Polyatomic Molecules Get Two Steps Closer to Quantum Horizon
Ultracold gases have been a cornerstone of modern physics, advancing fundamental physics, quantum simulation, quantum computation, atomic clocks, and quantum sensing. Recently, the family of ultracold gases has expanded to include diatomic molecules made of two atomic species. Their additional degrees of freedom of vibration and rotation add experimental complexity, while their unique features, such as strong dipolar interactions, expand their scientific scope far beyond ultracold atoms. Polyatomic molecules—those with three or more atoms—are the next frontier. Although they require control over more vibrational and rotational modes, they have properties that can be exploited to build the next generation of fundamental physics sensors, quantum computers, and simulators [1, 2]. Christian Hallas and Nathaniel Vilas from Harvard University and their colleagues have shown that they can trap polyatomic molecules at high densities and ultracold temperatures, enabling them to observe and control collisions between these molecules [3, 4]. These are important milestones toward creating gases of polyatomic molecules that are dense enough to unleash their full quantum nature.
To grasp the potential of polyatomic molecules, consider the simplest type: a triatomic molecule. The bending of such a molecule opens up a new degree of freedom that allows the molecule to rotate about the molecular axis in clockwise and counterclockwise directions (Fig. 1). The superposition of these two rotations forms two states—called a parity doublet—that have opposite symmetries under reflection. The energy spacing between these parity states in a triatomic molecule is orders of magnitude smaller than the closest parity-state splitting in a diatomic molecule. As a result, with a much smaller electric field, these two parity states become mixed, producing new states with unique field sensitivities [5], interaction potentials, and collisional properties [6]. These features are highly tunable with the electric field, which is advantageous for various applications including quantum computing [2] and precision measurements [1].
Realizing the promise of polyatomic molecules will require placing them in the quantum degenerate regime, where the particle wave functions in a gas begin to overlap. To create a degenerate gas, the steps typically include laser cooling, trapping, compressing, and evaporative cooling. Each step is more challenging for molecules than atoms because they have complex structures and because their collisions introduce additional chemical-reaction loss channels. Previously, cooling and trapping have been demonstrated for polyatomic molecules, but the achieved density was not high enough for the next steps.
Laser cooling involves repeatedly scattering photons off target particles. The scattering produces radiative forces that can remove energy and entropy from the system. Atoms are relatively easy to cool, as they always return to their initial state after each scattering event. For complex particles like polyatomic molecules, the extra degrees of freedom provide a host of vibrational and rotational states that particles can decay into. Therefore, more lasers are needed to address these states, so that photons can continue to scatter off the particles. In an experiment from 2022, Hallas, Vilas, and their Harvard colleagues needed 14 lasers to successfully cool and trap the polyatomic molecule calcium monohydroxide (CaOH) to a temperature of 100 µK in a magneto-optical trap (MOT) at a density of 106 cm–3[7].
Going to lower temperatures hits a roadblock. The reason is that in a MOT an applied magnetic field modifies the radiative forces to always point to the trap center. However, fluctuations in the photon-scattering process result in a temperature limit—called the Doppler limit—below which the radiative forces result in heating for most molecules. Further cooling requires loading into a trapping potential that does not rely on photon scattering, such as an optical dipole trap (ODT). For the loading to be efficient, the trap volume of the MOT needs to be squeezed down to better overlap with the ODT volume, which is much smaller than a MOT.
One way to achieve this trap overlap is to use a different type of MOT called a blue MOT. For a normal MOT, the laser frequencies are below the molecular transition frequencies. In a blue MOT, they are above the molecular transition frequencies. This “blue detuning” modifies the scattering process, making the radiative forces effective below the Doppler limit. For diatomic molecules, a blue MOT increases the molecule densities by a factor of 50 [8].
In the first of the two advances, the Harvard team implemented an improved configuration of the blue MOT for CaOH, achieving particle densities of 8 × 108 cm–3 [3]. These values, which are 400 times higher than in the normal MOT and 16 times higher than the original blue MOT, are sufficient for efficient loading of the gas into an ODT. The team’s simulation results, which show excellent agreement with experimental data, also suggest that this improved scheme could be applied to any molecular MOT.
The team’s second advance concerns control over collisional loss. This control is needed for evaporative cooling, where hotter particles preferentially leave the trap and the remaining particles rethermalize to a lower temperature. Thermalization requires collisions, which poses a problem for molecules, as their short-range collisions almost always lead to losses due to chemical processes. For diatomic molecules, the chemical processes can be shielded by shaping the long-range interaction potential between molecules to be repulsive using microwaves [9] or strong, static electric fields [10]. For their polyatomic molecule, the Harvard team utilized CaOH’s parity-doublet structure to bypass the experimental challenges of a strong electric field. By applying a relatively small electric field, the researchers were able to tune the CaOH properties so that short-range collisions were suppressed. Conceivably, this suppression would reduce losses and set the stage for evaporative cooling. It’s worth noting that this method should be applicable to other molecular species with parity doublets.
Having advanced both the trapping and cooling methods of polyatomic molecules, the Harvard team presents a clear road map for bringing polyatomic molecules to the quantum degenerate regime. This guideline is open to other polyatomic molecules, including those with parity doublets in their long-lived ground states—a scenario that could allow deep, evaporative cooling into the quantum degenerate regime. Polyatomic molecules offer a broad range of possibilities that could advance various applications. For example, heavier molecules such as ytterbium monohydroxide (YbOH) and radium monomethoxide (RaOCH3) are promising probes of new physics beyond the standard model of particle physics [1]. Closed shell molecules, such as formaldehyde (H2CO) and fluoromethane (CH3F), are ideal molecules for quantum information processing [2].
References
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- M. Löw et al., “Coherence of symmetry-protected rotational qubits in cold polyatomic molecules,” Phys. Rev. Lett. 134, 113402 (2025).
- C. Hallas et al., “High compression blue-detuned magneto-optical trap of polyatomic molecules,” Phys. Rev. Lett. 136, 133402 (2026).
- N. B. Vilas et al., “Quantum-state-controlled collisions of ultracold polyatomic molecules,” Phys. Rev. X 16, 021001 (2026).
- L. Anderegg et al., “Quantum control of trapped polyatomic molecules for eEDM searches,” Science 382, 665 (2023).
- L. D Augustovičová and J. L Bohn, “Ultracold collisions of polyatomic molecules: CaOH,” New J. Phys. 21, 103022 (2019).
- N. B. Vilas et al., “Magneto-optical trapping and sub-Doppler cooling of a polyatomic molecule,” Nature 606, 70 (2022).
- J. J. Burau et al., “Blue-detuned magneto-optical trap of molecules,” Phys. Rev. Lett. 130, 193401 (2023).
- L. Anderegg et al., “Observation of microwave shielding of ultracold molecules,” Science 373, 779 (2021).
- K. Matsuda et al., “Resonant collisional shielding of reactive molecules using electric fields,” Science 370, 1324 (2020).