High-density optical quantum sensors with pulsed probe read-out for correlated spin-noise reduction

Abstract Optical quantum sensors based on alkali-metal atoms enable highly sensitive magnetic-field measurements in warm vapor. Their performance can become limited by intrinsic spin noise once technical noise sources are sufficiently suppressed. Characterizing temporal spin correlations is therefore important both for optimizing sensor sensitivity and for investigating quantum-enhanced sensing protocols. Here we investigate correlated spin fluctuations in a radio-frequency optical quantum sensor based on a high-density potassium vapor cell using pulsed probe readout. Orthogonal pump and probe beams establish and detect the spin polarization, while a bias magnetic field defines the sensing frequency and the probe pulses are synchronized electronically with the driven spin response. The protocol combines periodic probing, phase cycling, and variable-delay window subtraction. Phase cycling suppresses reproducible probe-induced coherent transients between experimental cycles, whereas window subtraction probes correlations between two measurements within the same cycle. The measured difference noise increases toward an asymptotic value with delay, demonstrating temporal correlations in the processed signal. This behavior is compatible with correlations expected from a conditional quantum non-demolition mechanism, but the present experiment does not independently establish spin squeezing below the standard quantum limit or uniquely exclude residual technical correlations. The protocol provides a practical framework for studying correlated noise in warm-vapor radio-frequency optical quantum sensors.