Non-monochromatic optical pumping in spin-based quantum sensors

In spin-based quantum sensors, monochromatic resonant light is commonly used for optical pumping to initialize the quantum spin state. However, optical absorption within the atomic ensemble induces spatial polarization gradient, which thereby elevates the spin relaxation rate and significantly degrades sensitivity. The effect is particularly prominent in spin-exchange relaxation-free (SERF) comagnetometers operating at high atomic density. Here, we theoretically demonstrate that atomic polarization gradient can be substantially mitigated by non-monochromatic optical pumping (NMOP) which can be experimentally achieved through phase noise modulation of monochromatic resonant light via an electro-optic modulator (EOM). Experimental investigation within a hybrid K-Rb-Ne comagnetometer demonstrated that this non-monochromatic optical pumping yielded a 13.1% suppression of the Rb electron spin relaxation rate, which in turn produced a 6.5% enhancement in the transverse relaxation time of the Ne nuclear spins. Further improvements are observed in the nuclear spin self-compensation suppression factors, which increase by 9.3% and 16.2% along the x- and y-axes, respectively. This leads to a 43(13)% improvement in the averaged sensitivity over the 0.5–1 Hz band compared to conventional monochromatic pumping methods (MOP). The scheme facilitates integration into a range of quantum sensors based on hot atomic ensembles, including vapor cell atomic clocks, atomic magnetometers, and atomic gyroscopes.