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IEEE Transactions on Instrumentation and Measurement

Analysis and Suppression for Critical Optical Nonorthogonal Noise in SERF Comagnetometers

Zitong Xu, Kai Wei, Chang Liu, Xing Heng, Xiaofei Huang, Dinghui Gong, Fan Wang, Yueyang Zhai, Wei Quan

Published Jan 1, 2024• 16 citations
Publisher

Abstract

Ultrasensitive atomic comagnetometers have proven to be valuable tools for precise measurement and search for new physics. Besides, comagnetometers have potential for inertial navigation and quantum information. Despite their great potential, sensitivity improvement has been limited by different types of noise. To address this issue, the sources and magnitudes of noises are comprehensively analyzed and a suppression method is proposed for the most critical noise source, optical non-orthogonality. The optical non-orthogonality leads to critical transverse pumping effect, thus transverse light shift, and transverse effective magnetic noise. A direct correlation between the optical non-orthogonality and the electronic spin transverse relaxation rate is found, and a method for suppression of the optical non-orthogonality is proposed accordingly. Finally, the optical non-orthogonal noise is suppressed by 3.1 dB. Besides, an increase of 5.2 dB in the scale factor is achieved. With the combined effect of noise suppression and scale factor enhancement, the resulting inertial rotation sensitivity of 2.4×10-8rad/s/Hz1/2in the frequency range from 1 to 3 Hz is among the most sensitive comagnetometers, which opens up promising opportunities for new physics measurements, such as the search for ultralight dark matter and the fifth force.

DOI
10.1109/tim.2024.3385818

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Ultrasensitive atomic comagnetometers have proven to be valuable tools for precise measurement and search for new physics. Besides, comagnetometers have potential for inertial navigation and quantum information. Despite their great potential, sensitivity improvement has been limited by different types of noise. To address this issue, the sources and magnitudes of noises are comprehensively analyzed and a suppression method is proposed for the most critical noise source, optical non-orthogonality. The optical non-orthogonality leads to critical transverse pumping effect, thus transverse light shift, and transverse effective magnetic noise. A direct correlation between the optical non-orthogonality and the electronic spin transverse relaxation rate is found, and a method for suppression of the optical non-orthogonality is proposed accordingly. Finally, the optical non-orthogonal noise is suppressed by 3.1 dB. Besides, an increase of 5.2 dB in the scale factor is achieved. With the combined effect of noise suppression and scale factor enhancement, the resulting inertial rotation sensitivity of 2.4×10-8rad/s/Hz1/2in the frequency range from 1 to 3 Hz is among the most sensitive comagnetometers, which opens up promising opportunities for new physics measurements, such as the search for ultralight dark matter and the fifth force.
As a novel quantum sensor utilizing atomic spin coupling mechanisms, spin-exchange relaxation-free (SERF) comagnetometers demonstrate promising applications in precision metrology such as magnetic field and rotation sensing. However, the ultra-high precision measurement capability exhibits vulnerability to multiple disturbance factors, with pump light intensity fluctuations emerging as a critical instability source that compromises operational reliability during prolonged measurements. In this paper, a model-driven paradigm for pump light intensity error suppression in SERF comagnetometers is proposed. The nonlinear dynamics are rigorously linearized via Taylor expansion within their valid range to construct a state-space model, ultimately deriving the pump light intensity frequency response model. The proposed methodology undergoes experimental validation on a K-Rb-21Ne comagnetometry platform, confirming the predictive capability for stability degradation mechanisms. Through theoretical analysis and experimental verification, this study reveals that magneto-optical non-orthogonality, which introduces a differential component in the light intensity transfer function, creates an approximate proportional relationship between the output signal and pump light intensity within the operating bandwidth. Light intensity noise is decomposed, and light intensity errors are quantitatively evaluated via the frequency response model. To address this critical issue affecting inertial measurement accuracy, a closed-loop control-based suppression method with optimized feedback gain is proposed. By dynamically adjusting the closed-loop feedback gain through real-time intensity monitoring, the intensity instability associated with optical pumping is suppressed from 0.56% to 0.14%, concomitant with a 2.8-fold improvement in the long-term stability of the SERF comagnetometer.
Ultrasensitive atomic comagnetometers (ACMs) are invaluable tools for both inertial measurement and fundamental physics research. However, the misalignment between the pump light and the main magnetic field affects the polarization characteristics of nuclear spins of the noble gas atoms, making it a significant source of noise. In this article, an atomic magnetometer response model with the optical-magnetic misalignment error is established, and the mechanism of the influence of optical-magnetic misalignment on nuclear spin polarization is analyzed. An in situ evaluation method and a suppression method of optical-magnetic misalignment error are proposed, and their effectiveness is verified through experiments with varying pump light incidence directions. The experimental results indicate that after optical-magnetic alignment using the proposed method, the spin polarization of the coupled atomic ensemble is enhanced by 20.7%, and the longitudinal relaxation rate of the nuclear spin is reduced by 6.9%. Additionally, the amplitude-frequency response (AFR) of the x-axis magnetic field is reduced by 33.2% at 1 Hz and by 53.1% at frequencies below 0.002 Hz. This indicates an improvement in the transverse magnetic field error suppression capability of the ACMs.
Ultrasensitive spin exchange relaxation-free (SERF) comagnetometers have been widely applied in various new physics experiments and show great potential for inertial navigation. However, the spatially nonuniform spin polarization of alkali metal (AM) and noble gas (NG) atoms results in an effective magnetic field gradient on the order of 100 nT/cm which is much larger than that is produced by the coils and remanence on the magnitude of 1 nT/cm, thereby reducing the suppression ability of the magnetic noise and limiting the scale factor and noise floor of the comagnetometers. In this study, we establish a model of the effective magnetic field gradient caused by uneven spin polarization and analyze the influencing factors. Our results indicate that optimizing the density ratio of hybrid AM atoms can suppress the effective magnetic field gradient. Nevertheless, the available pump light power imposes a limitation on the density ratio. To further address this issue, we propose a balanced light-assisted hybrid spin exchange optical pumping (HSEOP) method. Our experimental results confirm the effectiveness of the mentioned approach, with 67% suppression of the nuclear transverse relaxation rate and a nearly threefold increase in the scale factor. Furthermore, the optimal density ratio in experimental applications can be reduced from 200 to below 80, thereby overcoming the limitation of laser power in SERF comagnetometers and providing the possibility for wider applications. Finally, the suppression factor of magnetic noise is enhanced by 89% by optimizing the density ratio and the balanced light-assisted HSEOP method. This research sheds light on the fundamental physics experiments and potential for inertial navigation.
Atomic spin sensors are essential for beyond-the-standard-model exploration, biomagnetic measurement, and quantum navigation. While the traditional DC mode spin-exchange relaxation-free (SERF) comagnetometer achieves ultrahigh sensitivity, further improvements require suppressing technical noise and surpassing standard quantum limit. In this work, we develop a K-Rb-21^{21}21Ne SERF atomic comagnetometer that dynamically polarizes the electron and nuclear spins, shielding signals from direct interference by pump light. We establish a three-phase evolutionary model for hybrid spin ensemble dynamics, yielding a complete analytical solution, and analyze the responses to various spin perturbations. Additionally, we achieve an averaged 38.5 %\%% suppression of the polarization noise and identify the key factors that limit sensitivity improvements. The dynamically polarized comagnetometer exhibits effective suppression of technical noise and holds the potential to overcome quantum noise limit, while offering promising applications in exploring new physics and precise magnetic field measurements.
The atomic comagnetometers (ACMs) based on the spin-exchange relaxation-free (SERF) regime exhibit extremely high sensitivity in angular velocity measurements, positioning them as leading candidates for next-generation precision inertial sensing. However, magnetic noise remains a significant factor that contributes significantly to long-term bias drift. Here, a comprehensive model in full-frequency band of the angular velocity measurement error induced by magnetic noise in ACMs is established. Based on the proposed model, what we believe to be a new method is proposed to reduce magnetic noise error by suppressing the magnetic field response in the mid-frequency range. After optimizing the temperature of the vapor cell to minimize the magnetic field response in the mid-frequency range, the angular velocity measurement error caused by magnetic noise was reduced by 20.4%, and the bias drift was improved by 34.8%. The theories and methods proposed in this study innovatively highlight the critical role of mid-frequency magnetic field responses in suppressing magnetic noise errors, and provide important guidance for improving bias drift in SERF ACMs.
Atomic spin sensors are essential for beyond-the-standard-model exploration, biomagnetic measurement, and quantum navigation. While the traditional DC mode spin-exchange relaxation-free (SERF) comagnetometer achieves ultrahigh sensitivity, further improvements require suppressing technical noise and surpassing standard quantum limit. In this work, we develop a K-Rb- 21 Ne SERF atomic comagnetometer that dynamically polarizes the electron and nuclear spins, shielding signals from direct interference by pump light. We establish a three-phase evolutionary model for hybrid spin ensemble dynamics, yielding a complete analytical solution, and analyze the responses to various spin perturbations. Additionally, we achieve an average 39% suppression of the polarization noise and identify the key factors that limit sensitivity improvements. The dynamically polarized comagnetometer exhibits effective suppression of technical noise and holds the potential to overcome quantum noise limit while offering promising applications in exploring new physics and precise magnetic field measurements.