Detection of spin coherence in cold atoms via Faraday rotation fluctuations

We report noninvasive detection of spin coherence in a collection of Raman-driven cold atoms using dispersive Faraday rotation fluctuation measurements, which opens possibilities of probing spin correlations in quantum gases and other similar systems. We demonstrate five orders of magnitude enhancement of the measured signal strength as compared to traditional spin noise spectroscopy with thermal atoms in equilibrium. Our observations are in good agreement with the comprehensive theoretical modeling of the driven atoms at various temperatures. The extracted spin relaxation rate of cold rubidium atoms with atom number density ∼10 ⁹/cm³ is of the order of 3 ×10 ³s⁻¹ at 150 μ K , two orders of magnitude less than 3 ×10 ⁵s⁻¹ of that of a thermal atomic vapor with atom number density ∼10 1²/cm³ at 373 K.