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arXiv · 2512.02521

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

Laura Zarraoa·Tomas Lamich·Sondos Elsehimy·Morgan W. Mitchell·Romain Veyron
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Abstract

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 $10^{10}$ 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 per symbol when sending 150 probe photons per 10 ms time-bin, embedded in 1 nW of sunlight (of order $10^{10}$ photons/s in the visible and near-infrared bands). The demonstration may benefit background-limited applications such as daytime light detection and ranging (LIDAR), remote magnetometry, and free-space classical and quantum optical communications.

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Laura Zarraoa, Tomas Lamich, Sondos Elsehimy, Morgan W. Mitchell, Romain Veyron. 2025-12-02. Detection of photon-level signals embedded in sunlight with an atomic photodetector. https://doi.org/10.1116/5.0314886

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