Phonon-Counting Experiments with Superfluid Helium Optomechanics

Superfluid Helium is a very good platform for quantum optomechanics owing to its extremely low optical and mechanical losses. In a device consisting of a Fabry-Perot cavity filled with superfluid helium, each acoustic mode of the helium is optomechanically coupled to a single optical mode of the cavity [1]. In previous work we used such a device to measure an acoustic mode's Gaussian quantum fluctuations [2]. Here we describe new measurements in which we have incorporated single-photon detectors (and multiple cascaded optical filters) to record individual sideband photons, each of which is associated with the creation (or annihilation) of a single phonon in the acoustic mode. We will describe the prospects for using this approach to prepare and characterize non-Gaussian states of the acoustic mode.

[1] A. D. Kashkanova et. al., Nat. Phys. 13, 74 (2017)
[2] A. B. Shkarin et. al., Phys. Rev. Lett. 122, 153601 (2019)