Physicists Wrap Molecules in Microwaves to Reach a Quantum State Nobody Could Cool Into Before

Researchers at the Chinese University of Hong Kong and the Institute of Theoretical Physics at the Chinese Academy of Sciences have produced a Bose-Einstein condensate out of ultracold polar molecules, clearing an obstacle that has blocked the field for years: the molecules keep annihilating each other on the way down.

A Bose-Einstein condensate forms when particles called bosons are chilled to a fraction of a degree above absolute zero, at which point they all pile into the same quantum state and behave collectively, like one enormous super-particle. Physicists first made them with atoms in 1995. Doing it with molecules is far harder, and doing it with polar molecules — molecules whose positive and negative charges are separated, giving them a permanent electric dipole — has been one of the field's standing goals.

The prize is what those dipoles make possible. "Polar molecules are especially interesting because, unlike atoms, they have rich internal degrees of freedom, particularly vibrational and rotational structure, as well as permanent electric dipole moments," said co-senior authors Tao Shi and Dajun Wang. "These features open the door to new physics that are difficult or impossible to access with typical atomic systems, especially strongly interacting and long-range many-body phenomena."

The obstacle is collisional loss. When two of these molecules approach, they can form a transient two-molecule complex lasting only microseconds, and that complex tends to vanish from the sample entirely. "Bringing ultracold polar molecules into the condensate regime has remained extremely challenging because of near-universal two-body collisional loss," Shi and Wang said. Before this work, only NaCs molecules had reached a condensate, using a dual-microwave suppression technique.

The team's fix was to build a wall the molecules never reach past. "We engineered a repulsive barrier between the molecules at long range so that they are less likely to reach the short-range region where loss occurs," they explained. They created it with blue-detuned microwave fields tuned near the J = 0 to J = 1 rotational transition of sodium-rubidium: one circularly polarized field to induce a rotating electric dipole, one linearly polarized field to induce an oscillating one. Together the two fields shaped the long-range potential and let the researchers tune how the molecules interact. That microwave "dressing" strongly suppressed the destructive collisions while preserving enough useful elastic ones to keep cooling working. "That balance is essential, because evaporative cooling works only when the ratio of good to bad collisions is large," they said. From there the process was conventional: lower the optical trap, let the hottest molecules escape, let the rest rethermalize, repeat until the gas crosses into condensation.

Inside the new condensate the team also spotted something they had not set out to make — a self-bound NaRb quantum droplet, a blob of matter held together by its own interactions rather than by the trap. "We have already observed a gas-to-droplet phase transition in this system," Shi and Wang said, adding that the nature of the droplet phase remains open and that theory suggests it may form a single-layer structure. Studying it in detail will require bigger samples than they can currently make, which is now a near-term goal.

The researchers argue the microwave-dressed interactions here are strange enough to break some standard assumptions. Because the repulsive barrier is comparable in size to the spacing between molecules, "some of the standard assumptions underlying dilute-gas BEC theory may no longer apply," they said — the system may have more in common with liquid helium than with a textbook dilute condensate. The work is published in Nature Physics, with Zhaopeng Shi as lead author.