LAUSR

In this Letter, we propose a single-beam nonlinear magneto-optical rotation (NMOR) magnetometer with a multipass 4He gas-discharged cell. In contrast to the single-pass cell, the multipass cell allowed laser beams to pass through the metastable-state atomic ensemble 22 times, which directly increases the optical path length and significantly enhances magneto-optical rotation in the 4He gas sample. Based on nonlinear Faraday rotation, the 4He magnetometer with the multipass cell demonstrates a noise floor of 9 fT/Hz1/2, which approaches the photon-shot noise floor limit of 6.4 fT/Hz1/2. In addition, the wider linewidth in metastable-state atoms realizes an NMOR 4He magnetometer with a 3 dB bandwidth of 4.3 kHz, in contrast to the ultranarrow linewidth in the antirelaxation-coated cells or spin-exchange relaxation-free regime alkali-metal cells with buffer gas. Since the 4He cell functions without heating or cryogenic cooling, the femtotesla sensitivity and kilohertz-bandwidth 4He magnetometer exhibits potential in biomagnetic applications such as magnetocardiography and magnetoencephalography.