Ultracold Molecules Normally Destroy Each Other in Essentially Every Collision. Columbia and Radboud Physicists Dressed a Gas of Sodium-Cesium Molecules at 100 Nanokelvin With Two Microwave Fields and Cut Those Losses More Than 10,000-Fold, So the Gas Lived for Seconds and Its Interactions Could Be Tuned From Zero to a Full Micrometer.

Gases of ultracold molecules are one of the newest forms of artificial quantum matter, and one of the most frustrating. Cooled to a few billionths of a degree above absolute zero, the molecules stop behaving like individual particles and act collectively, forming a Bose-Einstein condensate in which many share one quantum state. But unlike the atoms physicists have trapped for three decades, molecules tend to vanish the moment they touch: a close encounter between two of them almost always ends in a chemical reaction or a sticky complex that leaves the trap. Researchers at Columbia University and Radboud University in the Netherlands have now suppressed that loss by more than a factor of 10,000, producing a molecular gas that survives for seconds.

The work, published in Science, was done in the laboratory of Sebastian Will at Columbia with theory from Tijs Karman at Radboud. The team started with sodium-cesium molecules held in an optical trap at an initial temperature of about 100 nanokelvin and a density of roughly 0.7 trillion molecules per cubic centimeter, then bathed them simultaneously in two microwave fields, one circularly polarized and one linearly polarized. The fields dress the molecules' rotational states and reshape the effective force between them, creating a repulsive shield at short range that keeps pairs from reaching the distance at which they would react.

"By controlling the molecular dipole moments with microwaves, researchers were able to suppress these losses by more than a factor of 10,000," Karman said. "In fact, the suppression was so great that only an upper bound, set by the single-molecule lifetime, could be measured in the lab of Sebastian Will at Columbia." Three-body losses, in which a third molecule joins the collision, fell by more than a factor of 1,000.

Crucially, the shielding held across a wide range of interaction strengths. By adjusting the microwave parameters the team could tune the dipolar length, a measure of how far the molecules' electric dipoles reach, continuously from zero to about one micrometer, or roughly 20,000 times the Bohr radius. That is longer than the average spacing between molecules in the gas, which is the regime where a quantum fluid stops behaving like a weakly interacting gas and starts behaving like a liquid. "This combination makes it possible to realize stable quantum liquids, where the interaction range is larger than the average distance between molecules," Karman said. "Such systems provide a new platform for exploring strongly interacting quantum matter that is difficult to achieve with atomic gases."

The appeal of molecules over atoms is precisely their dipoles. A polar molecule carries a built-in separation of positive and negative charge, giving it long-range, direction-dependent interactions that resemble the forces between electrons in real materials far more closely than the short-range contact interactions of atoms. A dense, long-lived, tunable dipolar gas can therefore act as a controllable stand-in for exotic solids, letting physicists simulate phenomena such as unconventional superconductivity or quantum magnetism with every knob under their control.

The paper, whose authors include Weijun Yuan, Siwei Zhang, Niccolò Bigagli, Haneul Kwak, Claire Warner and Ian Stevenson alongside Karman and Will, follows the same group's earlier demonstration of Bose-Einstein condensation in dipolar molecules. Together, the authors write, the two results "open the door to the exploration of strongly dipolar quantum liquids." A week before this publication, Karman also reported a way to arrange molecules into highly ordered arrays, like eggs in a carton, a layout that could serve as a foundation for using individual molecules as quantum bits.