Increasing the length of free-falling path with homogeneous bias field for compact cold atom interferometers by configuring MOT coils

In a cold atom interferometer, a bias magnetic field, usually generated by a solenoid or a set of coils, is used to select specific Raman transitions of atoms in the interference region, and its inhomogeneity leads to a Zeeman-effect-induced error for the measurement. To improve the measurement’s accuracy further, we propose a simple method to compensate for the non-uniformity of the bias field by reversing the direction and tuning the value of the current running through the pair of quadrupole coils used for magneto-optical trap (MOT). To smoothen the field with MOT coils, the current applied should be controlled timely and precisely, which was done with a fast-responding electric circuit we designed and built. We also used Raman spectroscopy to characterize the magnetic field profile without and with our method applied, which shows that the length of the homogeneous field (where the fluctuation of magnetic strength is less than 10%) is expanded from 81.87 mm to 101.64 mm. From our calculations, with the free evolution time T of 30 ms, the quadratic Zeeman-effect-induced phase shift Δϕzeeman can be reduced by 76%, which reduces the quadratic Zeeman-effect-induced error of measured gravity Δgzeeman to a quarter of that without our method applied (from 1.745 mGal to 0.418 mGal). Simulation and experiment prove that this method is effective. This method can also be used in other atom interferometers, especially compact systems.