Atomic Lock: How to Catch Light in a Solar Storm express
A rubidium atom is like a lock that opens only with one key: a particle of light of a strictly defined color. Even under the fiery rain of the sun, where myriads of colorful 'keys' fall, the lock remains silent. But when the right one strikes, the atom gets excited for an instant, and the detector registers a click. This is how researchers managed to count individual photons against a background billions of times brighter.
The secret is extreme color selectivity: the atom lets through only one in a hundred million solar photons — like a whisper amid a roaring crowd. In the experiment, the atom was placed at the intersection of a laser and sunlight. Each rare click carried a bit of data, enabling signals to be transmitted through the day. The power was mere billionths of a watt, yet enough for stable communication.
The roots of the idea lie in the work of Roy Glauber and Serge Haroche on photon counting. Now this opens the door to daytime laser radars, magnetic sensors, and jamming-resistant optics. Meanwhile, the tiny atom doesn't wear out and can serve forever. Surprising fact: every second, 10¹⁸ photons hit a square millimeter, yet the atom picks out only the right ones — as if from all the sand on a beach we were to pluck a single golden grain.
Original abstract
as written by the authors arXiv:2512.02521
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.
Every second, about 10¹⁸ particles of sunlight hit each square millimeter of Earth's surface. The atom filter screens out all but a tiny fraction — like a sieve that catches only precious grains.