Photon burst detection of single atoms in an optical cavity

Many protocols in atomic physics and quantum information hinge on the ability to detect the presence of neutral atoms. Up to now, two avenues have been favoured: the direct detection of spontaneously emitted photons using high-quality optics, or the observation of changes in light transmission through cavity mirrors due to strong atom-photon coupling. Here, we present an approach that combines these two methods by detecting an atom in a driven cavity mode through the collection of spontaneous emission and forward scattering into an undriven, orthogonally polarized cavity mode. Moderate atom-cavity coupling enhances the signal, enabling the detection of multiple photons from the same atom. This real-time measurement can establish the presence of a single freely moving atom in less than 1 s with more than 99.7% confidence, using coincidence measurements to decrease the rate of false detections.