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EPJ Quantum Technology

Suppressing virtual magnetic field noise and enhancing spin coherence in hybrid optical pumping atomic magnetometers

Faming Wang, Xiujie Fang, Yueyang Zhai, Zhuo Wang, Zihan Ling, Ping Xu, Kun CHEN, Danyue Ma

Published Jun 2, 2026• 0 citations
Publisher

Abstract

A hybrid optical pumping method employing a broad linewidth laser is proposed to enhance spin coherence and suppress virtual magnetic field noise in atomic magnetometers. The central frequency of broad linewidth laser is tuned to the resonance peak of the alkali-metal atoms D1 line. Its multiple frequency components suppress AC-Stark shift fluctuations caused by pumping laser frequency variations, thereby reducing virtual magnetic field noise. In addition, these components decrease the average optical absorption cross-section, improving the uniformity of atomic spin polarization, further enhancing spin coherence and extending transverse relaxation time. Experimental results show that, compared with narrow linewidth pumping, the proposed method suppresses virtual magnetic field noise by 17.95% to 34.50%, and increases the magnetometer response by 1.62% to 30.31% at 5 Hz. The average sensitivity within the frequency range of 1-20 Hz is improved to 0.158 fT/Hz 1/2 , representing a significant enhancement of 30.70% compared to narrow linewidth pumping.

DOI
10.1140/epjqt/s40507-026-00529-4

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A hybrid optical pumping method employing a broad linewidth laser is proposed to enhance spin coherence and suppress virtual magnetic field noise in atomic magnetometers. The central frequency of broad linewidth laser is tuned to the resonance peak of the alkali-metal atoms D1 line. Its multiple frequency components suppress AC-Stark shift fluctuations caused by pumping laser frequency variations, thereby reducing virtual magnetic field noise. In addition, these components decrease the average optical absorption cross-section, improving the uniformity of atomic spin polarization, further enhancing spin coherence and extending transverse relaxation time. Experimental results show that, compared with narrow linewidth pumping, the proposed method suppresses virtual magnetic field noise by 17.95% to 34.50%, and increases the magnetometer response by 1.62% to 30.31% at 5 Hz. The average sensitivity within the frequency range of 1-20 Hz is improved to 0.158 fT/Hz 1/2 , representing a significant enhancement of 30.70% compared to narrow linewidth pumping.
A high-sensitivity atomic magnetometer based on broad linewidth laser pumping is demonstrated. The broad linewidth laser was obtained by broadening a narrow linewidth laser utilizing an external phase modulator driven by a pseudo-random-bit-sequence (PRBS) waveform. This ingenious optical pumping scheme can significantly enhance the signal response and suppress light shift noise, as well as can reduce the gradient of electron spin polarization and improve the stability of magnetometers. Compared with a narrow linewidth laser pumping scheme, the response of the magnetometer is increased by 56%, experimentally. The relative polarization gradient is reduced from 9.8%/cm to better than 4%/cm by setting PRBS with a bit rate of 16 GHz. This scheme can also be applied to many other fields such as noninvasive bio-magnetic imaging and atomic spin gyroscopes.
An optical pumping scheme is proposed for reducing the gradient of electron spin polarization and suppressing light source noise in a spin-exchange relaxation-free magnetometer. This is achieved by modulating only the phase of a narrow-linewidth pump light field with external Gaussian noise. Compared to the absence of phase modulation, the uniformity of electron spin polarization was improved by over 40%, and the light-frequency noise suppression ratio of the magnetometer was enhanced by 4.3 times. Additionally, the response of the magnetometer was increased by 54%, resulting in a sensitivity of 0.34 fT/Hz 1/2 at 30 Hz. The applicability of this scheme can extend to other optical pumping experiments involving large atom ensembles requiring uniform electron spin polarization distribution, which is beneficial for developing ultra-high sensitivity and high stability magnetometers essential for magneto-cardiography and magneto-encephalography research applications.
In a spin-exchange-relaxation-free (SERF) atomic magnetometer with ultrahigh sensitivity, the ac-Stark shift generated by circularly polarized light results in the attenuation of the SERF atomic magnetometer response, and the virtual magnetic field gradient induced by the ac-Stark shift can reduce the sensitivity of the SERF atomic magnetometer. In this paper, we use a hybrid optical pumping method to address the ac-Stark shift problem. By switching between left-hand and right-hand circularly polarized lights and compensating the residual magnetic field of the magnetometer, the ac-Stark shift could be reduced and then suppressed effectively.
Alkali-metal-vapor magnetometers, using coherent precession of polarized atomic spins for magnetic field measurement, have become one of the most sensitive magnetic field detectors. Their application areas range from practical uses such as detections of NMR signals to fundamental physics research such as searches for permanent electric dipole moments. One of the main noise sources of atomic magnetometers comes from the light shift that depends on the frequency of the pump laser. In this work, we theoretically study the light shift, taking into account the relaxation due to the optical pumping and the collision between alkali atoms and between alkali atoms and the buffer gas. Starting from a full master equation containing both the ground and excited states, we adiabatically eliminate the excited states and obtain an effective master equation in the ground-state subspace that shows an intuitive picture and dramatically accelerates the numerical simulation. Solving this effective master equation, we find that in the light-narrowing regime, where the line width is reduced while the coherent precession signal is enhanced, the frequency-dependence of the light shift is largely reduced, which agrees with experimental observations in cesium magnetometers. Since this effective master equation is general and is easily solved, it can be applied to an extensive parameter regime, and also to study other physical problems in alkali-metal-vapor magnetometers, such as heading errors.
Quantum manipulation technology and photoelectric detection technology have jointly facilitated the rapid development of ultra-sensitive atomic spin magnetometers. To improve the output signal and sensitivity of the spin-exchange-relaxation-free (SERF) atomic spin magnetometer, the noises influencing on the output signal and the sensitivity were analyzed, and the corresponding noise suppression methods were presented. The magnetic field noises, including the residual magnetic field noise and the light shift noise, were reduced to approximately zero by employing the magnetic field compensation method and by adjusting the frequency of the pump beam, respectively. With respect to the operation temperature, the simulation results showed that the temperature of the potassium atomic spin magnetometer realizing the spin-exchange relaxation-free regime was 180 °C. Moreover, the fluctuation noises of the frequency and the power were suppressed by using the frequency and the power stable systems. The experimental power stability results showed that the light intensity stability was enhanced 10%. Contrast experiments on the sensitivity were carried out to demonstrate the validity of the suppression methods. Finally, a sensitivity of 13 fT/Hz(1/2) was successfully achieved by suppressing noises and optimizing parameters.
An optically pumped atomic magnetometer using a hybrid cell of potassium and rubidium atoms was demonstrated to yield high sensitivity to magnetic fields. We operated the magnetometer with the four possible combinations of optically pumped and optically probed atoms and found that the combination of optically pumped potassium and optically probed rubidium showed the highest sensitivity among the four combinations because the rubidium atoms were denser than those of potassium. Furthermore, we investigated the dependence of the sensitivity on the power densities of the pump and probe beams and the wavelength of the probe beam. The magnetometer using the hybrid cell required higher pump-beam power and had narrower magnetic linewidth than those of the single alkali-metal cell. However, the magnetic linewidth was larger than the theoretical value, ignoring the spin relaxation caused by the spin-exchange collisions. By adjusting the laser conditions, the highest sensitivity approached 30 fTrms/Hz1/2.