An Atomic Sensor for Direct Detection of Weak Microwave Signals

This paper demonstrates direct detection of weak signals at microwave frequencies based on parametric frequency conversion. The atomic medium is optically pumped by a resonant light field and prepared in a coherent atomic superposition by a weak microwave field. The coherent atomic superposition causes a parametric modulation of the polarization of a probe light field at the microwave field frequency. An upper limit magnetic field component sensitivity of 1.2(1.0) pT/Hz^1/2, corresponding to 3.7(3.1)-μV/cm/Hz^1/2 electric field component sensitivity, is achieved at ~6.835 GHz with a 33-mm³ vapor cell.