• 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.
We demonstrate a paraffin-coated planar reflective multipass vapor cell for compact optical atomic magnetometry. The cell has an internal volume of 12×12×8mm3 and supports 20 optical passes with a total transmittance exceeding 65%, while maintaining a longitudinal spin-relaxation time of 87Rb longer than 1s. The planar geometry provides spatially separated input and output beams, enabling compact optical integration. A single-cell free-induction-decay (FID) magnetometer reaches 10pT/Hz in the geomagnetic-field range, presently limited by current-source noise in the field coils. A two-cell differential configuration achieves a sensitivity of ∼28fT/Hz over the 1--15Hz band for bias fields of 0.3--0.7G. These results establish that paraffin-coated planar multipass cells offer high optical depth, long coherence times, and an integration-friendly platform for ultrasensitive magnetometry.
In this Letter, we propose a single-beam nonlinear magneto-optical rotation (NMOR) magnetometer with a multipass 4 He gas-discharged cell. In contrast to the single-pass cell, the multipass cell allowed laser beams to pass through the metastable-state atomic ensemble 22 times, which directly increases the optical path length and significantly enhances magneto-optical rotation in the 4 He gas sample. Based on nonlinear Faraday rotation, the 4 He magnetometer with the multipass cell demonstrates a noise floor of 9 fT/Hz 1/2 , which approaches the photon-shot noise floor limit of 6.4 fT/Hz 1/2 . In addition, the wider linewidth in metastable-state atoms realizes an NMOR 4 He magnetometer with a 3 dB bandwidth of 4.3 kHz, in contrast to the ultranarrow linewidth in the antirelaxation-coated cells or spin-exchange relaxation-free regime alkali-metal cells with buffer gas. Since the 4 He cell functions without heating or cryogenic cooling, the femtotesla sensitivity and kilohertz-bandwidth 4 He magnetometer exhibits potential in biomagnetic applications such as magnetocardiography and magnetoencephalography.