Squeezed-light spin noise spectroscopy

We report quantum enhancement of Faraday rotation spin noise spectroscopy by polarization squeezing of the probe beam. Using natural abundance Rb in 100 Torr of N-2 buffer gas and squeezed light from a subthreshold optical parametric oscillator stabilized 20 GHz to the blue of the D-1 resonance, we observe that an input squeezing of 3.0 dB improves the signal-to-noise ratio by 1.5 to 2.6 dB over the combined (power)circle times(number density) ranges (0.5-4.0 mW)circle times(1.5 x 10(12) cm(-3) to 1.3 x 10(13) cm(-3)), covering the ranges used in optimized spin noise spectroscopy experiments. We also show that squeezing improves the tradeoff between statistical sensitivity and broadening effects, a previously unobserved quantum advantage.