Interfacing microwave qubits and optical photons via spin ensembles

TL;DR

This paper proposes a spin ensemble-based transducer protocol that enables efficient, reversible conversion of single photons between optical and microwave frequencies, facilitating integration of quantum networks.

Contribution

It introduces a novel protocol combining optical storage and controlled coupling to achieve high-efficiency frequency conversion between optical photons and superconducting qubits.

Findings

Efficiency exceeds 80% with NV centers

Approaches 99% with cold atomic ensembles

The protocol bridges optical and microwave quantum systems

Abstract

A protocol is discussed which allows one to realize a transducer for single photons between the optical and the microwave frequency range. The transducer is a spin ensemble, where the individual emitters possess both an optical and a magnetic-dipole transition. Reversible frequency conversion is realized by combining optical photon storage, by means of EIT, with the controlled switching of the coupling between the magnetic-dipole transition and a superconducting qubit, which is realized by means of a microwave cavity. The efficiency is quantified by the global fidelity for transferring coherently a qubit excitation between a single optical photon and the superconducting qubit. We test various strategies and show that the total efficiency is essentially limited by the optical quantum memory: It can exceed 80% for ensembles of NV centers and approaches 99% for cold atomic ensembles, assuming state-of-the-art experimental parameters. This protocol allows one to bridge the gap between the optical and the microwave regime so to efficiently combine superconducting and optical components in quantum networks.