Triatomic molecules cooled with lasers

Physics World - May 5, 2017

Molecules containing three atoms have been laser cooled to ultracold temperatures for the first time. The feat was achieved by John Doyle and colleagues at Harvard University in the US, who used a technique called Sisyphus cooling to chill an ensemble of about a million strontium-monohydroxide molecules to 750 μK. The team says the work opens the door to a range of applications, including quantum simulation and precision measurements.

First developed in the late 1970s, the laser cooling of atomic gases to ultracold temperatures has revolutionized the study of the quantum states of matter. Important milestones include the creation of the first-ever Bose–Einstein condensate in the lab in 1995 and the first Fermi–Dirac condensate in 2003. The technique relies on the fact that photons carry small amounts of momentum and – under certain conditions – the repeated absorption and re-emission of photons by an atom can reduce its random motion and hence its temperature.

Degrees of freedom

Laser cooling of molecules – rather than atoms – is complicated by their rotational and vibrational degrees of freedom, which affect how they absorb and emit photons. As a result, the absorption and emission of photons can put the molecules into “dark states” that no longer take part in the cooling process. Despite this and other challenges, however, David DeMille and colleagues at Yale University managed to laser-cool a collection of strontium-fluoride diatomic molecules in 2014.
In this latest work, John Doyle and colleagues at Harvard University have now cooled triatomic-strontium monohydroxide molecules using a method that is named after the doomed Greek hero Sisyphus, who was forced to push a boulder up a hill, only for it to roll down to the bottom and then repeat the task for eternity. Sisyphus cooling involves molecules losing kinetic energy by having to “climb” a hill of potential energy created by a standing wave of laser light.
The atoms reach the “peak” when they spontaneously transition to a state that no longer interacts with the light. At this point, an applied magnetic field puts the atoms back into the original state – ready to climb again. This process is repeated many times, with each cycle reducing the atoms’ kinetic energy – and thus their random motion and temperature too.

Rapid cooling

Key to the success of Doyle’s team is that the cooling was achieved very rapidly – in 100 μs – and only involved about 200 photons interacting with each molecule. This speed is critical as the molecules are therefore less likely to be put into dark states before the cooling finishes.
Writing in Physical Review Letters, Doyle and colleagues say that their technique could also be used to cool larger and more complicated strontium-based polyatomic molecules – for example by replacing the hydroxide with a methyl group. If the technique could be further extended to chiral molecules, it could also be used to investigate why some biological processes favour right- or left-handed molecules.

Hamish Johnston, Physics World