Laser cooling and control of excitations in superfluid helium

TL;DR

This paper demonstrates the use of cavity optomechanics to probe, cool, and control excitations in superfluid helium, enabling real-time thermodynamics studies and potential quantum optomechanics applications.

Contribution

It introduces a novel approach combining cavity optomechanics with superfluid helium to control and measure excitations at microscopic scales.

Findings

Real-time thermodynamic probing of superfluid excitations.

Laser cooling and amplification of superfluid phonons.

Potential for quantum optomechanics with superfluid films.

Abstract

Superfluidity is an emergent quantum phenomenon which arises due to strong interactions between elementary excitations in liquid helium. These excitations have been probed with great success using techniques such as neutron and light scattering. However measurements to-date have been limited, quite generally, to average properties of bulk superfluid or the driven response far out of thermal equilibrium. Here, we use cavity optomechanics to probe the thermodynamics of superfluid excitations in real-time. Furthermore, strong light-matter interactions allow both laser cooling and amplification of the thermal motion. This provides a new tool to understand and control the microscopic behaviour of superfluids, including phonon-phonon interactions, quantised vortices and two-dimensional quantum phenomena such as the Berezinskii-Kosterlitz-Thouless transition. The third sound modes studied here also offer a pathway towards quantum optomechanics with thin superfluid films, including femtogram effective masses, high mechanical quality factors, strong phonon-phonon and phonon-vortex interactions, and self-assembly into complex geometries with sub-nanometre feature size.