Older techniques inspire new discoveries for ultracold molecules

Friday, November 24, 2023

Sometimes, new scientific discoveries can be made from looking at well-known methods or experimental techniques in new ways. This is the basis for new research from Dr. Alan Jamison, a faculty member at the Institute for Quantum Computing (IQC) and the University of Waterloo’s Department of Physics and Astronomy, and his collaborators at the Massachusetts Institute of Technology (MIT).

Jamison researches ultra-cold molecules, which are made by cooling down atoms to nearly absolute zero in an atom trap. Once formed, these molecules can then be studied for applications including quantum-state-controlled chemistry, quantum simulations, and quantum information processing. One of the first great successes of cooling atoms to ultracold temperatures was the observation of the Bose-Einstein condensate. This was first achieved experimentally using magnetic atom traps in the mid 1990s by researchers including Jamison’s collaborator, Dr. Wolfgang Ketterle, for which Ketterle was awarded the 2001 Nobel Prize in Physics.

Since this time, however, while magnetic traps are sometimes used during the process of cooling atoms, it has become more common for researchers to use optical lasers to trap atoms during experiments. The optical traps are faster and can trap a wider range of atoms and molecules than just those with the specific magnetic properties needed to use the magnetic traps.

“When people started making ultracold molecules, they had to be in an optical trap to hold the right atomic states to make the molecules, and so you just naturally did the experiments with the molecules also in an optical trap,” said Jamison. “But it turns out that some ultracold molecules which were expected to be chemically stable seem to be undergoing chemical reactions caused by the light from the optical traps.”

Jamison and his collaborators reasoned that if they could remove the requirement of light from their experiments by using magnetic traps, they could then study these light-induced chemical reactions in controlled environments and explore new and exciting results.

“We study one of the few ultracold molecules that can be magnetically trapped, which gave us the freedom to study these older techniques in new ways,” said Jamison. “It’s exciting looking at these reactions without having to worry about what the light is doing. On one hand, it constrains us to only work with states that are magnetically trappable, but on the other hand it removes the constraint that we always need to have light on in the background.”

Alan Jamison headshot

To combine the best properties of magnetic and optical traps, their experiment used both trapping techniques in a new combined experimental design that removed the need for atoms to be moved between the different trap types. Atoms of sodium and lithium were cooled down to ultracold temperatures using a combination of magnetic and optical cooling techniques. To form the ultracold NaLi molecules, optical trapping was necessary, however, upon formation, the molecules can be trapped again by magnetism, so the laser light was removed.

The researchers used their newly developed trapping design to measure inelastic collisions of the molecules as a proof of concept. Their success is now inspiring studies focused on a variety of different effects, such as studying how molecules respond to the introduction of light, studying the previously problematic light-induced chemical reactions in controlled ways, or seeing if the lifetime of these ultracold molecules can be prolonged with the different trapping method.

“By looking at what's considered an older way of doing things, we're finding that we have new possibilities for the future and how we work with our molecules,” said Jamison. “It’s important to always be looking forward, but also not lose sight of what's been done in the past. People had different interests and different focus in the past, so a lot of times, they thought through things in a way you didn't, or they've done something that you forgot could be done.”

This research, Magnetic trapping of ultracold molecules at high density, was published in Nature Physics on July 31 by IQC’s Jamison, and MIT’s Ketterle, Juliana Park andYu-Kun Lu.

Four illustrated panels. First shows larger yellow circles and smaller orange circles contained in a parabola shaped well drawn with a dotted orange line. Second panel contains same dotted orange line but the yellow and orange circles are attached together to represent molecules made of one yellow circle and one orange circle. Third panel shows molecule representations in a parabola shaped well made from both dotted orange and a grey lines. Panel 4 shows molecules in just a solid grey line parabola.

Representations of sodium (yellow) and lithium (orange) atoms (graphic I) and molecules (graphic II) confined in an optical trap (orange dotted line). The optical trap is slowly replaced by the magnetic trap (graphic III), and then experiments were performed in a completely magnetic trap (graphic IV).