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

TL;DR

This paper demonstrates that a single rubidium atom can serve as a quantum jump photodetector to identify photon-level signals embedded in intense sunlight, with potential applications in LIDAR and quantum communications.

Contribution

The work extends quantum jump photodetectors to operate in strong sunlight, enabling detection of weak signals in high-background environments with modeled and experimental validation.

Findings

Successfully detected individual photons embedded in sunlight.

Achieved a channel capacity of 0.5 bits per symbol in sunlight conditions.

Developed a rate-equation model matching experimental results.

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.