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arXiv:2506.14160 · Optics and Lasers in Engineering

Spin correlations in recirculating multipass alkali cells for advancing quantum magnetometry

Qian Ling Kee, Lingyi Zhao, Ruvi Lecamwasam, Biveen Shajilal, Xinan Liang, Joel K. Jose, Yao Chen, Ping Koy Lam, Tao Wang

Published May 5, 2026• 0 citations
arXivHTMLPDF

Abstract

• Recirculating multipass cell enables improved beam coverage and active volume. • Analytical model predicts beam distribution and astigmatism effects. • General framework developed for spin noise in multipass alkali cells. • Beam geometry shown to influence spin diffusion noise. • Recirculating design demonstrates reduced spin noise under optimized conditions. 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.

DOI
10.1016/j.optlaseng.2026.109828

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• Recirculating multipass cell enables improved beam coverage and active volume. • Analytical model predicts beam distribution and astigmatism effects. • General framework developed for spin noise in multipass alkali cells. • Beam geometry shown to influence spin diffusion noise. • Recirculating design demonstrates reduced spin noise under optimized conditions. 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.
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Miniaturized high-sensitivity atomic magnetometers play a crucial role in biomagnetic measurements. The utilization of multipass cells enhances the sensitivity of atomic magnetometers. This study explores the benefits of employing a multipass cell in spin-exchange relaxation-free (SERF) atomic magnetometers through theoretical and experimental analysis. A signal-to-noise ratio (SNR) model was established by accounting for the optimal detuning frequency of the probe light and low-frequency 1/f noise. This enabled the optimal number of probe light passes through the vapor cell to be determined. Additionally, compact and symmetric reflective optical paths were designed to mitigate polarization gradients in atomic magnetometers with high atomic number density. Experimental results confirmed that the magnetic field sensitivity of the triple-pass configuration (1.8 fT/Hz1/2^{1/2}1/2) was 1.89 times that of the single-pass configuration (3.4 fT/Hz1/2^{1/2}1/2), consistent with the theoretical analysis. This study provides valuable theoretical guidance for analyzing the SNR performance of SERF atomic magnetometers and demonstrates the potential of multipass cells in enhancing their magnetic field sensitivity. The utilization of multipass cells presents a viable approach toward the realization of highly sensitive and portable miniaturized SERF atomic magnetometers.
Spin squeezing can suppress quantum projection noise via interparticle entanglement, therefore enabling measurement sensitivities beyond the standard quantum limit. In practice, however, the Gaussian and finite intensity profiles of the optical probe beam induce spatially inhomogeneous atom-light interactions. As polarized atoms move within a vapor cell, they experience position-dependent optical intensities, generating transit noise that limits spin squeezing performance. Here, we investigate the transit noise in a coated rubidium vapor cell through combined theoretical analysis and experimental measurements. By varying the probe beam diameter, we quantify the dependence of transit noise on beam size and atomic Larmor frequency. Our results show that, for a vapor cell with fixed dimensions, the transit noise increases as the probe beam spot area decreases. Moreover, when the Larmor frequency is below the characteristic linewidth of the transit noise, the noise contribution becomes larger. We further calculated and measured spin squeezing for different beam sizes and found an experimental difference of 2.7±0.22.7 \pm 0.22.7±0.2 dB between 2~mm and 0.6~mm, similar to the theoretical prediction of 3.0±0.33.0 \pm 0.33.0±0.3 dB. Theoretical analysis under conditions of stronger squeezing shows that transit noise becomes an even more critical limiting factor. These results provide practical guidance for optimizing probe beam parameters and suppressing transit noise in spin squeezing experiments.
This paper focuses on standardized fabrications of atomic vapor cells with multipass cells. For this purpose, we build a vacuum system that enables the sealing of the multipass-cavity-assisted cell under vacuum. Alkali atoms are prepared inside a glass holder, and the tip of the holder is broken by controlled collisions under vacuum. Atoms are then transferred to a cell glass body part by heating. Once enough atoms accumulate inside the glass part, buffer and quenching gases are filled into the system, and the glass body part is moved to contact the silicon wafer which is bonded with a Herriott-cavity. Then the cavity part and the glass part are sealed together using the anodic bonding technique. The resulting vapor cells provide enhanced measurement sensitivity and improved device standardization, which allows for seamless replacements of each other in practical applications. The performances of these cells are tested, including a test in a double-resonance alkali-metal atomic magnetometer. A magnetic field sensitivity of 95 fT/Hz1/2 is achieved in a frequency range from 10 to 20 Hz with a multipass cell filled with 400 Torr (1 Torr = 1.33×102 Pa) N2 and natural Rb atoms at 100 ℃. The technology and cells developed in this work are expected to have wide applications in atomic devices, especially in He magnetometers and nuclear-spin atomic co-magnetometers, which have special requirements for cell qualities.
We introduce a novel, to the best of our knowledge, compact multipass cell design for spin-exchange relaxation-free (SERF) atomic magnetometers, replacing conventional reflective cavities with a dual-parallel plate configuration. This adjustable planar reflector setup achieves precise control of reflection optical paths and dramatically enhances signal response, resolving the complexity of cavity design and volume increase caused by multiple reflectors in existing schemes, while having high-temperature adaptability and minimizing size and complexity. Additionally, combined with a flat-top light pump to boost atomic polarization uniformity, our system achieves 1 fT/Hz½ sensitivity, delivering 260% improvement in signal and 200% in sensitivity over conventional designs. This streamlined, high-temperature-compatible architecture marks a significant step toward scalable, next-generation quantum sensors that unite exceptional sensitivity with practical miniaturization.
Abstract : We investigated the feasibility of a femto-Tesla level scalar atomic magnetometer operating in Earths field. Our approach is unique in using multi-pass alkali-metal cells, quantum-non-demolition measurements, and suppression of spin-exchange relaxation to achieve the highest sensitivity per unit volume. We developed a new multi-pass cell geometry with a small internal volume and experimentally verified the predictions of our model for quantum spin noise. We also investigated the availability of pulsed pump and probe lasers to operate a high-sensitivity atomic magnetometer.