High-sensitivity atomic magnetometry in Earth's magnetic field

Measurements of small magnetic fields in an unshielded environment present several challenges due Earth's static magnetic field as well as ambient field noise and gradients. To overcome these problems we are developing a scalar magnetic gradiometer using Rb atoms. The atoms are initialized by a pulsed pump laser and then precess freely in the magnetic field while being interrogated by a multi-pass probe laser. We realize a differential magnetic field sensitivity of 14 fT/Hz$^{1/2}$ in a 50 $\mu$T bias magnetic field. We also characterize the absolute accuracy of magnetic field measurements and the common mode rejection ratio of the magnetic gradiometer. Development of a sensitive and robust magnetic gradiometer operating under ambient conditions will enable detection of biomagnetic signals in unshielded environments as well as many other magnetometry applications.