Molecular One

Molecular quantum gases (that is, ultracold and dense molecular gases) have many potential applications, including quantum control of chemical reactions, precision measurements, quantum simulation and quantum information processing. For molecules, to reach the quantum regime usually requires efficient cooling at high densities. In new research, physicists from the James Franck Institute at the University of Chicago and the Shanxi University’s Institute of Opto-Electronics have successfully created a two-dimensional Bose-Einstein condensate of spinning cesium molecules.

Zhang et al. demonstrate the long-sought transition between atomic and molecular condensates, the bosonic analogue of the crossover from a Bose-Einstein condensate to a Bardeen-Cooper-Schrieffer superfluid in a Fermi gas. Image credit: Zhang et al., doi: 10.1038/s41586-021-03443-0.

Zhang et al. demonstrate the long-sought transition between atomic and molecular condensates, the bosonic analogue of the crossover from a Bose-Einstein condensate to a Bardeen-Cooper-Schrieffer superfluid in a Fermi gas. Image credit: Zhang et al., doi: 10.1038/s41586-021-03443-0.

Because of their rich energy structure, cold molecules may enable advances in quantum engineering and quantum chemistry.

A wide variety of platforms have been developed to trap and cool the cold molecules.

The same rich energy structure, however, also causes complex reactive collisions that obstruct experimental attempts to cool molecules towards quantum degeneracy.

One successful strategy towards preparing molecular quantum gas is to begin with an atomic quantum gas, and then to pair the atoms into molecules.

“Atoms are simple spherical objects, whereas molecules can vibrate, rotate, carry small magnets,” said Professor Cheng Chin, a researcher in the James Franck Institute at the University of Chicago.

“Because molecules can do so many different things, it makes them more useful, and at the same time much harder to control.”

“People have been trying to do this for decades, so we’re very excited,” he added.

“I hope this can open new fields in many-body quantum chemistry. There’s evidence that there are a lot of discoveries waiting out there.”

First, Professor Chin and colleagues created a Bose-Einstein condensate of cesium (Cs) atoms and cooled it down to 10 nanokelvins.

Then they produced Cs2 molecules by pairing Bose-condensed Cs atoms in a two-dimensional, flat-bottomed trap.

“Typically, molecules want to move in all directions, and if you allow that, they are much less stable,” Professor Chin said.

“We confined the molecules so that they are on a two-dimensional surface and can only move in two directions.”

The result was a set of virtually identical molecules — lined up with exactly the same orientation, the same vibrational frequency, in the same quantum state.

“In the traditional way to think about chemistry, you think about a few atoms and molecules colliding and forming a new molecule,” Professor Chin said.

“But in the quantum regime, all molecules act together, in collective behavior. This opens a whole new way to explore how molecules can all react together to become a new kind of molecule.”

“This has been a goal of mine since I was a student, so we’re very, very happy about this result.”

The results appear in the journal Nature.

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Z. Zhang et al. 2021. Transition from an atomic to a molecular Bose–Einstein condensate. Nature 592, 708-711; doi: 10.1038/s41586-021-03443-0

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