# Coherent conversion between microwave and optical photons -- an overview of physical implementations

## TL;DR

This paper reviews various physical implementations for converting microwave photons to optical photons, highlighting their theoretical foundations, experimental progress, limitations, and future prospects in quantum communication.

## Contribution

It provides a comprehensive overview of the theoretical and experimental approaches for microwave-to-optical photon conversion, emphasizing recent progress and challenges.

## Key findings

- Optomechanical and electro-optic interactions are the most promising methods.
- Current systems face limitations in efficiency and noise levels.
- Future developments aim to improve fidelity and scalability.

## Abstract

Quantum information technology based on solid state qubits has created much interest in converting quantum states from the microwave to the optical domain. Optical photons, unlike microwave photons, can be transmitted by fiber, making them suitable for long distance quantum communication. Moreover, the optical domain offers access to a large set of very well developed quantum optical tools, such as highly efficient single-photon detectors and long-lived quantum memories. For a high fidelity microwave to optical transducer, efficient conversion at single photon level and low added noise is needed. Currently, the most promising approaches to build such systems are based on second order nonlinear phenomena such as optomechanical and electro-optic interactions. Alternative approaches, although not yet as efficient, include magneto-optical coupling and schemes based on isolated quantum systems like atoms, ions or quantum dots. In this Progress Report, we provide the necessary theoretical foundations for the most important microwave-to-optical conversion experiments, describe their implementations and discuss current limitations and future prospects.