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IEEE Transactions on Instrumentation and Measurement

Ultrasensitive Optical Rotation Detection With Closed-Loop Suppression of Spin Polarization Error

Xing Heng, Kai Wei, Tian Zhao, Zitong Xu, Qian Cao, Xiaofei Huang, Yueyang Zhai, Mao Ye, Wei Quan

Published Jan 1, 2023• 19 citations
Publisher

Abstract

Optical rotation detection system (ORDS) utilizing quantum nondemolition (QND) measurement methods is widely applied in the field of quantum metrology and quantum information. However, the sensitivity of ORDS is limited by the uncertainty from optical-couple noise during the measurement of the atomic spin ensemble. In this study, we specifically analyze the mechanism of optical-couple noise caused by the fluctuations of probe light’s polarization in the modulated ORDS with a new model established to describe atomic spin precession in this particular condition. It is discovered that transverse electron-spin polarization errors are generated by the residual probe photon spin polarization in the ORDS, which results in extra coupling magnetic noise. In order to suppress this noise, a novel in situ method is proposed that the resultant electron-spin errors are reduced by a specifically designed closed-loop system. The results are verified through the ORDS in a co-magnetometer. After zeroing the extra probe photon spin polarization, an angular sensitivity better than$1 \times 10 ^{\mathrm {-8}}$rad/Hz$^{\mathrm {1/2}}$is achieved for frequencies higher than 5 Hz, demonstrating a probe background noise of 0.26 fT/Hz$^{\mathrm {1/2}}$@14.5 Hz, approaching electronic noise and photon shot noise (PSN). With closed-loop control, the optical rotation bias instability is promoted by 4.2 times (from 107.9 to$25.7 \mu $rad/h), and the angular noise of the ORDS is reduced by 2.4 times at 1–100 Hz. The measurement uncertainty of the realized ORDS nears the standard quantum limit, paving the road for long-term ultrasensitive measurements for new physics explorations.

DOI
10.1109/tim.2023.3237208

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We analyze and suppress the magnetic noise response in optical rotation detection system (ORDS) in atomic magnetometers in this study. Because of the imperfections of the optical elements, the probe light is actually elliptically polarized in ORDS, which can polarize the atom ensemble and cause the responses to the three-axis magnetic noise. We theoretically analyze the frequency responses to the magnetic noise, and prove that the responses are closely associated with the DC magnetic field. The values of the DC magnetic fields are calculated with special frequency points, called 'break points', in the transverse responses. We reveal the relationships between the DC magnetic field and the sensitivities of ORDS, and effectively suppress the magnetic noise responses with the residual magnetic field compensation. Finally, the sensitivity of ORDS is improved by approximately two times at 10-20 Hz.
Quantum non-demolition measurement with an off-resonance polarized probe is a widely utilized technique for atomic sensors. Typically, QND instruments read out spin states through optical polarimetry using a linearly off-resonance probe in atomic media. This optical read-out process offers high accuracy, with potential applications in biomagnetism, inertial navigation and new physics exploration beyond the standard model. It was observed that previously unexplored spin-alignment polarization induced by the far-detuned off-resonance linear probe in an ultrasensitive optical polarimetry. The evolution of probe-generated multipole moments exacerbates probe noise due to strong magnetic couplings. A method was demonstrated to decouple spin-alignment from magnetic fields by manipulating the multipole moments in zero fields. A suppression of the probe noise by 8.2 dB at 1 Hz and 10.4 dB at 10 Hz with the proposed method, reaching the standard quantum limit, was achieved. The realized shot-noise-limited optical polarimetry shows significant potential for advancing the performance of atomic sensors, particularly in applications requiring ultra-high sensitivity, such as precision measurements and quantum metrology. These findings provide a pathway toward enhanced sensor precision in diverse scientific and technological applications.
This paper investigates the laser polarization error in the optical rotation detection system (ORDS) of an atomic comagnetometer (ACM), which will seriously degrade the long-term performance of the ORDS. We first establish an optical transmission model of the ORDS by using Jones matrix concerning the optical imperfection of polarizers. Then, we analyze the polarization error based on this model and propose a novel error suppression method. Finally, we experimentally test the long-term performance of the ORDS and the ACM before and after the polarization error suppression to verify the effectiveness of the proposed method. The experimental results show that the long-term performance of the ORDS and the ACM can be improved by approximately 3.4 times with the proposed polarization error suppression method.
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.
We propose a high-sensitivity spin polarization detection scheme based on optical amplitude modulation in atomic co-magnetometers, which is different from the traditional configuration.A linearly polarized laser with intensity modulation interacts with electron spins, generating optical rotation angles.Through differential amplification and demodulation, atomic spin polarization information is extracted with high precision.The effectiveness of the proposed method is verified by applying external perturbations during testing.Compared to traditional detection methods, the proposed approach effectively enhances inertial measurement sensitivity.Specifically, at 1 Hz, the measurement sensitivity has improved from 9.7 × 10 -6 to 3.25 × 10 -6 deg/(s Hz 1/2 ).Furthermore, the proposed scheme is easily integrable and conducive to future research on miniaturizing co-magnetometers and can also be applied to many other related fields.
Rb optically pumped magnetometer (OPM) based on electron paramagnetic resonance (EPR) show advantages to measure the nuclear magnetization and have succeeded in fundamental physics and rotation sensing, etc . The magnetometry sensitivity is a key performance of these Rb OPMs which should be improved. In this study, a high sensitivity Rb OPM is demonstrated theoretically and experimentally. To improve the sensitivity, acousto-optic modulation based on balanced detection is applied to suppress the probe noises. Compared with the conventional optical rotation detection for this OPM configuration, the probe noise shows a significant suppression especially in low frequencies. Eventually, a simultaneous dual-axis transverse measurement with 30 fT/Hz 1/2 sensitivity is achieved in a 200 Hz bandwidth and a 250nT linear working range. In addition, we utilize a closed-loop feedback to improve the stability and enlarge the transverse measurement range to 10µT order of magnitude while maintain the open-loop performances. A quasi-static magnetic field measurement can also be achieved in the longitudinal direction in the closed-loop mode. This OPM can serve for the nuclear magnetization measurement with a high sensitivity especially in environments with a large magnitude of the external magnetic field.