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EPJ Quantum Technology

Suppression of spin-exchange decoherence for zero-field parametric modulation magnetometers

Shushan Gao, Bangcheng Han, Z. X. Li, Ziao Liu, Zhongyu Wang, Jianwei Sheng, Jixi Lu

Published Mar 10, 2026• 2 citations
Publisher

Abstract

The zero-field atomic magnetometer is a type of high-sensitivity and miniaturized quantum sensing instrument, typically using continuous radio-frequency fields for parametric modulation to extract weak magnetic signals. However, this modulation induces additional spin-exchange relaxation that degrades spin coherence. Here we report a weak bias magnetic field-assisted pulsed field modulation scheme that significantly suppresses spin-exchange decoherence. The pulsed magnetic field reduces asynchronous phase accumulation by shortening the interaction time between the modulation field and the atomic spin ensemble, while the perpendicular bias field induces spin Larmor precession to periodically average modulation-induced relaxation, thereby suppressing the spin-exchange relaxation rate by a factor approximately equal to the pulse duty cycle. We develop a perturbative spin dynamics model to analyze the spin polarization response under this modulation. The experimental results demonstrate that this scheme overcomes the limitation imposed by the duty cycle threshold, suppresses the spin-exchange relaxation rate by more than 50%, and significantly enhances the magnetic response and sensitivity compared to conventional continuous modulation. This method can be extended to various alkali-metal spin systems relying on radio-frequency field operation, providing a new scheme for reducing modulation-induced spin-exchange decoherence.

DOI
10.1140/epjqt/s40507-026-00488-w

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The zero-field atomic magnetometer is a type of high-sensitivity and miniaturized quantum sensing instrument, typically using continuous radio-frequency fields for parametric modulation to extract weak magnetic signals. However, this modulation induces additional spin-exchange relaxation that degrades spin coherence. Here we report a weak bias magnetic field-assisted pulsed field modulation scheme that significantly suppresses spin-exchange decoherence. The pulsed magnetic field reduces asynchronous phase accumulation by shortening the interaction time between the modulation field and the atomic spin ensemble, while the perpendicular bias field induces spin Larmor precession to periodically average modulation-induced relaxation, thereby suppressing the spin-exchange relaxation rate by a factor approximately equal to the pulse duty cycle. We develop a perturbative spin dynamics model to analyze the spin polarization response under this modulation. The experimental results demonstrate that this scheme overcomes the limitation imposed by the duty cycle threshold, suppresses the spin-exchange relaxation rate by more than 50%, and significantly enhances the magnetic response and sensitivity compared to conventional continuous modulation. This method can be extended to various alkali-metal spin systems relying on radio-frequency field operation, providing a new scheme for reducing modulation-induced spin-exchange decoherence.
Abstract Atomic magnetometers serve as a type of quantum sensing instrument, offering extensive applications in the field of extremely weak magnetic field measurement. Miniaturized zero‐field atomic magnetometers typically use continuous high‐frequency magnetic fields to modulate atomic spin precession for multi‐axis measurements. However, this modulation induces additional spin‐exchange relaxation, which significantly broadens the resonance linewidth. In this study, the pulsed magnetic field modulation is introduced instead of continuous modulation to reduce the effective duration of the magnetic field. This operation significantly diminishes perturbations of steady‐state spin distribution in Zeeman sublevels caused by Larmor precession during each modulation cycle, thus effectively mitigating decoherence effects. The relationship between spin‐exchange relaxation rate and pulsed field parameters is studied based on the generalized hyperfine Bloch equation, and a spin dynamics model under the pulsed modulation is developed. Agreement between experimental and theoretical results validates the suppression effect of spin‐exchange relaxation and the accuracy of this model. Experimental results indicate that the pulsed modulation scheme results in a 30% reduction in the spin‐exchange relaxation rate and achieves dual‐axis sensitivities of 1.5 and 2.5 fT Hz −1/2 , respectively, representing improvements of 25% and 20% over conventional continuous modulation methods.
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.
We demonstrate that spin-exchange dephasing of Larmor precession at near-Earth-scale fields is effectively eliminated by dressing the alkali-metal atom spins in a sequence of ac-coupled 2π pulses, repeated at the Larmor precession frequency. The contribution of spin-exchange collisions to the spectroscopic linewidth is reduced by a factor of the duty cycle of the pulses. We experimentally demonstrate resonant transverse pumping in magnetic fields as high as 0.1 G, present experimental measurements of the suppressed spin-exchange relaxation, and show enhanced magnetometer response relative to a light-narrowed scalar magnetometer.
Emerging quantum sensing devices namely atomic magnetometers have become promising candidate for future biomagnetism imaging. However, the limitation of detection bandwidth exists a major drawback for devices of this kind. Conventional approaches, including transverse magnetic field modulation and close-loop control, have seen challenging tradeoffs between size, resolution and bandwidths. In this study, a wide bandwidth zero-field magnetometer with a miniaturized light source has been developed for alternating current (AC) magnetic field detection. A dual-beam Spin-Exchange Relaxation-Free (SERF) magnetometer employing a 4×4×4mm3 alkali metal vapor cell achieves a bandwidth of 188 Hz. This represents a threefold increase in bandwidth compared to non-modulated mode. The sensitivity under modulated mode demonstrated a sensitivity of 55fT/Hz1/2within the frequency range of 10-188Hz. Our results also demonstrated that the sensitivity is constrained by relaxation induced by the modulation of pumping source, so possible tradeoffs can be made based on various applications. The proposed method provides novel approach for bandwidth extension of atomic magnetometers which further pave the way for future applications including magnetoencephalography (MEG), particularly in reducing magnetic noise crosstalk and enhancing spatial resolution.
Abstract High-sensitivity triaxial magnetic field measurements are crucial in various scientific and industrial applications, with spin-exchange relaxation-free (SERF) atomic magnetometers (AMs) standing out for their exceptional sensitivity. However, conventional methods for achieving triaxial measurements in SERF magnetometers typically involve multiple modulated magnetic fields, which introduce crosstalk challenges in array-based configurations. We propose a static bias magnetic field assisted optical modulation triaxial AM to address this limitation. In this configuration, the detuned circularly polarized light is utilized to achieve synchronous modulation of the fictitious magnetic field and pumping rate while incorporating a static bias magnetic field to generate a transverse projection component. We develop a comprehensive triaxial response model that fully considers the effects of the longitudinal spin polarization deflection. Building upon this model, we derive the relationship between the transverse spin component and the triaxial magnetic field and introduce parameter optimization methods to obtain optimal parameters, thus enabling triaxial measurements of 12 fT Hz −1/2 , 10 fT Hz −1/2 , and 15 fT Hz −1/2 along the x-, y -, and z -axes. Our approach offers significant advantages over existing methods, particularly in minimizing crosstalk and mitigating spin-exchange relaxation introduced by multiple modulated magnetic fields. Moreover, the effective detection of simulated magnetocardiography signals highlights the practical applicability and reliability of the proposed method in real-world scenarios.
We demonstrate an optically pumped (87)Rb magnetometer in a microfabricated vapor cell based on a zero-field dispersive resonance generated by optical modulation of the (87)Rb ground state energy levels. The magnetometer is operated in the spin-exchange relaxation-free regime where high magnetic field sensitivities can be achieved. This device can be useful in applications requiring array-based magnetometers where radio frequency magnetic fields can induce cross-talk among adjacent sensors or affect the source of the magnetic field being measured.