Atomic magnetometers based on Zeeman shift measurement have the potential for high sensitivity and long-term stability. Like other atomic sensors including atomic clocks and atom interferometers, the atomic magnetometer could in principle be augmented with spin squeezing for further sensitivity enhancement. However, existing atomic magnetometers are not compatible with spin squeezing because the atoms can hardly be in a pure quantum state during operation. A natural challenge is the arbitrary direction of the magnetic field. In this paper, we propose a cold-atom-based magnetometer with spin squeezing that can measure both the magnitude and the direction of an arbitrary magnetic field. For experimentally accessible parameters, we show that the technique described above could achieve a sensitivity nearly three orders of magnitude higher than that of the best existing magnetometers.
Under negative feedback, the quality factor Q of a radio-frequency atomic magnetometer can be decreased by more than two orders of magnitude, so that any initial perturbation of the polarized spin system can be rapidly damped, preparing the magnetometer for detection of the desired signal. We find that noise is also suppressed under such spin damping, with a characteristic spectral response corresponding to the type of noise; therefore magnetic, photon shot, and spin-projection noise can be measured distinctly. While the suppression of resonant photon shot noise implies the closed-loop production of polarization-squeezed light, the suppression of resonant spin-projection noise does not imply spin squeezing, rather simply the broadening of the noise spectrum with Q. Furthermore, the application of spin damping during phase-sensitive detection suppresses both signal and noise in such a way as to increase the sensitivity bandwidth. We demonstrate a threefold increase in the magnetometer's bandwidth while maintaining 0.3 fT/Hz sensitivity.
The deterioration of spin coherence has been a significant challenge in the field of quantum magnetic measurements based on optically pumped atomic magnetometers (OPAMs). However, measurement and evaluation methods for specific relaxation components of OPAMs under working conditions remain lacking, which impedes further improvement in OPAM performance. To address this, we propose a methodology for quantifying the relaxation components of an OPAM under realistic operating conditions. Using this method, we confirm the existence of an anomalous relaxation mechanism induced by radio-frequency (RF) magnetic fields and develop an empirical expression to describe its behavior. The results experimentally demonstrate that, in addition to the well-known spin-exchange relaxation, the dominant contributions to spin relaxation arise from optical pumping and RF magnetic fields. This work paves the way for sensitivity enhancement in quantum magnetometry and refines the theoretical understanding of relaxation mechanisms in OPAMs.