The "triggering" of transient maser systems using excited Rydberg atoms as active medium is investigated theoretically and experimentally. We show how the maser pulse characteristics are modified by the interaction of the active medium with small amounts of resonant mm-wave radiation. The phase, polarization, and emission delay of the radiated field are modified by this triggering: this can be used to detect very small powers of mm-or sub-mm-wave radiation. A complete discussion of the quantum and thermal noise of these detectors is presented. A demonstration of the feasibility of this technique is reported. Triggering signals corresponding to a detectivity of ${3\times 10}^{-17\mathrm{}}$ ${\mathrm{W}/\mathrm{H}\mathrm{z}}^{1/2}$ at 108 GHz ($\lambda =2.8\mathrm{}$ mm) have been measured at room temperature. The quantum-noise limit—corresponding to ${6\times 10}^{-19\mathrm{}}$ ${\mathrm{W}/\mathrm{H}\mathrm{z}}^{1/2}$—should be reached with liquid-helium-temperature-cooled detectors.

• Received 16 March 1982

DOI:https://doi.org/10.1103/PhysRevA.27.2065