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 at room temperature. Their ultimate performance is limited by intrinsic spin noise once technical noise sources are sufficiently suppressed. Reducing and characterizing this noise is therefore essential for improving 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 a pulsed probe readout scheme. The system employs orthogonal pump and probe laser beams, while a static magnetic field defines the sensing frequency and synchronizes the probe pulses with the spin precession. To detect weak spin correlations, we implement a measurement protocol that combines periodic probing, phase cycling, and window-shifted acquisition. Phase cycling suppresses reproducible probe-induced coherent transients, while subtraction of temporally correlated signals acquired within the spin-relaxation time reduces the measured spin-noise level. The observed noise reduction is consistent with temporal spin correlations, including those expected in spin-squeezing protocols, although the present measurements do not uniquely distinguish this interpretation from other correlated-noise mechanisms. The demonstrated measurement protocol provides a practical route toward operating warm-vapor optical quantum sensors closer to their fundamental spin-noise limit and may benefit radio-frequency magnetometry and sensing of weak coherent signals, including emerging dark-matter detection schemes.