Spin Correlations in Recirculating Multipass Alkali Cells for Advancing Quantum Magnetometry

TL;DR

This paper introduces a recirculating multipass alkali cell design that improves optical coverage and spin correlation in quantum magnetometry, validated by analytical models and simulations.

Contribution

It presents a novel recirculating multipass cell architecture and analytical framework for spin noise, enhancing quantum sensing capabilities.

Findings

Increased active volume ratio and minimized beam overlap improve sensitivity.

Analytical and simulation models accurately predict beam trajectories and astigmatism.

Design choices like long focal length mirrors reduce spin diffusion noise.

Abstract

Multipass cells enable long optical path lengths in compact volumes and are central to quantum technologies such as atomic magnetometers and optical quantum memories. In optical magnetometry, multipass geometries enhance sensitivity by increasing optical depth, reducing photon shot noise, and enabling quantum non-demolition detection. However, in conventional cylindrical multipass cells, Lissajous beam trajectories lead to repeated revisiting and incomplete mirror coverage, limiting effective volume utilization. Here we present a recirculating multipass alkali cell that overcomes these limitations by increasing the active-to-cell volume ratio and minimizing beam spot overlap. We develop an analytical ABCD-matrix model to predict beam trajectories, spot sizes, and astigmatism, validated by Zemax simulations. We further introduce a general analytical framework for spin correlation noise that incorporates astigmatism and spatial intensity distributions. By deriving the spin-noise time-correlation function and spectrum, we show how beam intensity profiles influence spin diffusion noise. Our results demonstrate improved beam coverage, reduced spot overlap, and enhanced spin correlation, particularly for concave mirrors with long focal lengths, while showing that avoiding tightly-focused regions significantly suppresses spin diffusion noise. These findings establish recirculating multipass cells as a practical, high-performance platform for precision atomic sensing and other multipass-cavity-based quantum devices.