Atomic Lock: How to Catch Light in a Solar Storm ⚡ экспресс
Detecting weak signals in broadband background radiation is an extremely difficult task for photon counting, requiring narrowband filtering with strong out-of-band rejection. It was shown that the quantum jump detector method based on a single rubidium atom can register photon signals submerged in intense sunlight. Individual photons from a narrowband laser were experimentally counted against a solar background of around 10¹⁰ photons/s. A rate-equation model of the atom's internal state dynamics in sunlight was constructed, demonstrating quantitative agreement with experiment. Based on the model, the capacity of a noisy communication channel for transmitting weak coherent states was calculated: 0.5 bits per symbol under conditions of sending 150 probe photons over a 10 ms interval with a background of 1 nW (10¹⁰ photons/s in the visible and near-IR ranges). The results may find application in background-limited tasks such as daytime lidars, remote magnetometry, and free-space optical communication (both classical and quantum).
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.
🎯 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.