Cavity quantum electrodynamics (QED) allows the study of light-matter interactions at the most basic level, through precise identification of the coherent and incoherent (dissipative) parts of the system evolution. We present measurements of light from a cavity QED system consisting of a high-finesse optical resonator coupled to a beam of cold Rb atoms. The novelty of the design lies in the interplay of two degenerate and orthogonal polarization modes. One mode (driven) behaves as the canonical cavity mode of the Jaynes-Cummings Hamiltonian, coherently exciting the atoms with a modest coupling strength; the other mode (undriven) collects a small fraction of spontaneously emitted light and provides a probe of the dissipative processes.

We first demonstrate the ability to detect individual atoms passing through the cavity modes in real time by coincidence detection of photons from the undriven mode. Calculation of statistics and correlation functions from the complete photon detection record allows the determination of detection probabilities and the reconstruction of atomic trajectories. We next present evidence of quantum coherence that is created, modied, and measured in the excitation-spontaneous emission cycle. The coherence appears as a long-lived quantum beat at the ground-state Larmor frequency, visible in the intensity autocorrelation function of the undriven mode. Quantum jumps of the atomic state, occurring in between the detections of photons from the cavity, result in substantial changes in the frequency and spectral width of the beats. We present the results of a full quantum Monte Carlo calculation in order to quantitatively explain the measurements.