High-precision magnetic field locking system for cold atoms
Authors: Liu Xue-Mei , Rui Yang , Zhang Liang , Wu Yue-Long , Wu Hai-Bin ,   State Key Laboratory of Precision Spectroscopy, East China Normal University, Shanghai 200241, China
Published: March 2022

In ultracold atomic experiments, evaporative cooling is usually achieved by using Feshbach resonance magnetic fields on the order of Gauss to hundreds of Gausses. The frequency of resonant transition induced by the optical field or radiofrequency is directly affected by the stability of the quantum axis. For example, the phase between two linearly independent vectors of a qubit is affected by the magnetic field noise. Based on the Feshbach resonance technique, magnetic field regulation has become a basic tool to control the interaction between atoms. Narrow Feshbach resonance shows unique advantages in high-temperature superconducting, superfluidity, neutron star state simulation, etc. However, since its resonance width and Fermi energy can be compared with each other, the scattering characteristics are greatly disturbed by the magnetic field. Therefore, a stable and uniform magnetic field is a prerequisite for studying the narrow Feshbach resonances. In experiment, Helmholtz coils are usually used to provide the magnetic field for cold atomic gas, and the magnetic field noise is generally determined by the coil current noise and other magnetic field noises of the environment. However, there are relatively few researches of the high-precision control of large magnetic fields above hundreds of Gausses. With a larger coil current required, the coil current noise contributes more to the magnetic field noise, thus high-precision control of large magnetic fields is still challenging. In this paper, a magnetic field locking system is used to realize a <inline-formula><tex-math id="Z-20220704103959">\begin{document}$2.27 \times 10^{-6} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220399_Z-20220704103959.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="14-20220399_Z-20220704103959.png"/></alternatives></inline-formula> level locking of the Feshbach magnetic field. A feedback locking system is used to achieve the stability by shunting the magnetic field coil current noise. Compared with the non-locked magnetic field, the low-frequency current noise is suppressed by more than 45 dB. To assess the stability of the actual magnetic field at the atoms, the Rabi oscillation is measured, the coherence time increases nearly 9.6 times, which effectively improves the stability of the ultracold atomic system. Furthermore, we measure the atom number fluctuation at the Gaussian inflection point of the loss spectrum under different Raman pulse widths to evaluate the noise of the magnetic field. Roman pulse duration up to a 24 μs is used to increase the sensitivity of atom number fluctuation in loss spectrum relative to magnetic field noise, of which the root mean square (RMS) noise is suppressed from 20.66 mGs to 1.2 mGs, a 16-fold reduction of the noise is obtained. Such a magnetic field locking system can provide an accurate and stable background magnetic field for ultracold atomic gases, which is of great significance for extending quantum storage time, precisely controlling atomic scattering, and simulating of condensed matter and other ultracold quantum gas in experiment.