide resonators are investigated. Surprisingly, the loss of the resonators is observed to decrease with a lowering temperature at low excitation powers and temperatures below the TLS saturation. This behavior is attributed to the reduction of the TLS resonant response bandwidth with decreasing temperature and power to below the detuning between the TLS and the resonant photon frequency in a discrete ensemble of TLS. When response bandwidths of TLS are smaller than their detunings from the resonance, the resonant response and thus the loss is reduced. At higher excitation powers, the loss follows a logarithmic power dependence, consistent with predictions from the generalized tunneling model (GTM). A model combining the discrete TLS ensemble with the GTM is proposed and matches the temperature and power dependence of the measured internal loss of the resonator with reasonable parameters.

This is a brief review on the principle, categories, and applications of quantum metrology. Special attention is paid to different quantum resources that can bring quantum superiority in enhancing sensitivity. Then, the paper reviews the no-go theorem of noisy quantum metrology and its active control under different noise-induced decoherence situations.

Abstract

Quantum metrology pursues the physical realization of higher-precision measurements to physical quantities than the classically achievable limit by exploiting quantum features, such as entanglement and squeezing, as resources. It has potential applications in developing next-generation frequency standards, magnetometers, radar, and navigation. However, the ubiquitous decoherence in the quantum world degrades the quantum resources and forces the precision back to or even worse than the classical limit, which is called the no-go theorem of noisy quantum metrology and greatly hinders its applications. Therefore, how to realize the promised performance of quantum metrology in realistic noisy situations attracts much attention in recent years. The principle, categories, and applications of quantum metrology are reviewed. Special attention is paid to different quantum resources that can bring quantum superiority in enhancing sensitivity. Then, the no-go theorem of noisy quantum metrology and its active control under different kinds of noise-induced decoherence situations are introduced.

This paper tackles light detection and ranging challenges, delving into laser rangefinder principles, optical modulation, and advancements in laser tech. The promising photonic crystal surface emitting laser is a standout. Progress in scanning stability and angles is discussed as well, along with advancements in metasurface technology, enhancing beam deflection and field of view.

Abstract

Light detection and ranging (LiDAR) sensor is widely recognized as a critical component for accurate perception. However, there are a host of challenges that impede their performance, including low spatial resolution, high costs, large size, low reliability, and susceptibility to interference. It is challenging to overcome these issues using a single LiDAR module, necessitating the need for a review of current LiDAR technologies. The paper commences by introducing the fundamental principles of various laser rangefinders and discussing the optical modulation technologies used to prevent interference and ghost images. Next, the paper delves into the latest developments in laser technology, with a focus on enhancing the switching rate, compliance with eye safety regulations, miniaturization, and improving stability. One highly promising innovation is the photonic crystal surface emitting laser (PCSEL), a novel light source that boasts high-speed, small divergence angles, and high-power output. Finally, the paper discusses the advancements made in non-solid-state scanning and solid-state scanning, such as improving stability, increasing scanning angles, and optimizing the manufacturing of mechanical and micro-electromechanical systems (MEMS). Additionally, the paper highlights the recent advancements in nanotechnology, specifically metasurface technology, which offers superior capabilities such as beam deflection, enhanced field-of-view (FOV), and dynamic modulation.

In general, a bulk diamond sensor requires a high-power laser, which hinders the growth of NV numbers and thus limits the final sensitivity. Here, a relaxometry-based microwave magnetometer is given, which shows that the power density is T1-limited. By cooling the diamond sensor, the required power density reduces to 0.077 Wcm⁻², 10⁻⁶ of the saturation value.

Abstract

The nitrogen-vacancy (NV) center in diamond is a unique magnetometer. Its atomic size enables integrations of a tremendous amount (n _NV) of NV centers in a bulk diamond with a sensitivity scaling as 1/nNV$1/\sqrt {n_{\rm NV}}$. However, such a bulk sensor requires a high-power laser to polarize and read out the NV centers. The increasing thermal damage and additional noises associated with high-power lasers hinder the growth of n _NV, and thus limit the sensitivity at picotesla level. Here, it shows a relaxometry-based microwave magnetometer that the power density is determined by the relaxation time T ₁. By cooling the diamond sensor to prolong the T ₁ (≈s), the required power density further reduces to 0.077Wcm−2$0.077\nobreakspace {\rm Wcm^{-2}}$, ≈10−6$\approx \ 10^{-6}$ of the saturation value. This work paves the way for the utilization of large-size diamond to promote the sensitivity of diamond magnetometer to femtotesla level and beyond.

Presenting a novel multi-class quantum kernel-based classifier. With this classifier, the number of qubits required, the measurement strategy, and the topology of the circuits used are invariant to the number of classes. Analytical results and numerical simulations show that this classifier is not only effective when applied to diverse classification problems but also robust under certain noise conditions.

Abstract

Multi-class classification problems are fundamental in many varied domains in research and industry. A popular strategy for solving multi-class classification problems involves first transforming the problem into many binary classification problems. However, this requires the number of binary classification models that need to be developed to grow with the number of classes. Recent work in quantum machine learning has seen the development of multi-class quantum classifiers that circumvent this growth by learning a mapping between the data and a set of label states. This work presents the first multi-class SWAP-Test classifier inspired by its binary predecessor and the use of label states in recent work. With this classifier, the cost of developing multiple models is avoided. In contrast to previous work, the number of qubits required, the measurement strategy, and the topology of the circuits used is invariant to the number of classes. In addition, unlike other architectures for multi-class quantum classifiers, the state reconstruction of a single qubit yields sufficient information for multi-class classification tasks. Both analytical results and numerical simulations show that this classifier is not only effective when applied to diverse classification problems but also robust to certain conditions of noise.

Quantum biology promises to synthesize two until now rather disjoint fields of science: quantum science and life science. To promote this synthesis, this work takes advantage of concepts used by scientists interested in building quantum computers and applies them to biochemical scenarios relevant to measurements performed by biologists in cellular environments.

Abstract

A fundamental concept of quantum physics, the Wigner–Yanase information, is used here as a measure of quantum coherence in spin-dependent radical-pair reactions pertaining to biological magnetic sensing. This measure is connected to the uncertainty of the reaction yields and, further, to the statistics of a cellular receptor-ligand system used to biochemically convey magnetic-field changes. Measurable physiological quantities, such as the number of receptors and fluctuations in ligand concentration, are shown to reflect the introduced Wigner–Yanase measure of singlet-triplet coherence. A quantum-biological uncertainty relation connecting the product of a biological resource and a biological figure of merit with the Wigner–Yanase coherence is arrived at. This approach can serve as a general search for quantum-coherent effects within cellular environments.

Polymer nanowires (PNWs) are high index-contrast cylindrical waveguides that can directly couple a quantum dot's (QD's) emission into the HE11$HE_{11}$ optical mode, which improves collection efficiency into a single mode fiber. PNWs can be used as standalone devices or in conjunction with top-down fabricated QD devices. Strategies for successful PNW fabrication are presented, and enhancement from a PNW is demonstrated.

Abstract

Single epitaxial quantum dots (QDs) embedded in nanophotonic geometries are a leading technology for quantum light generation. However, efficiently coupling their emission into a single mode fiber or Gaussian beam often remains challenging. Here, direct laser writing (DLW) is used to address this challenge by fabricating 1 µm diameter polymer nanowires (PNWs) in-contact-with and perpendicular-to a QD-containing GaAs layer. QD emission is coupled to the PNW's HE11$HE_{11}$ waveguide mode, enhancing collection efficiency into a single-mode fiber. PNW fabrication does not alter the QD device layer, making PNWs well-suited for augmenting pre-existing in-plane geometries. Standalone PNWs and PNWs in conjunction with metallic nanoring devices that have been previously established for increasing extraction of QD emission are studied. Methods that mitigate standing wave reflections and heat, caused by GaAs's absorption/reflection of the lithography beam, and which otherwise prevent PNW fabrication, are also reported. A maximum improvement of (3.0±0.7)×$3.0\nobreakspace \pm \nobreakspace 0.7)\times$ in a nanoring system with a PNW compared to the same system without a PNW is observed, in line with numerical results, and highlighting the PNW's ability to waveguide QD emission and increase collection efficiency simultaneously. These results demonstrate new DLW functionality in service of quantum emitter photonics that maintains compatibility with existing top-down fabrication approaches.

A topological waveguide–cavity coupling system is constructed, realizing the energy coupling between the waveguide and cavity, Fano resonance phenomenon can be observed in this system. Moreover, an extended waveguide–cavity coupling system is proposed, having stronger electric field intensity, narrower resonance bandwidth, and higher quality factor. This work provides a new path to design high-performance micro-nano-integrated optical devices.

Abstract

In recent years, the topological edge states (TESs) and topological corner states (TCSs) based on high-order topological photonic insulators have been researched and applied in various fields. Among them, the combination of topological photonics and waveguide–cavity coupling system has attracted much attention of researchers. In this paper, TESs and TCSs based on Kagome lattice photonic crystals (PCs) with rectangular dielectric columns are obtained. In addition, a topological waveguide–cavity coupling system is proposed, realizing the energy coupling between the TES waveguide and the TCS cavity. To further improve the performance of the system, a sandwich-like waveguide and an extended waveguide are used to replace the original TES waveguide. Interestingly, the proposed extended waveguide–cavity coupling system cannot only achieve the excitation of two TCSs at different frequencies, but also has stronger electric field intensity, narrower resonance bandwidth, and higher quality factor than the other structures. This work provides a new path to design high-performance micro-nanointegrated optical devices such as filters, lasers, and resonators.

Extending the T ₂ ^* of perfectly aligned nitrogen-vacancy (NV) centers in large-volume chemical vapor deposition (CVD) diamonds leads to enhanced magnetic sensitivity. This study found that the stress distribution in the CVD films are mitigated at high misorientation angle θmis${\theta _{mis}}$, leading to a T ₂ ^* extension of the NV centers. This study provides an important method for realizing highly sensitive quantum sensors.

Abstract

Extending the spin-dephasing time (T ₂ ^*) of perfectly aligned nitrogen-vacancy (NV) centers in large-volume chemical vapor deposition (CVD) diamonds leads to enhanced DC magnetic sensitivity. However, T ₂ ^* of the NV centers is significantly reduced by the stress distribution in the diamond film as its thickness increases. To overcome this issue, they developed a method to mitigate the stress distribution in the CVD diamond films, leading to a T ₂ ^* extension of the ensemble NV centers. CVD diamond films of ≈60 µm thickness with perfectly aligned NV centers are formed on (111) diamond substrates with misorientation angles of 2.0°, 3.7°, 5.0°, and 10.0°. The study found that T ₂ ^* of the ensemble of NV centers increased to approach its value limited only by the electron and nuclear spin bath with increasing the misorientation angle. Microscopic stress imaging revealed that the stress distribution is highly inhomogeneous along the depth direction in the CVD diamond film at low misorientation angles, whereas the inhomogeneity is largely suppressed on highly misoriented substrates. The reduced stress distribution possibly originates from the reduction of the dislocation density in the CVD diamond. This study provides an important method for synthesizing high-quality diamond materials for use in highly sensitive quantum sensors.

Neutral atoms devices represent a promising technology that uses optical tweezers to arrange atoms and modulated laser pulses to control the quantum states. Two machine learning approaches are proposed to estimate the noise parameters and to mitigate the noise effects on such devices. The former is evaluated on simulated and real data and the latter do not require ancilla qubits.

Abstract

Neutral atoms devices represent a promising technology using optical tweezers to geometrically arrange atoms and modulated laser pulses to control their quantum states. They are exploited as noisy intermediate-scale quantum (NISQ) processors. Indeed, like all real quantum devices, they are affected by noise introducing errors in the computation. Therefore, it is important to understand and characterize the noise sources and possibly to correct them. Here, two machine-learning based approaches are proposed respectively to estimate the noise parameters and to mitigate their effects using only measurements of the final quantum state. Our analysis is then tested on a real neutral atom platform, comparing our predictions with a priori estimated parameters. It turns out that increasing the number of atoms is less effective than using more measurements on a smaller scale. The agreement is not always good but this may be due to the limited amount of real data that are obtained from a still under development device. Finally, reinforcement learning is employed to design a pulse that mitigates the noise effects. Our machine learning-based approach is espected to be very useful for the noise benchmarking of NISQ processors and, more in general, of real quantum technologies.

Primitive polynomials over finite fields are essential across various computer science domains, including pseudo-random number generation, coding theory, and post-quantum cryptography. Despite their significance, random generation of primitive polynomials remains a challenge for classical computers. This work presents two efficient quantum algorithms to address this task, which paves the way for future quantum communication and computation applications.

Abstract

Primitive polynomials over finite fields are crucial resources with broad applications across various domains in computer science, including classical pseudo-random number generation, coding theory, and post-quantum cryptography. Nevertheless, the pursuit of an efficient classical algorithm for generating random primitive polynomials over finite fields remains an ongoing challenge. In this work, it shows how this problem can be solved efficiently with the help of quantum computers. Moreover, the designs of specific quantum circuits to implement them are also presented. The research paves the way for the rapid and real-time generation of random primitive polynomials in diverse quantum communication and computation applications.

The gravity inversion method is used to map the subsurface density distributions starting from surface measurements. The article investigates a quantum-enhanced binary inversion formulation, addressing a novel application of quantum annealing to geophysics. The algorithm is tested in two different realistic use cases. When non-convexity of the associated optimization problem prevails, a hybrid classical-quantum approach provides an improved subsurface reconstruction.

Abstract

A quantum-enhanced implementation of the binary inversion method for gravity data acquisition is discussed. The subsurface structure of a single density anomaly with an assigned density contrast is calculated by using a D-Wave adiabatic quantum computer. In particular, an iterative heuristic based on quantum annealing that recovers a sharp shape of the subsurface anomaly is developed. Such a task is accomplished by collecting partial images obtained by quantum annealing processes for optimal Lagrange penalty coefficients. The results are compared with those obtained according to the same cost function minimized via genetic algorithms by conventional hardware on a realistic 2D dataset. The outcomes of this work are promising as the reconstructed model is obtained in tenths of iterations instead of the hundreds required in conventional methods. Moreover, for the part of the computation that resides in the quantum processing unit, the computational cost of the single quantum annealing descent is constant with respect to the number of degrees of freedom of the subsurface grid. The implemented method is likely to reveal its full potential on forthcoming quantum annealing devices, outperforming existing techniques.

The combination of a single-photon source in a ridge waveguide with a photonic wire bond (PWB) connected to the end-facet is demonstrated. After cooling the sample to 1.6 K, decay time and second-order autocorrelation under pulsed, resonant optical excitation are analyzed. The findings herein highlight the possibility of using PWBs at low temperatures as an interface for single-photon collection.

Abstract

The collection of single-photon emission from a quantum dot (QD) in a Bragg waveguide through a photonic wire bond (PWB) via free-space resonant frequency pumping at 1.6 K is demonstrated. The in-fiber single photons show a small multiphoton contribution, quantified by a low second order photon autocorrelation value of gcorr(2)(0)=(5.9±0.8)×10−3$g_{{\mathrm{corr}}}^{( 2 )}\;( 0 ) = ( {5.9 \pm 0.8} )\; \times {10^{ - 3}}$ (background-corrected) or graw(2)(0)=(9.5±1.4)×10−2$g_{{\mathrm{raw}}}^{( 2 )}( 0 )\; = ( {9.5 \pm 1.4} )\; \times {10^{ - 2}}{\mathrm{\;}}$(raw data). The decay time of the QD is measured to be τ=440${\mathrm{\tau \;}} = {\mathrm{\;}}440$ ps. The PWB obviates the need for in-cryostat alignment of the single-photon source with an optical fiber and thus offers a route to scalable integration of quantum photonic devices in a cryogenic environment. Uniquely, the approach combines the QD-waveguide technique, enabling resonant driving of individual QDs without the need for cross-polarization filtering, and the PWB for deterministic, alignment-free coupling of single-photon sources to optical fibers.

In article number 2300210, researchers from SIMIT propose an integrated diamond quantum sensor for high-precision current sensing, which may be potentially applicable to monitor grid current. The cover is an artistic view of the sensor device, where the diamond is encapsulated in a microfabricated silicon structure.

In article number 2300099 by Vladimir M. Shalaev and co-workers, the fundamental photophysical properties of intrinsic single-photon emitters in silicon nitride are probed through measurements of optical transition wavelengths, linewidths, and photon antibunching as a function of temperature from 4.2 to 300 K. The cover shows a confocal microscope exciting single-photon emitters in silicon nitride. The emitters are represented as blue dots and the emitted photons are shown as blue wave packets. The thermometer and “4K” text indicate that these are cryogenic measurements. Finally, the inset figure shows the change in emission properties as the emitters are cooled down to low temperatures.

This paper investigates the cavity-assisted generation of highly indistinguishable and at the same time entangled photon pairs from semiconductor quantum dots, resulting in high-quality photons for quantum information processing. The authors combine Maxwell simulations of the optical resonator with quantum simulations of the quantum dot and find optimal parameters for the implementation of such emitter devices.

Abstract

The biexciton-exciton emission cascade commonly used in quantum-dot systems to generate polarization entanglement yields photons with intrinsically limited indistinguishability. In the present work, it focuses on the generation of pairs of photons with high degrees of polarization entanglement and simultaneously high indistinguishability. It achieves this goal by selectively reducing the biexciton lifetime with an optical resonator. It demonstrates that a suitably tailored circular Bragg reflector fulfills the requirements of sufficient selective Purcell enhancement of biexciton emission paired with spectrally broad photon extraction and twofold degenerate optical modes. The in-depth theoretical study combines (i) the optimization of realistic photonic structures solving Maxwell's equations from which model parameters are extracted as input for (ii) microscopic simulations of quantum-dot cavity excitation dynamics with full access to photon properties. It reports non-trivial dependencies on system parameters and use the predictive power of the combined theoretical approach to determine the optimal range of Purcell enhancement that maximizes indistinguishability and entanglement to near unity values, here specifically for the telecom C-band at 1550 nm.