We describe a direct gradiometer using optical pumping with opposite circular polarization in two ${}^{87}\mathrm{Rb}$ atomic ensembles within a single multipass cell. A far-detuned probe laser undergoes a near-zero paramagnetic Faraday rotation due to the intrinsic subtraction of two contributions exceeding 3.5 rad from the highly polarized ensembles. We develop analysis methods for the direct gradiometer signal and measure a gradiometer sensitivity of 10.1 $\mathrm{fT}/\mathrm{cm}\sqrt{\mathrm{Hz}}$ . We also demonstrate that our multipass design, in addition to increasing the optical depth, provides a fundamental advantage due to the significantly reduced effect of atomic diffusion on the spin-noise time-correlation, in excellent agreement with the theoretical estimate.We describe a direct gradiometer using optical pumping with opposite circular polarization in two ${}^{87}\mathrm{Rb}$ atomic ensembles within a single multipass cell. A far-detuned probe laser undergoes a near-zero paramagnetic Faraday rotation due to the intrinsic subtraction of two contributions exceeding 3.5 rad from the highly polarized ensembles. We develop analysis methods for the direct gradiometer signal and measure a gradiometer sensitivity of 10.1 $\mathrm{fT}/\mathrm{cm}\sqrt{\mathrm{Hz}}$. We also demonstrate that our multipass design, in addition to increasing the optical depth, provides a fundamental advantage due to the significantly reduced effect of atomic diffusion on the spin-noise time-correlation, in excellent agreement with the theoretical estimate.

arXiv:2009.13212Physical Review AppliedJan 6, 202172 citations

Quantum sensors based on atomic coherence

Irina V. Novikova

Optical and Quantum Sensing and Precision Metrology IIMar 5, 20260 citations

Subfemtotesla Scalar Atomic Magnetometry Using Multipass Cells

D. Sheng, Lun Li, Nezih Dural +1 more

Scalar atomic magnetometers have many attractive features but their sensitivity has been relatively poor. We describe a Rb scalar gradiometer using two multipass optical cells. We use a pump-probe measurement scheme to suppress spin-exchange relaxation and two probe pulses to find the spin precession zero crossing times with a resolution of 1 psec. We realize a magnetic field sensitivity of 0.54 fT/Hz(1/2), which improves by an order of magnitude the best scalar magnetometer sensitivity and exceeds, for example, the quantum limit set by the spin-exchange collisions for a scalar magnetometer with the same measurement volume operating in a continuous regime.Scalar atomic magnetometers have many attractive features but their sensitivity has been relatively poor. We describe a Rb scalar gradiometer using two multipass optical cells. We use a pump-probe measurement scheme to suppress spin-exchange relaxation and two probe pulses to find the spin precession zero crossing times with a resolution of 1 psec. We realize a magnetic field sensitivity of 0.54 fT/Hz(1/2), which improves by an order of magnitude the best scalar magnetometer sensitivity and exceeds, for example, the quantum limit set by the spin-exchange collisions for a scalar magnetometer with the same measurement volume operating in a continuous regime.

arXiv:1208.1099Physical Review LettersApr 18, 2013365 citations

Spin correlations in recirculating multipass alkali cells for advancing quantum magnetometry

Qian Ling Kee, Lingyi Zhao, Ruvi Lecamwasam +6 more

• 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.• 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.

arXiv:2506.14160Optics and Lasers in EngineeringMay 5, 20260 citations

We describe a direct gradiometer using optical pumping with opposite circular polarization in two

{}^{87}\mathrm{Rb}

atomic ensembles within a single multipass cell. A far-detuned probe laser undergoes a near-zero paramagnetic Faraday rotation due to the intrinsic subtraction of two contributions exceeding 3.5 rad from the highly polarized ensembles. We develop analysis methods for the direct gradiometer signal and measure a gradiometer sensitivity of 10.1

\mathrm{fT}/\mathrm{cm}\sqrt{\mathrm{Hz}}

. We also demonstrate that our multipass design, in addition to increasing the optical depth, provides a fundamental advantage due to the significantly reduced effect of atomic diffusion on the spin-noise time-correlation, in excellent agreement with the theoretical estimate.