Calibration of magnetic field measurement capability of rubidium-xenon vapor cell atomic magnetometer

Abstract

<sec>The precise measurement of weak magnetic fields by using high-sensitivity magnetometers is not only widely used, but also promotes the development of many research fields. The magnetic field measurement capability of the magnetometer determines the potential and scope of its application, which means that research on its magnetic field measurement capability is essential.</sec><sec>In this work, we develop a rubidium-xenon vapor cell atomic magnetometer. The cell filled with 5-torr <sup>129</sup>Xe, 250-torr N<sub>2</sub> and a droplet of enriched <sup>87</sup>Rb is placed in the center of a five-layer magnetic shield with four sets of inner coils to control the internal magnetic field environment. In the cell, <sup>129</sup>Xe is polarized by spin exchange collisions with <sup>87</sup>Rb atoms, which are pumped with a circularly polarized laser beam at the D<sub>1</sub> transition. If magnetic fields or pulses are applied to the cell, the polarization state of <sup>87</sup>Rb and <sup>129</sup>Xe will change and evolve, whose evolution process can be described by a pair of Bloch equations. The analysis of the Bloch equations indicates that the rubidium-xenon vapor cell atomic magnetometer can measure magnetic fields by two different methods. The magnetic field measurement capabilities of the two methods are experimentally calibrated respectively. The first method is to measure the alternating current (AC) magnetic fields by measuring the influence of the external magnetic fields on the polarization of the <sup>87</sup>Rb atoms. The experimental results show that the sensitivity of the AC magnetic field measurement is about <inline-formula><tex-math id="M1000">\begin{document}$1.5\;{{{\rm{pT}}} / {\sqrt {{\rm{Hz}}} }} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20190868_M1000.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="16-20190868_M1000.png"/></alternatives></inline-formula> in a frequency range of 2100 Hz, and the bandwidth is about 2.8 kHz. The second method is to measure the static magnetic fields by measuring the Larmor frequency of the hyperpolarized <sup>129</sup>Xe in the cell. Considering that its measurement accuracy is limited by the relaxation of the hyperpolarized <sup>129</sup>Xe, the transverse and longitudinal relaxation time are measured to be about 20.6 s and 21.5 s, respectively. Then, the experimental calibration results indicate that the static magnetic field measurement precision is about 9.4 pT and the measurement range exceeds 50 μT, which prove that the static magnetic field measurement can still be performed under geomagnetic field (50 μT). The rubidium-xenon vapor cell atomic magnetometer enables the measurement of AC magnetic fields and static magnetic fields in the same system. Compared with the spin exchange relaxation free (SERF) atomic magnetometer, the rubidium-xenon vapor cell atomic magnetometer has some unique advantages. For AC magnetic field measurement, it has a wider frequency range. For static magnetic field measurement, it can be performed under geomagnetic field and can give the magnetic field measurement value without using the calibration parameters of the system. These characteristics make the rubidium-xenon vapor cell atomic magnetometer have broad application prospects. It is expected to be applied to geomagnetic surveys, basic physics and other aspects of research.</sec>