  The phase noise of incident light fields can be converted into amplitude
noise via absorption and dispersion effects in media under electromagnetically
induced transparency. The conversion process is sensitive to two-photon
detuning. This effect can be used to perform a novel type of Rydberg
spectroscopy. We present experimental measurements of the phase noise spectrum
(PNS) with Rydberg atoms in a cesium vapor cell. Furthermore, to explore
microwave field measurements, a modulation-enhanced PNS approach is proposed.
