Concurrent Measurement of Photon Shot Noise and Spin-Projection Noise via Spatial Light Modulation in an Operating Atomic Magnetometer

We propose a method for simultaneously measuring photon shot noise (PSN), spin projection noise (SPN), and technical noise in an atomic magnetometer using a spatial light modulator. A digital micromirror device (DMD) is placed behind the cell to modulate the area of the probe beam, thereby enabling the detected optical power and the number of polarized atoms to be varied while keeping the light intensity inside the cell constant—thus avoiding additional perturbations. Based on this experimental approach, we adapt the conventional models and fit the total equivalent magnetic-field noise power spectral density as a function of the modulation area to obtain the key model parameters. Using the effective transverse relaxation time T₂ extracted from the free-induction-decay (FID) signal, the three noise components are then separated. The method is validated on a Bell-Bloom magnetometer operated with a bias magnetic field of 2–3 µT. The results show that our model can simultaneously measure PSN, SPN, and technical noise with high accuracy, and outperforms conventional methods in terms of fitting robustness and goodness of fit. This work provides a practical approach for quantum-noise diagnostics in operating atomic magnetometers without requiring ideal conditions such as weak perturbations or low technical noise.