Two-photon gateway in one-atom cavity quantum electrodynamics

The authors report on the direct observation of a higher-order dressed state in the transmission spectrum of an optical cavity QED system. A single trapped rubidium atom is coupled to the mode of a high-finesse optical resonator. When the system is probed at low input power, only single photon transitions are driven and the transmission spectrum shows the normal modes. By increasing the probe power, however, photon pairs start to excite the two-photon resonance of the higher-order dressed states, which results in an additional resonance at a characteristic frequency in the transmission spectrum. At the frequency of the two-photon resonance the transmission increases quadratically with input power, in a regime where a classical field assumption predicts a linear increase. In a second experiment, a photon correlation measurement using a Hanbury Brown and Twiss configuration demonstrates that the system transforms an incoming random stream of laser photons into a correlated beam of output photons, thereby acting as a two-photon gateway. The Poissonian distributed input light changes into super-Poissonian, bunched light at the output.