# Microwave-to-optics conversion using a mechanical oscillator in its quantum groundstate

## TL;DR

This paper demonstrates a highly efficient, low-noise microwave-to-optical signal conversion using a quantum groundstate mechanical oscillator, advancing quantum communication technology.

## Contribution

It introduces an integrated on-chip electro-opto-mechanical device capable of coherent conversion with minimal thermal noise, operating in the quantum groundstate.

## Key findings

- Achieved microwave-to-optical conversion with less than one phonon thermal background.
- Maintained coherence of microwave signals during transduction.
- Operated with an optomechanical cooperativity greater than 1.

## Abstract

Conversion between signals in the microwave and optical domains is of great interest both for classical telecommunication, as well as for connecting future superconducting quantum computers into a global quantum network. For quantum applications, the conversion has to be both efficient, as well as operate in a regime of minimal added classical noise. While efficient conversion has been demonstrated using mechanical transducers, they have so far all operated with a substantial thermal noise background. Here, we overcome this limitation and demonstrate coherent conversion between GHz microwave signals and the optical telecom band with a thermal background of less than one phonon. We use an integrated, on-chip electro-opto-mechanical device that couples surface acoustic waves driven by a resonant microwave signal to an optomechanical crystal featuring a 2.7 GHz mechanical mode. We initialize the mechanical mode in its quantum groundstate, which allows us to perform the transduction process with minimal added thermal noise, while maintaining an optomechanical cooperativity >1, so that microwave photons mapped into the mechanical resonator are effectively upconverted to the optical domain. We further verify the preservation of the coherence of the microwave signal throughout the transduction process.