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AIP Advances

The influence of longitudinal magnetic-field gradient on the transverse relaxation rate of cesium atoms

Y. Fu, Jiayue Yuan

Published Nov 1, 2017• 9 citations
Publisher

Abstract

Reducing the transverse relaxation rate of alkali atoms is crucial to improve the sensitivity of the magnetometers. In a perturbative treatment of the effects of the magnetic-field gradients, the transverse relaxation rates of alkali atoms are predicted to be proportional to the square of the longitudinal magnetic-field gradient. The theoretical predictions are confirmed in our experiments. The magnetic-field gradients in the vapor cell are measured by experiments. The magnetic-field gradient in the cell is suppressed drastically through compensating it with the compensation coils. Experimental results show that suppressing the magnetic-field gradient can reduce the transverse relaxation rate of alkali atoms significantly. This study contributes to the development of ultra-high sensitivity alkali atomic magnetometers.

DOI
10.1063/1.5005956

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Reducing the transverse relaxation rate of alkali atoms is crucial to improve the sensitivity of the magnetometers. In a perturbative treatment of the effects of the magnetic-field gradients, the transverse relaxation rates of alkali atoms are predicted to be proportional to the square of the longitudinal magnetic-field gradient. The theoretical predictions are confirmed in our experiments. The magnetic-field gradients in the vapor cell are measured by experiments. The magnetic-field gradient in the cell is suppressed drastically through compensating it with the compensation coils. Experimental results show that suppressing the magnetic-field gradient can reduce the transverse relaxation rate of alkali atoms significantly. This study contributes to the development of ultra-high sensitivity alkali atomic magnetometers.
Magnetic field gradients interfere with the coherence of the atomic ensemble and degrade the performance of the spin-exchange relaxation-free (SERF) atomic magnetometer. In this paper, the influence of magnetic field gradients is incorporated into the response model of single-beam atomic magnetometer, and the theoretical models of transverse relaxation rate, spin polarization and scale factor are established. Based on this model,we find that magnetic field gradients in different directions can cause varying degrees of degradation in the performance parameters of the magnetometer. The magnetic linewidths under different light power and magnetic field gradients are measured using the designed dedicated magnetic field gradients coils, and the spin polarization and magnetic field gradients relaxation rate are obtained through fitting. An unevenness and deviation of the curve are also observed in the experiments, indicating the presence of the magnetic field gradient of approximately 2 nT/cm in the magnetic shields. Furthermore, in a magnetic field gradient environment of 20 nT/cm, the mean of the long-term sensitivity decreased by 3.5 times and the standard deviation increased by 11 times. It shows that the magnetic field gradients will not only affect the signal-to-noise ratio, but also make the magnetometer more vulnerable to external interference when working. The sensitivity and stability of magnetometer will be greatly reduced. The research in this article provides a theoretical and experimental basis for eliminating the influence of magnetic field gradients and improving the accuracy of magnetic field measurements.
In order to improve the sensitivity of radio-frequency atomic magnetometer, two methods to extend the transverse relaxation time of polarized alkali atoms are investigated. One way is the application of a non-relaxing coating, another way is the introduction of buffer gas into cell. A theory relaxation model is built, and the dependences of transverse relaxation time on buffer gas pressure, laser power, cell temperature, and magnetic-field gradient are obtained. The measurements of transverse relaxation time are carried out in a cesium atomic radio-frequency magnetometer. The optical experiment conditions which yield the highest sensitivity are determined in experiments. Experiments results indicate that the optimal pressure for neon is 200 Torr. The experimental results are in agreement with the theoretical ones. An ultimate sensitivity of 100 fT/Hz1/2 of the cesium atomic radio-frequency magnetometer has been achieved. This study helps further optimization of the cell performance and the sensitivity of radio-frequency atomic magnetometer.
In this work, we experimentally measure the broadened linewidth of magneto-optical resonance signal due to magnetic-field gradient with an alkali buffer-gas-free anti-relaxation-coated cell. We also study how the temperature of the atomic vapor cell affects the magneto-optical resonance signal linewidth under different magnetic-field gradients. Theoretical calculations show that the relaxation caused by magnetic-field gradient could be suppressed with increased temperature, which could be useful to optimize the working parameters for unshielded atomic magnetometer based on anti-relaxation-coated cells.
Alkali-metal atomic magnetometers employing longitudinal carrier magnetic field have ultrahigh sensitivity to measure transverse magnetic fields and have been applied in a variety of precise-measurement science and technologies. In practice, the magnetometer response is not rigorously proportional to the measured transverse magnetic fields and the existing fundamental analytical model of this magnetometer is effective only when the amplitudes of the measured fields are very small. In this paper, we present a modified analytical model to characterize the practical performance of the magnetometer more definitely. We find out how the longitudinal magnetization of the alkali metal atoms vary with larger transverse fields. The linear-response capacity of the magnetometer is determined by these factors: the amplitude and frequency of the longitudinal carrier field, longitudinal and transverse spin relaxation time of the alkali spins and rotation frequency of the transverse fields. We give a detailed and rigorous theoretical derivation by using the perturbation-iteration method and simulation experiments are conducted to verify the validity and correctness of the proposed modified model. This model can be helpful for measuring larger fields more accurately and configuring a desirable magnetometer with proper linear range.
The magnetic field gradient within optical pumping magnetometers (OPMs) suppresses sensitivity improvement. We investigated the effects of the magnetic field gradient along the x-, y-, and z-axes on the limiting factors of magnetometers under extremely low magnetic field conditions. We modified the magnetic field gradient relaxation model such that it can be applied to atoms in the spin exchange relaxation free (SERF) regime. The gradient relaxation time and spin polarizations, combined with fast spin-exchange interaction, were determined simultaneously using the oscillating cosine magnetic field excitation and amplitude spectrum analysis method. During the experiments, we eliminated the errors caused by the temperature and pumping power, and considered different isotope spin exchange collisions in naturally abundant Rb during the data analysis to improve the fitting accuracy. The experimental results agreed well with those of theoretical calculations and confirmed the accuracy of the improved model. The contribution of the transverse magnetic field gradient to the relaxation of the magnetic field gradient cannot be ignored in the case of small static magnetic fields. Our study provides a theoretical and experimental basis for eliminating magnetic gradient relaxation in atomic sensors in the SERF region.
The accuracy of atomic sensor is determined by the transverse nuclear spin relaxation time (T 2 ). As the longitudinal nuclear spin relaxation time (T 1 ) is the upper limit of T 2 , it becomes an important metric for evaluating the merits of alkali vapor cells. However, the conventional method for measuring T 1 of pulse delay takes dozens of minutes, which will increase the effect of random errors during the testing process. In order to solve this problem, a method to quickly estimate the value of T 1 is proposed by suppressing the influence of spin exchange relaxation and magnetic field gradient relaxation terms on T 2 . Experimental results indicate that the accuracy is maintained at above 95%, and the testing time is reduced to less than 5%.