Measuring High-Order Phonon Correlations in an Optomechanical Resonator

TL;DR

This paper demonstrates the measurement of high-order phonon correlations in a superfluid helium resonator using single photon detectors, revealing detailed quantum coherence properties of the mechanical mode.

Contribution

It introduces a method to probe and measure high-order phonon coherences in an optomechanical resonator via photon detection and post-selection, advancing quantum state characterization techniques.

Findings

Measured phonon coherences up to the fourth order.

Confirmed thermal equilibrium with a Markovian bath.

Demonstrated control over phonon addition and subtraction.

Abstract

We use single photon detectors to probe the motional state of a superfluid $^4$He resonator of mass $\sim1$ ng. The arrival times of Stokes and anti-Stokes photons (scattered by the resonator's acoustic mode) are used to measure the resonator's phonon coherences up to the fourth order. By post-selecting on photon detection events, we also measure coherences in the resonator when $\leq3$ phonons have been added or subtracted. These measurements are found to be consistent with predictions that assume the acoustic mode to be in thermal equilibrium with a bath through a Markovian coupling.