Fundamental limits of free-space microwave-to-optical frequency conversion efficiency using Rydberg atoms

TL;DR

This paper establishes the fundamental efficiency limits of free-space microwave-to-optical conversion using Rydberg atoms, highlighting the impact of diffraction and proposing near-field antennas to improve performance.

Contribution

It develops a theoretical framework for evaluating conversion efficiency, identifying fundamental limits, and suggests using near-field antennas to surpass these constraints.

Findings

Maximum conversion efficiency is about 3/16 due to diffraction limits.

Focusing of microwave fields fundamentally restricts efficiency.

Near-field antennas can potentially overcome the diffraction limit.

Abstract

Efficient microwave-to-optical frequency conversion (MOC) is crucial for applications such as radiometry, electrometry, quantum microwave illumination and quantum networks. Rydberg atoms provide a unique platform for realizing free-space MOC, promising wide-bandwidth, scalable, and flexible quantum interfaces. Here, we develop a theoretical framework to evaluate the system conversion efficiency, accounting for the mismatch between microwave and optical wavelengths comparing with the atomic ensemble size. Our analysis reveals that the conversion efficiency is fundamentally limited by the focusing of the free-space microwave field, with an upper bound of about 3/16 for diffraction-limited focusing. We propose using a microwave near-field antenna to overcome this limit. Our work provides a foundation for assessing and optimizing free-space MOC, paving the way for a variety of applications based on free-space MOC.