A team at the Chinese University of Hong Kong has done what physicists spent two years trying to do since the first molecular Bose-Einstein condensate was created: not just condense polar molecules, but give experimenters a knob to turn. In a paper published in Nature Physics on July 9, 2026, lead authors Zhaopeng Shi and Zerong Huang, working under senior physicist Dajun Wang, report the first Bose-Einstein condensate of sodium-rubidium (NaRb) molecules — and, more critically, the first demonstration of a continuously adjustable dipolar interaction dial that can switch the condensate between two fundamentally different phases of quantum matter. The paper attracted wide popular science attention as of July 30, 2026, when coverage of the Nature Physics result went broad.

The dial works by layering two microwave fields of different polarizations on top of the trapped molecules. Tune one field’s parameters and the condensate is a diffuse, expanding quantum gas. Tune them differently and the condensate collapses — not catastrophically, but into a compact, self-bound ball of quantum matter called a quantum droplet, held together not by any external trap but by the quantum fluctuations of the molecules themselves. The team demonstrated both phases cleanly and mapped the boundary between them, completing a phase diagram that theorists had predicted but no molecular experiment had previously charted.

Why Getting Polar Molecules to Condense Is So Hard

Every BEC experiment starts the same way: load atoms or molecules into a magnetic or optical trap, then gradually remove the fastest-moving particles in a process called evaporative cooling, allowing the remaining particles to rethermalize at progressively lower temperatures until they reach the quantum ground state and condense. With atoms, this works well — atomic BECs have been routine since 1995 and can contain millions of atoms. With polar molecules, it had never worked at all, for a simple and brutal reason: when two polar molecules collide at close range, they almost always react chemically or scatter inelastically, releasing energy that heats and destroys the gas before it can cool.

The problem is called two-body collisional loss, and for ground-state bosonic polar molecules like NaRb, it is nearly universal. The molecules are chemically reactive at short range regardless of how cold the gas is. Every attempt to run evaporative cooling produced a dead gas long before quantum degeneracy was reached.

Dual Microwave Shielding: How Two Fields Solve What One Cannot

The key to the CUHK experiment is a technique called dual microwave shielding, which applies two simultaneous microwave fields to the trapped molecules, each with a different polarization. The first field, circularly polarized (σ+), dresses the molecules’ internal rotational states in a way that creates a long-range repulsive barrier between approaching molecules. This had been demonstrated before — the Wang group itself published a 2023 study showing single-field shielding of NaRb that reduced two-body losses by two orders of magnitude. But single-field shielding alone was not enough to enable efficient evaporative cooling to a BEC in NaRb, because three-body recombination losses — three molecules colliding simultaneously — remained too high.

The second field, linearly polarized (π), changes the situation because of a fundamental property of dipole-dipole interactions: rotating dipoles (from the σ+ field) and oscillating dipoles (from the π field) have opposite signs in their dipole-dipole interaction (DDI). When both fields are applied together, their contributions partially cancel or add depending on the specific field parameters. This gives experimenters unprecedented control — they can tune the net DDI across a wide range, suppressing both two-body and three-body losses simultaneously while keeping elastic collisions high enough for effective evaporative cooling to work.

The theory of dual microwave shielding was developed by Tijs Karman and colleagues at Radboud University, in collaboration with Sebastian Will’s group at Columbia University, and published in PRX Quantum in 2025. The CUHK team’s NaRb experiment is the second independent implementation of this technique in a different molecular species, validating that dual microwave dressing is a general method transferable across the field.

About 500 Molecules, but Definitively a Condensate

The resulting condensate contains roughly 500 molecules — a small number by atomic BEC standards, but unmistakably quantum-degenerate. The team confirmed condensation using the standard signature: time-of-flight (TOF) expansion. When the trap is switched off, a thermal (non-condensate) molecular cloud expands isotropically. A BEC expands anisotropically, reflecting the shape of the trap and the quantum nature of the state. The team observed the characteristic bimodal distribution — a narrow condensate peak sitting on top of a broader thermal cloud — that is the definitive signature of a BEC phase transition.

NaRb has a permanent electric dipole moment of approximately 3.3 Debye (D), which is more moderate than the NaCs dipole moment of about 4.75 D used in the first molecular BEC at Columbia University. The smaller dipole moment of NaRb makes interaction tuning more tractable: large dipole moments produce strong, hard-to-control interactions, while NaRb’s more modest moment responds more precisely to the microwave field parameters. This makes NaRb a particularly attractive species for systematic studies of how dipolar interactions shape quantum many-body phases.

Gas-to-Droplet Boundary Is the Map for a Molecular Supersolid Search

The most scientifically significant part of the CUHK experiment is not the condensate itself but what the interaction dial reveals: a clearly mapped boundary between two distinct quantum phases.

At weaker dipolar coupling — repulsive DDI settings — the condensate behaves as an ordinary gas-phase BEC. It expands freely when the trap is released, and its behavior matches mean-field theory.

At stronger dipolar coupling — as the DDI is tuned toward attractive — the condensate transitions into a quantum droplet: a compact, self-bound state that does not expand when the trap is switched off. Quantum droplets are stabilized against collapse by beyond-mean-field quantum fluctuations (Lee-Huang-Yang corrections), which provide a residual repulsive pressure that balances the attractive DDI. The team identified the gas-to-droplet transition point precisely by watching the TOF expansion velocities change sharply from expanding to non-expanding behavior as the microwave parameters were varied.

This phase diagram is not merely a curiosity. The gas-phase BEC and the quantum droplet are the two flanking states around the theoretically predicted supersolid — a state of matter that is simultaneously crystalline (with periodic density modulations) and superfluid (zero-viscosity flow). Supersolids have been observed in dipolar atomic BECs using dysprosium and erbium atoms, but never in a molecular system. A molecular supersolid would be dramatically more tunable than its atomic equivalent, because the molecular interaction dial can shift the DDI strength over a much wider range. The NaRb phase diagram already charted is the roadmap for finding it.

What This Adds to the First Molecular BEC

The first molecular BEC — sodium-cesium (NaCs) molecules condensed by Sebastian Will’s group at Columbia University in 2024 — was itself a landmark. That experiment demonstrated that evaporative cooling of bosonic polar molecules to quantum degeneracy was possible using enhanced microwave shielding. But the NaCs paper did not demonstrate a continuously tunable interaction regime or a gas-to-droplet phase transition; those remained goals.

Will’s group subsequently observed self-bound droplets of ultracold dipolar molecules in a study published in Nature in March 2026. The CUHK NaRb result — produced independently in a different molecular species, using a specifically optimized dual-field shielding implementation — provides a second experimental platform for studying molecular droplet physics. Two independent platforms make reproducibility real and comparisons across molecular species possible for the first time.

Dajun Wang and Tao Shi described the achievement’s significance in statements reported by Phys.org in late July 2026: polar molecules are especially interesting because they combine rich internal structure — vibrational and rotational degrees of freedom — with permanent electric dipole moments, giving experimenters more handles to control interactions than any atomic system offers.

What Dual Microwave Dressing Means for the Broader Field

The technique validated by the NaRb result is not species-specific. The theoretical framework for dual microwave dressing, published by Karman, Bigagli, Will, and colleagues in PRX Quantum in 2025, applies to any polar molecular species for which ground-state samples can be prepared. Several other bi-alkali polar molecules are currently being prepared in laboratories worldwide — NaK, RbCs, KCs, and others. Each of those species now has a validated roadmap for reaching quantum degeneracy and achieving interaction tunability.

The near-term experimental agenda at CUHK and at competing labs is now focused on searching for supersolid phases, mapping the full droplet-array phase diagram, loading molecules into optical lattices to simulate quantum magnetism models beyond classical computation, and probing the equation of state and collective excitation spectrum of strongly dipolar molecular quantum matter.

The research was conducted by Zhaopeng Shi, Zerong Huang, Fulin Deng, Wei-Jian Jin, Su Yi, Tao Shi, and Dajun Wang, with affiliations at the Chinese University of Hong Kong and the Institute of Theoretical Physics at the Chinese Academy of Sciences. The work was supported by the Hong Kong Research Grants Council, China’s National Science and Technology Major Project (2024ZD0300600), the National Natural Science Foundation of China, and the Guangdong Provincial Quantum Science Strategic Initiative.

Frequently Asked QuestionsWhat is a molecular Bose-Einstein condensate and why is it different from an atomic BEC?

An atomic BEC forms when identical atoms — which must be bosons — are cooled to near absolute zero (0 K, or −459.67°F / −273.15°C) and collectively fall into the same quantum ground state, behaving as a single macroscopic quantum object. A molecular BEC does the same, but with molecules instead of atoms. The difference matters because polar molecules carry permanent electric dipole moments — uneven charge distributions that create long-range, directional attractive and repulsive forces between molecules. Atoms interact only through short-range contact forces. Long-range dipole-dipole interactions produce qualitatively new phases of quantum matter — including supersolids and self-bound droplets — that are inaccessible with atomic BECs. The challenge is that molecules lose energy in inelastic collisions far more readily than atoms do, which is why molecular BECs required entirely new cooling and loss-suppression techniques.

What are quantum droplets, and why does the NaRb experiment produce them?

A quantum droplet is a self-bound state of ultracold matter that forms when the attractive component of the dipole-dipole interaction becomes dominant enough that the gas would normally collapse — but is instead stabilized by beyond-mean-field quantum fluctuations, a correction to standard quantum mechanics known as the Lee-Huang-Yang correction. The result is a compact droplet that holds together even without an external trap. In the NaRb experiment, researchers tuned the microwave field parameters to increase the attractive DDI strength until the condensate crossed the gas-to-droplet phase boundary, confirmed by watching the molecules stop expanding after the trap was released. The ability to cross this boundary controllably — and map it precisely — is what makes the NaRb platform valuable for supersolid physics research.

How does dual microwave shielding suppress the collisional losses that previously blocked molecular BEC?

Dual microwave shielding applies two microwave fields simultaneously to the trapped molecules: a circularly polarized (σ+) field that creates a long-range repulsive barrier between approaching molecules, and a linearly polarized (π) field whose oscillating-dipole contribution has the opposite DDI sign from the rotating-dipole contribution of the σ+ field. Together, the two fields can be tuned to cancel the attractive part of the DDI at short range while preserving long-range repulsion — suppressing both two-body inelastic collisions and three-body recombination losses that prevented earlier evaporative cooling attempts. Elastic collisions (which drive cooling) are preserved, allowing the molecular gas to cool efficiently all the way to quantum degeneracy.

What would a molecular supersolid be, and is the NaRb experiment close to finding one?

A supersolid is a state of matter that is simultaneously crystalline — with periodic spatial density modulations, like a solid — and superfluid, meaning it flows with zero viscosity. The coexistence of these properties, long thought impossible, has been demonstrated in dipolar atomic BECs since around 2019–2021 using dysprosium and erbium atoms. A molecular supersolid would be far more tunable than its atomic counterpart: the same microwave field dial that switches NaRb between gas and droplet phases can be adjusted to target the intermediate regime where a supersolid is theoretically predicted to exist. The CUHK team has not yet observed a molecular supersolid — but the phase diagram mapped in this experiment is the experimental roadmap for finding one.