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Chinese Control Conference (CCC)

The influence of spin-destruction collisions on the precision of atomic SERF magnetometer

Lijun Liu, Shuming Cheng, Zairong Xi, Bo Qi

Published Jul 1, 2014• 0 citations
Publisher

Abstract

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.

DOI
10.1109/chicc.2014.6895948

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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.
Ultra sensitive detection and high precision measurement of magnetic fields have been of great importance for both fundamental physics and practical applications. Recent developments in the technology of atomic magnetometers have enabled the spin-exchange relaxation free (SERF) atomic magnetometers as the most sensitive devices for detecting and measuring static or quasi-static magnetic fields. In this paper, we analyze the effect of the most prominent relaxation mechanism (i.e., the spin-destruction collisions) on the precision of SERF magnetometers. For static magnetic fields, it is explicitly demonstrated how the spin-destruction collisions degrade the precision of the SERF magnetometers from the Heisenberg limit to the standard limit even with quantum resources being employed.
Abstract Spin-exchange (SE) and spin-destruction (SD) collisions play a central role in determining the coherence properties of alkali-metal vapors and the performance of alkali-metal magnetometers in external magnetic fields. In this work, we investigate how these collisions influence spin relaxation dynamics and magnetometric response using a generalized Bloch equation that fully accounts for the coupled evolution of spin polarizations in different hyperfine manifolds. Our results show that in the low-field regime, both SE and SD collisions contribute to the suppression of transverse spin relaxation and the slowdown of spin precession, whereas longitudinal relaxation depends weakly on the SE rate and is predominantly governed by SD interactions. Building on the established relationship between spin relaxation rates and magnetometric response, we further analyze how SE and SD collisions affect the performance of two representative magnetometer architectures: the spin-exchange relaxation-free (SERF) magnetometer and the magnetic-resonance Mx-type magnetometer. The SERF magnetometer’s response is found to be primarily determined by the SD rate, with only minor dependence on the nuclear spin quantum number. In contrast, the magnetic-resonance Mx-type magnetometer’s sensitivity is significantly affected by both the magnetic field strength and the SE rate. While the resonance linewidth decreases with reduced magnetic field, the associated measurement accuracy deteriorates markedly. Consequently, reliable magnetic field reconstruction is only achievable in the high-field regime, where the Larmor frequency greatly exceeds the SE rate.
The spin-exchange relaxation-free (SERF) atomic magnetometer (AM) has vital importance in advanced research and has been widely applied in different fields. To improve the sensitivity and robustness of SERF magnetometers, understanding the relaxation mechanism is a significant approach. By controlling the amplitude and direction of external magnetic fields, and switching statuses of the pump and probe laser beam, we measured both the transverse and longitudinal relaxation rate in two experimental methods, respectively, and obtained the Larmor precession frequency, the spin-exchange relaxation rate, the spin-destruction relaxation rate, and the relaxation rate due to collisions between atoms and vapor cell walls. All experimental results are verified with the theoretical calculations. The final longitudinal relaxation rate of our magnetometer is 53 Hz. Furthermore, we measured the sensitivity of our SERF magnetometer reaching 5 fT/Hz1/2 from 5 to 70 Hz after differential measurement with the 3-dB bandwidth of 70 Hz. Our work helps to improve the detection capability of SERF magnetometers, providing performance assurance for searching for exotic spin-dependent interactions and applications, including magnetoencephalograms and magnetocardiograms.
Spin-exchange-relaxation-free (SERF) atomic magnetometer is the most sensitive magnetic detector in the world at present due to the fact that the broadening of spin-exchange collision is eliminated by operating at high alkali-mental density and low magnetic field. On that condition, the precession frequency of high-density alkali vapor becomes a constant fractional value of Larmor frequency, and the width of magnetic-resonance line gets narrower. This paper analyzes the principle of SERF regime and proposes an innovation based on measuring the slowing-down Larmor frequency directly instead of the complicated conventional method based on the width of magnetic resonance line. We calculated and simulated the slowing-down precession frequency and linewidth of magnetic resonance on certain conditions. The experimental results have verified that the method is valid to identify the regime of SERF of atomic magnetometer via detecting the slowing-down precession frequency.
Spin-exchange relaxation-free magnetometers based on dense ensembles of alkali-metal spins are precision quantum sensors that hold the record of measured and projected sensitivity to magnetic fields, in the microgauss to milligauss range. At geomagnetic fields however, these sensors quickly lose their magnetic sensitivity due to spin decoherence by random spin-exchange collisions. Here we discover that atoms with nuclear spin I=12I=\frac{1}{2}I=21​ can operate in the spin-exchange relaxation-free regime even at high magnetic field. We counterintuitively show that frequent collisions between a dense and optically inaccessible (I=12)(I=\frac{1}{2})(I=21​) gas with another optically accessible spin gas (I>12I>\frac{1}{2}I>21​) improve the fundamental magnetic sensitivity of the latter. We analyze the performance of a dual-species potassium and atomic hydrogen magnetometer and project a fundamental sensitivity of about 10aT(cm3/Hz)1/210\phantom{\rule{0.28em}{0ex}}\mathrm{aT}\phantom{\rule{0.16em}{0ex}}{({\mathrm{cm}}^{3}\text{/}\mathrm{Hz})}^{1/2}10aT(cm3/Hz)1/2 at geomagnetic fields for feasible experimental conditions.
Spin-exchange relaxation-free (SERF) atomic magnetometers seem to be the state-of-the-art magnetic sensors today as they have achieved a sensitivity of femtotesla in the elimination of broadening contribution of spin-exchange collisions operated in the mode of SERF regime. In order to extend the practicability of SERF atomic magnetometry in superb performances in a movable platform, ensuring and testing the working status of alkali atoms are of great importance. Given the fact that in the premise of SERF regime, the linewidth of magnetic resonance would be narrowing down and the precession frequency would become a constant fractional value of the Larmor frequency for alkali atoms, we measure the gyromagnetic ratio of weakly polarized alkali-metal vapor by yielding the damped oscillating curves and fitting measured increments of frequency and magnetic field to identify the working regime of alkali atoms in a fast and straightforward manner. The experimental results have good agreements with theoretical analysis and simulation, and the whole measurement process lasts milliseconds with relatively measured error ±1% in the temperature range of T = 156 °C-184 °C. To testify the validity of the innovation, magnetic resonance line is obtained in the same condition proffering the width comparative with the outcome from simulation, indicating the spin-exchange broadening has been eliminated, presenting the same conclusion with innovation, and providing systematic sensitivity 30 fT/Hz1/2.