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arXiv:2609.25714

Enhanced Atomic Magnetometry with a Pinched Spin State

Tingxuan Guo, Xiangyu Li, Yang Li, Kaifeng Zhao

quant-phSubmitted Sep 22, 2026• Announced Sep 23, 2026• 0 citations
arXivHTMLPDF

Abstract

The sensitivity of an atomic spin sensor is fundamentally constrained by the quantum Fisher information (QFI) of the probe state. The pinched state, $|{F,0}\rangle$, has an $(F+1)$-fold larger single-particle QFI for transverse-field sensing than the stretched state, $|{F,F}\rangle$, corresponding to a predicted $\sqrt{F+1}$ reduction in the spin-projection-noise-limited field uncertainty. We show that this advantage can be accessed in an rf sensing scheme by engineering a spectrum symmetric about $m=0$, yielding an ideal $(F+1)$-fold enhancement of magnetic-field response. For $^{87}\mathrm{Rb}$ with $F=2$, we observe up to 2.7-fold response enhancement. Implementing this approach in a compact $0.8~\mathrm{cm}^3$ anti-relaxation-coated multipass cell, we realize a single-beam, dual-axis, zero-field magnetometer, achieving $13$--$23$~$\mathrm{fT}/\sqrt{\mathrm{Hz}}$ over $3$--$100$~Hz at room temperature.

Comments
5 pages, 5 figures

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The sensitivity of an atomic spin sensor is fundamentally constrained by the quantum Fisher information (QFI) of the probe state. The pinched state, ∣F,0⟩|{F,0}\rangle∣F,0⟩, has an (F+1)(F+1)(F+1)-fold larger single-particle QFI for transverse-field sensing than the stretched state, ∣F,F⟩|{F,F}\rangle∣F,F⟩, corresponding to a predicted F+1\sqrt{F+1}F+1​ reduction in the spin-projection-noise-limited field uncertainty. We show that this advantage can be accessed in an rf sensing scheme by engineering a spectrum symmetric about m=0m=0m=0, yielding an ideal (F+1)(F+1)(F+1)-fold enhancement of magnetic-field response. For 87Rb^{87}\mathrm{Rb}87Rb with F=2F=2F=2, we observe up to 2.7-fold response enhancement. Implementing this approach in a compact 0.8 cm30.8~\mathrm{cm}^30.8 cm3 anti-relaxation-coated multipass cell, we realize a single-beam, dual-axis, zero-field magnetometer, achieving 131313--232323~fT/Hz\mathrm{fT}/\sqrt{\mathrm{Hz}}fT/Hz​ over 333--100100100~Hz at room temperature.
Atomic magnetometers operated in spin-exchange relaxation-free (SERF) have the potential to replace superconducting quantum interference devices (SQUIDs) in the field of biomagnetism. Here, we demonstrate the dual-axis closed-loop control of a single-beam SERF magnetometer that allows sensing of two orthogonal magnetic fields simultaneously and independently with suppressed cross-talk effect, achieved by applying a rotating modulation field and using of the negative feedback. Operating in the dual-axis closed-loop mode with a miniature 4 × 4 × 4 mm 87Rb cell, we achieve magnetic field sensitivity of 20 fT/Hz1/2 and a bandwidth of 1.8 kHz. In addition to the experimental study of the sensitivity and bandwidth, we perform a theoretical analysis to the dual-axis response and develop an effective decoupling method. The implementation of closed-loop control enables the magnetometer to be robust against the disturbance of the ambient field, which could enlarge the dynamic range, enhance the systematic stability, and suppress the cross-talk effect. Such a simple, yet high-performance SERF magnetometer could be suitable as a promising candidate in the field of biomagnetism, allowing for a denser sensor array and opening up new possibilities for the noninvasive biomagnetic imaging.
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.
Zero-field atomic ensembles enable the development of ultrahigh-sensitivity magnetometry. However, their intrinsic insensitivity to longitudinal magnetic fields along the pumping direction poses a fundamental limitation, restricting precise weak-field quantification and complex system decoupling. Herein, we propose a novel zero-field weak parametric modulation scheme capable of ultrahigh-sensitivity extraction of longitudinal magnetic fields. Transverse modulation is employed to manifest the electron spin polarization projection induced by the longitudinal fields. Based on the perturbation model established, first-order parametric resonance associated with the longitudinal magnetic field yields significant signal enhancement under low-frequency and weak modulation conditions, while simultaneously suppressing spin-exchange relaxation caused by magnetic field modulation. We experimentally achieved a longitudinal measurement sensitivity of 16 fT / Hz 1 / 2 , more than an order of magnitude improved over traditional schemes, with a bandwidth of 70 Hz. Benefiting from low spin relaxation, the noise floor of the transverse magnetic field measurement is preserved at 4 fT / Hz 1 / 2 . The proposed scheme pushes the boundaries of existing weak magnetic field measurement, providing critical capabilities for state-of-the-art precision experiments including new physical quantity detection.
This thesis presents advancements in radio-frequency atomic magnetometry utilising cold atoms as sensors. While cold atom magnetometers demonstrate lower absolute sensitivity compared to conventional vapour cell systems due to reduced atom numbers, they offer exceptional spatial resolution at the micrometre scale. The experimental apparatus features a cold atom production system capable of reaching quantum degeneracy through a refined cooling sequence that involves optical molasses, magnetic trapping and evaporation, and optical evaporative cooling in the dipole trap, culminating in a 87Rb Bose-Einstein condensate. This work details the development and characterisation of magnetometer set-ups with progressively enhanced performance. The first magnetometer utilised atoms in the optical molasses phase as a sensor, achieving an AC sensitivity of 330 pT/√Hz with approximately 108 atoms at 20 μK. Comprehensive magnetometry optimisation methods were developed to enhance sensitivity. Following laboratory interruptions due to the COVID-19 pandemic, system rebuilding incorporated significant upgrades, including a new probe laser and gradient compensation systems. The improved magnetometer prototype achieved a substantially enhanced sensitivity of 0.72±0.33 pT/√Hz, approaching the fundamental sensitivity limit of the system. Using atoms in the magnetic trap as a sensor, a sensitivity of 1.85 pT/√Hz was demonstrated. The system was verified to be photon-shot noise limited. Investigations into a double-pass probe configuration showed that, while the measured sensitivity was similar to that of the single-pass (4.3 pT/√Hz compared to 3.1 pT/√Hz at a probe power of 6.4 μW measured at the detector), the rotation angle increased by a factor of 2.2, with significantly reduced noise levels. These findings suggest that multi-pass strategies hold promise for enhanced performance in future implementations. In addition, the first signal detection from a BEC magnetometer, achieving 4.5 nT/√Hz with only 95,000 atoms at 35 nK was demonstrated.
A scheme of an ultra-sensitive magnetometer in the cavity quantum electromagnonics where the intracavity microwave mode coupled to a magnonic mode via magnetic dipole interaction is proposed. It is shown that by driving both magnonic and microwave modes with external classical fields and controlling the system parameters, one can reduce the added noise of magnetic field measurement below the standard quantum limit (SQL). Surprisingly, we show that beyond the rotating wave approximation (RWA), not only the added noise can be suppressed, but also the output cavity response to the input signal can be substantially amplified in order to achieve a precise magnetic-field measurement. The sensitivity of the proposed magnetic amplifier-sensor is approximately in the order of 10−18T/Hz10^{-18}T/\sqrt{Hz}10−18T/Hz​ which is competitive compared to the current state-of-the-art magnetometers like superconducting quantum interference devices (SQUIDs) and atomic magnetometers. The advantage of the proposed sensor in comparison with the other magnetometers is its high sensitivity at room temperature and sensing in a wide range of frequency up to MHz as well as its capability to signal-response amplification.
Three-axis atomic magnetometers have great advantages for interpreting information conveyed by magnetic fields. Here, we demonstrate a compact construction of a three-axis vector atomic magnetometer. The magnetometer is operated with a single laser beam and with a specially designed triangular 87Rb vapor cell (side length is 5 mm). The ability of three-axis measurement is realized by reflecting the light beam in the cell chamber under high pressure, so that the atoms before and after reflection are polarized along two different directions. It achieves a sensitivity of 40 fT/Hz in x-axis, 20 fT/Hz in y-axis, and 30 fT/Hz in z-axis under spin-exchange relaxation-free regime. The crosstalk effect between different axes is proven to be little in this configuration. The sensor configuration here is expected to form further values, especially for vector biomagnetism measurement, clinical diagnosis, and field source reconstruction.