Quantum drops are spotted in ultracold gas of molecules
Two years ago, Sebastian Will of Columbia University and colleagues cooled a gas of sodium cesium molecules to nanokelvin temperatures inside an optical trap and formed the first Bose–Einstein condensate of dipolar molecules. 1 A key motivation of the research was exploiting dipole–dipole interactions—at such cold temperatures, they’re more energetic than the molecules’ kinetic energies—to form ordered structures such as supersolids, which flow with no viscosity. (For more on the 2024 findings, see the PT story “A Bose–Einstein condensate of dipolar molecules ,” by Daniel Garisto.) But the dipolar interactions in that initial demonstration were too weak to realize such phenomena.
Now, using a pair of linearly and elliptically polarized microwave fields, Siwei Zhang (also at Columbia), Will, and colleagues have tuned the strength of the interactions between NaCs molecules to be much stronger than that in previous experiments. 2 As in the group’s 2024 work, the resulting enhancements to the repulsive van der Waals forces stabilize the condensates by keeping the molecules from getting too close to one another and colliding.
As a consequence of the strengthened dipole–dipole interactions, the NaCs condensate transitions to the droplet phase. Dense single droplets and droplet arrays, shown in the figure, appear for various strengths and orientations of the applied fields. The most dense, strongly interactive molecular clouds persist for about 100 ms, long enough for the researchers to observe the effects of the molecular interactions.
Absorption images show a droplet and a droplet array in a Bose–Einstein condensate (BEC) of ultracold dipolar sodium cesium molecules. Each picture is a three-image average taken along the z-direction. The strength of the molecular dipole–dipole interactions is controlled by an elliptically polarized microwave field. A large negative value of the ellipticity leads to the formation of a single self-bound droplet; a large positive value, to a droplet array.
(Figure adapted from ref. 2 .)
Will and colleagues are investigating whether the droplets could be a superfluid, which flows without a loss of kinetic energy. If the droplet arrays have crystalline order, they could be supersolids. The new results may have broader applications. Ultracold dipolar molecules could be a test platform for researchers to study many-body systems with long-range interactions, novel self-organization processes, and other condensed-matter phenomena.
References
1. N. Bigagli et al., “Observation of Bose–Einstein condensation of dipolar molecules ,” Nature 631, 289 (2024).
2. S. Zhang et al., “Observation of self-bound droplets of ultracold dipolar molecules ,” Nature 651, 601 (2026).
