Quantum jump photodetector for narrowband photon counting with a single atom

Using a single neutral 87Rb atom held in an optical trap, and "quantum jump" detection of single-photon-initiated state changes, we demonstrate an intrinsically-narrowband single-photon detector, of interest for separating weak signals from strong optical background. Using novel statistical analysis, we measure quantum efficiency of 2.9+-0.2e-3, a record for single-pass quantum jump production, and dark counts of 9+-20e-3counts during passive accumulation plus 1.8+-0.1e-2counts per readout, orders of magnitude below those of traditional single-photon detectors. The 6 detection bandwidth is orders of magnitude narrower than existing atomic filters. Available methods can substantially improve quantum efficiency, dark counts, bandwidth, and tunability.