Since the original work on Bose–Einstein condensation^1,2, the use of quantum degenerate gases of atoms has enabled the quantum emulation of important systems in condensed matter and nuclear physics, as well as the study of many-body states that have no analogue in other fields of physics³. Ultracold molecules in the micro- and nanokelvin regimes are expected to bring powerful capabilities to quantum emulation⁴ and quantum computing⁵, owing to their rich internal degrees of freedom compared to atoms, and to facilitate precision measurement and the study of quantum chemistry⁶. Quantum gases of ultracold atoms can be created using collision-based cooling schemes such as evaporative cooling, but thermalization and collisional cooling have not yet been realized for ultracold molecules. Other techniques, such as the use of supersonic jets and cryogenic buffer gases, have reached temperatures limited to above 10 millikelvin^7,8. Here we show cooling of NaLi molecules to micro- and nanokelvin temperatures through collisions with ultracold Na atoms, with both molecules and atoms prepared in their stretched hyperfine spin states. We find a lower bound on the ratio of elastic to inelastic molecule–atom collisions that is greater than 50—large enough to support sustained collisional cooling. By employing two stages of evaporation, we increase the phase-space density of the molecules by a factor of 20, achieving temperatures as low as 220 nanokelvin. The favourable collisional properties of the Na–NaLi system could enable the creation of deeply quantum degenerate dipolar molecules and raises the possibility of using stretched spin states in the cooling of other molecules.

自从玻色-爱因斯坦凝聚^1,2的最初工作以来，原子的量子简并气体的使用使得凝聚态和核物理中重要系统的量子仿真成为可能，以及对没有类似物的多体态的研究在其他物理领域³。由于与原子相比具有丰富的内部自由度，微和纳米开尔文体系中的超冷分子有望为量子仿真⁴和量子计算^{5带来强大的能力，并促进精确测量和量子化学研究6}. 超冷原子的量子气体可以使用基于碰撞的冷却方案（例如蒸发冷却）来产生，但尚未实现超冷分子的热化和碰撞冷却。其他技术，例如使用超音速喷气机和低温缓冲气体，已达到限制在 10 毫开尔文以上的温度^7,8. 在这里，我们展示了通过与超冷 Na 原子的碰撞将 NaLi 分子冷却到微米和纳米开尔文温度，分子和原子都以拉伸的超精细自旋状态制备。我们发现弹性与非弹性分子-原子碰撞比率的下限大于 50，足以支持持续的碰撞冷却。通过采用两级蒸发，我们将分子的相空间密度提高了 20 倍，实现了低至 220 纳开尔文的温度。Na-NaLi 系统的有利碰撞特性可以产生深度量子简并偶极分子，并提高了在冷却其他分子时使用拉伸自旋态的可能性。