Phys. Rev. Lett. 122, 153601 (2019)

Signatures of quantum phenomena have been seen before in optomechanics, but, so far, the mechanical resonators have been composed of gases or solids. Now, researchers in the US and France have observed telltale signs of zero-point motion and quantum back-action in a liquid. The team monitored an acoustic standing wave in superfluid liquid He, which was coupled to an optical cavity, enabling Stokes and anti-Stokes scattering of light. The authors highlighted a number of possible advantages of using liquids. Superfluid He can carry ‘impurities’, for example electrons, ions and excimers, enabling hybrid quantum systems. From an optical standpoint, there is the tantalizing fact that superfluids can conformally coat, or fill, a photonic cavity, and alignment of the two systems (acoustic and optical) may not be needed.