Conditional measurements on the undriven mode of a two-mode cavity QED systemprepare a coherent superposition of ground states which generate quantum beats.The continuous system drive induces decoherence through the phase interruptionsfrom Rayleigh scattering, which manifests as a decrease of the beat amplitudeand an increase of the frequency of oscillation. We report recent experimentsthat implement a simple feedback mechanism to protect the quantum beat. Wecontinuously drive the system until a photon is detected, heralding thepresence of a coherent superposition. We then turn off the drive and let thesuperposition evolve in the dark, protecting it against decoherence. At a latertime we reinstate the drive to measure the amplitude, phase, and frequency ofthe beats. The amplitude can increase by more than fifty percent, while thefrequency is unchanged by the feedback.
展开▼
