Many protocols in atomic physics and quantum information hinge on the ability to detect the presence of neutral atoms 1-4. Up to now, two avenues have been favoured: the direct detection of spontaneously emitted photons using high-quality optics 5-7 , or the observation of changes in light transmission through cavity mirrors due to strong atom-photon coupling 8-11. 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 µs with more than 99.7% confidence, using coincidence measurements to decrease the rate of false detections. Direct detection of single atoms and molecules through the collection of resonance fluorescence requires excellent optics, very good background rejection and typical integration times of tens of milliseconds, even for trapped atoms 5-7. Faster results are possible with fluorescence burst detection 12 , which looks for above-average count rates over short time intervals 13,14 ; a recent example 15 showed detection of freely falling atoms in 60 µs using highly efficient mirrors and lenses. Alternatively, one can collect fluorescence in an optical cavity with the axis perpendicular to the driving laser, gaining the benefit of Purcell-enhanced emission into the cavity mode 9-11. Experiments based on changes in cavity transmission, which require strong atom-cavity coupling 8 , have achieved singleatom detection times of 20 µs for moving atoms 16,17 , and as low as 10 µs for trapped atoms 18,19. All of these techniques gather data (photon flux) until a targeted confidence level is reached: detection of fluorescence requires the building up of a signal against background, whereas detection through cavity transmission requires the averaging of shot noise until a change in intensity level is discernible. The resultant signalto-noise and signal-to-background ratios set the probability of obtaining a false positive for atom detection. Here, we present a new approach that achieves high-fidelity single-atom detection in a short time. We use a cavity with two modes of orthogonal linear polarization (H and V), while driving the cavity on-axis with light of only one of these polarizations (H), a technique introduced in ref. 20. With a weak magnetic field set parallel to the incident polarization, the light drives π (m = 0) transitions in 85 Rb atoms traversing the cavity mode. An excited atom can return to the ground state in one of two ways: by emitting light of the same polarization (H) through a spontaneous or stimulated emission transition that preserves

We study the light generated by spontaneous emission into a mode of a cavity QED system under weak excitation of the orthogonally polarized mode. Operating in the intermediate regime of cavity QED with comparable coherent and decoherent coupling constants, we find an enhancement of the emission into the undriven cavity mode by more than a factor of 18.5 over that expected by the solid angle subtended by the mode. A model that incorporates three atomic levels and two polarization modes quantitatively explains the observations.