# Unconventional Cavity Optomechanics: Nonlinear Control of Phonons in the Acoustic Quantum Vacuum

## TL;DR

This paper explores nonlinear control of phonons in the acoustic quantum vacuum through unconventional cavity optomechanics, enabling quantum effects beyond traditional systems by using an intermediate qubit and modulating boundary conditions.

## Contribution

It introduces a novel method to enhance nonlinear phonon interactions using an intermediate qubit, enabling testing of quantum effects at unprecedented parameters.

## Key findings

- Demonstrates modulation of phonon resonator frequency.
- Shows mechanical parametric amplification.
- Realizes the dynamical Casimir effect for phonons.

## Abstract

We study unconventional cavity optomechanics and the acoustic analogue of radiation pressure to show the possibility of nonlinear coherent control of phonons in the acoustic quantum vacuum. Specifically, we study systems where a quantized optical field effectively modifies the frequency of an acoustic resonator. We present a general method to enhance such a nonlinear interaction by employing an intermediate qubit. Compared with conventional optomechanical systems, the roles of mechanical and optical resonators are interchanged, and the boundary condition of the phonon resonator can be modulated with an ultrahigh optical frequency. These differences allow to test some quantum effects with parameters which are far beyond the reach of conventional cavity optomechanics. Based on this novel interaction form, we show that various nonclassical quantum effects can be realized. Examples include an effective method for modulating the resonance frequency of a phonon resonator (e.g., a surface-acoustic-wave resonator), demonstrating mechanical parametric amplification, and the dynamical Casimir effect of phonons originating from the acoustic quantum vacuum. Our results demonstrate that unconventional optomechanics offers a versatile hybrid platform for quantum engineering of nonclassical phonon states in quantum acoustodynamics.