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arXiv:1901.02683 · Physical Review Applied

Closed-Loop Control of Compensation Point in the K − Rb − 21 Ne Comagnetometer

Liwei Jiang, Wei Quan, Feng Liu, Wenfeng Fan, Li Xing, Lihong Duan, Wuming Liu, Jiancheng Fang

Published Aug 9, 2019• 77 citations
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Abstract

Although spin-exchange relaxation-free (SERF) comagnetometers have been widely used for fundamental physics tests and rotation sensing, their applications encounter considerable obstacles, due to the uncontrolled compensation magnetic field in such a system. The authors demonstrate a real-time closed-loop control method to stabilize the compensation point of a K-Rb-${}^{21}$Ne comagnetometer, to improve systematic stability, enlarge the linear measuring range, and suppress cross-talk error.

DOI
10.1103/physrevapplied.12.024017

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Although spin-exchange relaxation-free (SERF) comagnetometers have been widely used for fundamental physics tests and rotation sensing, their applications encounter considerable obstacles, due to the uncontrolled compensation magnetic field in such a system. The authors demonstrate a real-time closed-loop control method to stabilize the compensation point of a K-Rb-21{}^{21}21Ne comagnetometer, to improve systematic stability, enlarge the linear measuring range, and suppress cross-talk error.
Zero‐Position Closed‐Loop Control of Nuclear Spin in a K‐Rb‐21Ne21Ne^{21}{\rm Ne}21Ne Comagnetometer
Abstract In SERF (spin‐exchange relaxation‐free) comagnetometers, the fluctuation of the atomic ensemble's polarization significantly impacts the long‐term stability of the output signal. A real‐time closed‐loop control method for the nuclear spin polarization is proposed, and the measurement model is established using Kalman state observer. Furthermore, magnetic field control signals are generated using a proportional‐integral‐derivative (PID) strategy, which is then applied to the atomic ensemble to dynamically adjust the nuclear spin polarization. Finally, closed‐loop control of nuclear spin polarization is achieved through continuous feedback and control. Moreover, experiments have demonstrated that implementing a closed‐loop system for nuclear spin zero‐position is conducive to reducing the sensitivity of the output signal to fluctuations in the external environment. The experimental results show that the Allan deviation at 100 s is reduced by , and the sensitivity of inertial rotation measurement is significantly improved, that is, the noise level at 1 Hz is suppressed by approximately .
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.
A compact dual-axis $\mathrm{K}\ensuremath{-}\mathrm{Rb}\ensuremath{-}^{21}\mathrm{Ne}$ comagnetometer with one circularly polarized pump beam and two orthogonal linearly polarized probe beams is presented. It can be exploited for rotation sensing by operating in the spin-exchange relaxation-free regime. Due to the light shift arising from the pump laser, there is a cross-talk effect between the two sensitive axes, which limits the comagnetometer applications. To eliminate this effect, an external field parallel to the direction of the pump beam is used to compensate the light-shift field. It is validated theoretically and experimentally in the dual-axis $\mathrm{K}\ensuremath{-}\mathrm{Rb}\ensuremath{-}^{21}\mathrm{Ne}$ comagnetometer. With the cross-talk effect suppressed, the comagnetometer can carry out high-precision rotation sensing along two sensitive axes simultaneously and independently.
Abstract The fluctuation of the vapor cell temperature leads to the variations of the density of the alkali metal atoms, which seriously damages the long-term stability of the spin-exchange relaxation-free (SERF) comagnetometer. To address this problem, we propose a novel method for suppressing the cell temperature error by manipulating the probe laser frequency. A temperature coefficient model of the SERF comagnetometer is established based on the steady-state response, which indicates that the comagnetometer can be tuned to a working point where the output signal is insensitive to the cell temperature fluctuation, and the working point is determined by the relaxation rate of the alkali metal atoms. The method is verified in a K−Rb− 21Ne comagnetometer, and the experimental results are consistent with the theory. The theory and method presented here lay a foundation for the practical applications of the SERF comagnetometer.
A spin-exchange relaxation-free comagnetometer is a promising high-precision gyroscope as its inertial measurement sensitivity can rival that of existing technologies. The magnetic field error is one of the main factors that limits the long-term stability of the comagnetometer. The steady-state response and frequency response models of a K-Rb-21Ne comagnetometer are modified by considering the spin-exchange rate between the electron spins and the nuclear spins. The modified models are used to redefine the bias magnetic sensitivity of the K-Rb-21Ne comagnetometer and the suppression method of low-frequency magnetic field error is proposed. We demonstrate the magnetic drift suppression method proposed in the article by optimizing the cell temperature and pump laser power density. This study is helpful to improve the rotation measurement performance of the K-Rb-21Ne comagnetometer.
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.