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Frontiers in Physics

The suppression of the probe laser pumping effect in SERF atomic magnetometer

Qian Cao, Yueyang Zhai, Binyue Huang, Renjie Li, Jiajie Li

Published Jun 1, 2023• 1 citation
Publisher

Abstract

Introduction: A spin exchange relaxation free atomic magnetometer, as an ultra-highly sensitive magnetic field sensor, is limited by the performance of the probe laser system. The probe laser pumping effect (PLPE) hinders the increase in the performance of probe laser system. Methods: This study investigated the PLPE and proposed a method for suppressing the same. Through changes to the angle of a quarter wave plate and the addition of a triangular modulated magnetic field to the alkali atoms, the suppression point was determined. Results and discussion: Further, related parameters were measured for different degree of polarizations of the probe laser, which confirmed that the influence of PLPE on the magnetic field was the least at the suppressed point.

DOI
10.3389/fphy.2023.1181399

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Introduction: A spin exchange relaxation free atomic magnetometer, as an ultra-highly sensitive magnetic field sensor, is limited by the performance of the probe laser system. The probe laser pumping effect (PLPE) hinders the increase in the performance of probe laser system. Methods: This study investigated the PLPE and proposed a method for suppressing the same. Through changes to the angle of a quarter wave plate and the addition of a triangular modulated magnetic field to the alkali atoms, the suppression point was determined. Results and discussion: Further, related parameters were measured for different degree of polarizations of the probe laser, which confirmed that the influence of PLPE on the magnetic field was the least at the suppressed point.
The magnetic field gradient within optical pumping magnetometers (OPMs) suppresses sensitivity improvement. We investigated the effects of the magnetic field gradient along the x-, y-, and z-axes on the limiting factors of magnetometers under extremely low magnetic field conditions. We modified the magnetic field gradient relaxation model such that it can be applied to atoms in the spin exchange relaxation free (SERF) regime. The gradient relaxation time and spin polarizations, combined with fast spin-exchange interaction, were determined simultaneously using the oscillating cosine magnetic field excitation and amplitude spectrum analysis method. During the experiments, we eliminated the errors caused by the temperature and pumping power, and considered different isotope spin exchange collisions in naturally abundant Rb during the data analysis to improve the fitting accuracy. The experimental results agreed well with those of theoretical calculations and confirmed the accuracy of the improved model. The contribution of the transverse magnetic field gradient to the relaxation of the magnetic field gradient cannot be ignored in the case of small static magnetic fields. Our study provides a theoretical and experimental basis for eliminating magnetic gradient relaxation in atomic sensors in the SERF region.
In the optical pumping systems based on the pump-probe arrangement, the spin polarization of the atoms is generally monitored utilizing the probe laser beam, in which way an extra perturbation must be introduced and thus affects the normal operation of the sensors. By investigating the absorption rate of the circularly polarized pump laser, here we demonstrate the feasibility of extracting the electron-spin polarization from the transmitted pump laser intensity. We experimentally validate the method in a spin-exchange relaxation free (SERF) magnetometer and the results are in excellent agreement with the theory. The scheme operates in a silent mode and features a real-time observation. We also study the corresponding magnetic field response of the SERF magnetometer and a term arising from the diffusion effects has been added to the original model to explain the discrepancy of the response.
High precision measurement of magnetic fields has been of great importance for both fundamental physics and practical applications. Magnetometers based on atomic spin effect provide a new technique for high precision measurement and have been enabled as the most sensitive magnetic sensors at present. However, the atomic spins used as probes to sense the magnetic fields are prone to the decoherence effects. For the atomic spin-exchange relaxation free (SERF) magnetometer which operates with high alkali atoms density in very low magnetic fields, the main relaxation mechanism is the spin-destruction collisions between atoms. In this paper, it is demonstrated by numerical simulations that the precision of the atomic SERF magnetometer is significantly affected by this spin-destruction collisions.
Weak measurement techniques have been extensively applied in the field of quantum precision measurement to detect ultra-small signals due to the amplification effect. In this work, we propose an optical detection system for a spin-exchange relaxation-free (SERF) magnetometer based on the inverse weak measurement (IWM) framework. By using the spatial pattern of a probe laser as the measurement pointer, we successfully detect ultra-weak magnetic fields. In our model, the spatial pattern of the probe laser is weakly coupled to its polarization, which is sensitive to external magnetic fields. Through post-selection on the optical polarization, the ultra-small magnetic field is significantly amplified with the amplification factor inversely proportional to the coupling strength, as reflected in the measured displacement of the final spatial pattern. By analysing the response curve of the probe laser displacement to the magnetic field, we identify the point of maximum sensitivity, achieving a magnetic field sensitivity of 182.8 fT/Hz1/2. Furthermore, in the IWM scheme, the detected signals depend only on the internal degrees of freedom of the probe laser, making the system robust against the fluctuations in laser power. To demonstrate this advantage, we compute the Allan standard deviation of the output signals for both conventional and IWM detection methods. The results indicate that the IWM-based method improves stability of detection by one to two orders of magnitude. This work presents a novel detection approach that integrates weak measurement techniques, offering a significant enhancement in the performance of SERF magnetometers.
Weak measurement techniques have been extensively applied in the field of quantum precision measurement to detect ultra-small signals due to the amplification effect. In this work, we propose an optical detection system for a spin-exchange relaxation-free (SERF) magnetometer based on the inverse weak measurement (IWM) framework. By using the spatial pattern of a probe laser as the measurement pointer, we successfully detect ultra-weak magnetic fields. In our model, the spatial pattern of the probe laser is weakly coupled to its polarization, which is sensitive to external magnetic fields. Through post-selection on the optical polarization, the ultra-small magnetic field is significantly amplified with the amplification factor inversely proportional to the coupling strength, as reflected in the measured displacement of the final spatial pattern. By analyzing the response curve of the probe laser displacement to the magnetic field, we identify the point of maximum sensitivity, achieving a magnetic field sensitivity of 182.8 fT / Hz 1/2 . Furthermore, in the IWM scheme, the detected signals depend only on the internal degrees of freedom of the probe laser, making the system robust against the fluctuations in laser power. To demonstrate this advantage, we compute the Allan standard deviation of the output signals for both conventional and IWM detection methods. The results indicate that the IWM-based method improves stability of detection by one to two orders of magnitude. This work presents, to our knowledge, a novel detection approach that integrates weak measurement techniques, offering a significant enhancement in the performance of SERF magnetometers.
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.