Anomalous noise spectra in a spin-exchange-relaxation-free alkali-metal vapor

TL;DR

This study investigates unexpected features in the noise spectra of unpolarized rubidium vapor in the SERF regime, revealing deviations from traditional models and implications for quantum sensing.

Contribution

It provides the first detailed experimental observation of anomalous noise spectra in high-density SERF vapor, confirming recent theoretical models involving hyperfine correlations.

Findings

Observed asymmetric spin-noise distribution at high density and low magnetic field.

Depletion of resonance line power and broad spectral components identified.

Results align with models including hyperfine state correlations.

Abstract

We perform spin-noise spectroscopy on an unpolarized $^{87}\mathrm{Rb}$ vapor in the spin-exchange-relaxation-free (SERF) regime. We observe noise spectral distributions that deviate strongly from Lorentzian models that accurately describe lower-density regimes. For example, at magnetic fields of $\sim 1 \mathrm{\mu T}$ and $^{87}\mathrm{Rb}$ densities $\gtrsim 1 \times 10^{14} \rm{atoms/cm^{3}}$ we observe an asymmetric spin-noise distribution in which the resonance line is depleted by about half its power, with the diverted power becoming a broad spectral component that could be mistaken for optical shot noise. The results are in good agreement with recent models accounting for correlations between the ground hyperfine states. We discuss implications for quantum sensing and absolute noise calibration in spin-squeezing and entanglement detection.