Detection of photon-level signals embedded in sunlight with an atomic photodetector

The detection of few-photon signals in a broadband background is an extreme challenge for photon counting, requiring filtering that accepts a narrow range of optical frequencies while strongly rejecting all others. Recent work [Zarraoa et al., Phys. Rev. Res. 6, 033338 (2024)] demonstrated that trapped single atoms can act as low dark-count narrow-band photodetectors. Here, we show that this “quantum jump photodetector” (QJPD) approach can also detect photon-level signals embedded in strong sunlight. Using a single rubidium atom as a QJPD, we count arrivals of individual narrow-band laser photons embedded in sunlight powers of order 1010 photons/s. We derive a rate-equation model for the atom's internal-state dynamics in sunlight and find quantitative agreement with experiment. Using this model, we calculate the channel capacity over a noisy communication channel when sending weak coherent states and detecting them in the presence of sunlight, achieving a representative rate of 0.5 bits/symbol when sending 150 probe photons per 10 ms time bin, embedded in 1 nW of sunlight (of order 1010 photons/s in the visible and near-infrared bands). The demonstration may benefit background-limited applications such as daytime light detection and ranging, remote magnetometry, and free-space classical and quantum optical communications.