Phys. Rev. Lett. 116, 063005 (2016)

While laser cooling is a well-established technique to produce atoms at very low temperatures, researchers have lacked an analogously robust and versatile approach for cooling molecules. The capability to cool molecules would advance investigations in fundamental physics and spectroscopy. Now, Alexander Prehn and co-workers from Germany have developed a scheme to cool molecules of gaseous formaldehyde down to microkelvin temperatures. Their approach is based on optoelectrical Sisyphus cooling, which involves a combination of laser, microwave and radiofrequency sources and an electrostatic trap. The molecules lose kinetic energy in a closed cooling cycle that promotes them to strongly trapped rotational states first, and then lowers them to states with weaker Stark interaction in the strong-field region at the edges of the trap. The researchers report the production of an ensemble of approximately 3 × 105 cooled molecules with a temperature of about 420 μK. Moreover, more than 80% of the ensemble populates a single rotational state. When compared with an uncooled sample, these results indicate a reduction of kinetic energy by a factor of 1,000.