Magneto-optical trapping and sub-Doppler cooling of a polyatomic molecule

Laser cooling and trapping^1,2, and magneto-optical trapping methods in particular², have enabled groundbreaking advances in science, including Bose-Einstein condensation^3-5, quantum computation with neutral atoms^6,7 and high-precision optical clocks⁸. Recently, magneto-optical traps (MOTs) of diatomic molecules have been demonstrated^9-12, providing access to research in quantum simulation¹³ and searches for physics beyond the standard model¹⁴. Compared with diatomic molecules, polyatomic molecules have distinct rotational and vibrational degrees of freedom that promise a variety of transformational possibilities. For example, ultracold polyatomic molecules would be uniquely suited to applications in quantum computation and simulation^15-17, ultracold collisions¹⁸, quantum chemistry¹⁹ and beyond-the-standard-model searches^20,21. However, the complexity of these molecules has so far precluded the realization of MOTs for polyatomic species. Here we demonstrate magneto-optical trapping of a polyatomic molecule, calcium monohydroxide (CaOH). After trapping, the molecules are laser cooled in a blue-detuned optical molasses to a temperature of 110 μK, which is below the Doppler cooling limit. The temperatures and densities achieved here make CaOH a viable candidate for a wide variety of quantum science applications, including quantum simulation and computation using optical tweezer arrays^15,17,22,23. This work also suggests that laser cooling and magneto-optical trapping of many other polyatomic species^24-27 will be both feasible and practical.