A Highly Sensitive In Situ Magnetic Field Fluctuation Measurement Method Based on Nuclear-Spin Depolarization in an Atomic Comagnetometer

Abstract

In this article, a highly sensitive <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> magnetic field fluctuation measurement method is proposed based on nuclear-spin depolarization in a spin-exchange relaxation-free (SERF) atomic comagnetometer (ACM). The steady-state model of the hybrid atomic spin ensemble is systematically studied. According to the theoretical model, two different states of nuclear-spin polarization and unpolarization lead to different sensitivities of the system to rotation and magnetic fields. The gradient magnetic field (GMF) is employed to change the state of nuclear-spin polarization of the noble gas, transforming the ACM into an atomic magnetometer (AM). The polarization and relaxation rates of the electron spin and nuclear spin under the two measurement modes are measured and compared by the magnetic resonance method and transient response method. At the same time, the performances in the ACM and AM are tested. Detailed experiments verify that the AM can achieve sensitivity for the magnetic field measurement with an average value of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$53.6~\mathbf {fT/Hz^{1/2}}$ </tex-math></inline-formula> in the frequency range of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10\sim 30$ </tex-math></inline-formula> Hz. The Allan variance method is adopted to evaluate the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$x$ </tex-math></inline-formula> - and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$y$ </tex-math></inline-formula> -axis magnetic field stabilities of the ACM, and the magnitudes are 61.6 and 55.9 fT at 100 s, contributing about 4.9 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10\mathbf {^{-3}}\,\,\mathbf {^{\circ }/h}$ </tex-math></inline-formula> and 2.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\times 10\mathbf {^{-4}}\,\,\mathbf {^{\circ }/h}$ </tex-math></inline-formula> to the long-term stability of the ACM. This method can not only realize the high-sensitivity <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">in situ</i> measurement of the close-to-zero magnetic field and its fluctuation but also realize the switching of the two measurement modes of rotation and magnetic field measurements, which is essential for the evaluation of magnetic field error.