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A frequency-tunable source of squeezed light is employed for spectroscopic measurements of atomic cesium. Relative to the usual quantum limit associated with vacuum-state fluctuations, enhanced sensitivity is demonstrated for the detection of Doppler-free resonances in saturation spectroscopy, with an improvement of 3.1 dB directly observed and 3.8 dB inferred after correcting for detector thermal noise.
Polzik et al. (Mon,) studied this question.