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Journal of Physics D Applied Physics

Ultra-sensitive all-optical comagnetometer with laser heating

Tian Zhao, Ying Liu, Kai Wei, Hong-Tai Xie, Tangjie Mu, Xiujie Fang, Zitong Xu, Yueyang Zhai

Published Jan 10, 2022• 14 citations

Abstract

Abstract Ultra-sensitive comagnetometers, which are designed to detect nuclear- and electron-spin-dependent interaction, have important applications ranging from basic research to inertial navigation systems (INSs). Unfortunately, electric heating, which is typically used in comagnetometers, introduces noise and instability because of the unavoidable generation of a modulated magnetic field. In this study, we investigate and introduce a K-Rb-21Ne comagnetometer that uses laser heating for the first time, when operated in the spin exchange relaxation free regime. The performance of the comagnetometer, which is equipped with both laser heating and electric heating, is investigated, and the two heating modes are compared. The temperature characteristics of the comagnetometer are studied theoretically and experimentally. By optimizing the operating temperature and power density of the pump-light, an equivalent rotation sensitivity of 2.5 × 10 − 7 rad−1 s−1 Hz 1 / 2 @1 Hz is achieved in laser heating mode. The improvement of laser-heating technology could prove essential to reduce electron relaxation and increase the low-frequency sensitivity of comagnetometers. Our results indicate that laser heating can make comagnetometers more suitable for applications in basic research (fifth forces, dark matter, etc), INSs, and other accurate measurements of electronic and nuclear precession.

DOI: 10.1088/1361-6463/ac49b5

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Abstract Ultra-sensitive comagnetometers, which are designed to detect nuclear- and electron-spin-dependent interaction, have important applications ranging from basic research to inertial navigation systems (INSs). Unfortunately, electric heating, which is typically used in comagnetometers, introduces noise and instability because of the unavoidable generation of a modulated magnetic field. In this study, we investigate and introduce a K-Rb-21Ne comagnetometer that uses laser heating for the first time, when operated in the spin exchange relaxation free regime. The performance of the comagnetometer, which is equipped with both laser heating and electric heating, is investigated, and the two heating modes are compared. The temperature characteristics of the comagnetometer are studied theoretically and experimentally. By optimizing the operating temperature and power density of the pump-light, an equivalent rotation sensitivity of 2.5 × 10 − 7 rad−1 s−1 Hz 1 / 2 @1 Hz is achieved in laser heating mode. The improvement of laser-heating technology could prove essential to reduce electron relaxation and increase the low-frequency sensitivity of comagnetometers. Our results indicate that laser heating can make comagnetometers more suitable for applications in basic research (fifth forces, dark matter, etc), INSs, and other accurate measurements of electronic and nuclear precession.

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.

We investigate the effects of pump laser power density on the hybrid optically pumped comagnetometer operated in the spin-exchange relaxation-free (SERF) regime. The analytic steady-state output model for the comagnetometer considering two alkali metal species and one nuclear species is presented for the first time. And the effects of pump laser power density on the rotation sensitivity, suppression of low-frequency magnetic noise and long-term stability of the comagnetometer are studied experimentally. The results indicate that when the product of pumping rate and density ratio of pumped atom to probed atom is equal to the spin relaxation rate of the probed atom, the maximum response and highest sensitivity of the comagnetometer are achieved. However, the suppression of low-frequency magnetic noise and long-term stability improve with the increasing of pump laser power density due to the increasing of nuclear spin polarization. Our focus is to optimize the performance of the comagnetometer for rotation sensing, but the theory and method presented here are relevant to all applications of the hybrid optical pumping technique.

We describe a dual-axis atomic spin comagnetometer for rotation sensing using a single laser beam. A circularly polarized laser is used to pump the co-located atomic ensembles of alkali metal and noble gas and probe their gyroscopic precession at the same time, which is different from the traditional two-beam orthogonal pump-probe configuration. Combined with magnetic field modulations perpendicular to the laser propagating direction, the comagnetometer could measure dual-axis rotation rates simultaneously. This system is demonstrated theoretically and experimentally. A rotation sensitivity of 5.1 × 10–7 rad s−1 Hz−1∕2 and a bias instability of 0.016 °/h have been achieved. The theory and method presented here shed light on the way to miniaturize the comagnetometer with high precision for practical applications.

The atomic spin co-magnetometer operating in the spin-exchange relaxation-free regime is a promising rotation sensor for its ultrahigh sensitivity to inertial forces. However, it was found that the bias magnetic sensitivity (BMS) of the comagnetometer was much larger than theoretically expected, making magnetic noise a dominant error. This paper reports on the experimental results of the effect of cell temperature on the BMS. It is found that the BMS increases rapidly as the cell temperature rises. This is attributed to the polarization gradient of atomic spins induced by the absorption of the pump beam by the atoms. The experiments and theory reveal that a high-power laser source for pumping is essential for the suppression of BMS and the improvement of bandwidth in the co-magnetometer.

Atomic magnetometers based on spin-exchange relaxation free (SERF) regime typically operate with the high optical density and low magnetic fields. We present SERF magnetometer with a compact heating system for compact and efficient temperature control. A resistive heating system with high frequency alternating current was used to achieve high optical density. The current frequency was carefully chosen to avoid resonance with the atomic magnetometer. This resulted in a compact heating system that also allows independent operation of multiple heating systems. Optical rotation signal was obtained at 200 oC and width was about 3 nT with pump laser power of 55 mW. Bandwidth of the magnetometer with the compact heater was 40 Hz. Consequently, the compact resistive heating system proved to be a good alternative to the air-flow heating system. Additionally, residual field effect and signal-to-noise yielded a sensitivity of 50 fT/Hz1/2 at 10 Hz.

Achieving high energy resolution in spin systems is important for fundamental physics research and precision measurements, with alkali-noble-gas comagnetometers being among the best available sensors. We found a new relaxation mechanism in such devices, the gradient of the Fermi-contact-interaction field that dominates the relaxation of hyperpolarized nuclear spins. We report on precise control over spin distribution, demonstrating a tenfold increase of nuclear spin hyperpolarization and transverse coherence time with optimal hybrid optical pumping. Operating in the self-compensation regime, our $^{21}\mathrm{Ne}\text{\ensuremath{-}}\mathrm{Rb}\text{\ensuremath{-}}\mathrm{K}$ comagnetometer achieves an ultrahigh inertial rotation sensitivity of $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}\text{ }\text{ }\mathrm{rad}/\mathrm{s}/{\mathrm{Hz}}^{1/2}$ in the frequency range from 0.2 to 1.0 Hz, which is equivalent to the energy resolution of $3.1\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}23}\text{ }\text{ }{\mathrm{eV}/\mathrm{Hz}}^{1/2}$. We propose to use this comagnetometer to search for exotic spin-dependent interactions involving proton and neutron spins. The projected sensitivity surpasses the previous experimental and astrophysical limits by more than 4 orders of magnitude.