# Synopsis: Out of many atoms, one photon

A gas of excited-state atoms could perform as a single-photon detector.

Devices that count discrete quanta of light could be the building blocks of sophisticated quantum circuits. Most such counters based on single atoms register a photon only half the time, but in a paper appearing in Physical Review Letters, Jens Honer at the University of Stuttgart, Germany, and his colleagues propose a theoretical multi-atom system that could do the job with a nearly $100%$ success rate.

Honer et al.’s idea takes advantage of interactions between Rydberg atoms confined to a small trap. In a Rydberg atom, at least one valence electron is in a highly excited state, circling the nucleus with a large radius that mimics a classical orbit. These excited state atoms interact strongly with one another, such that one Rydberg atom in a trap can block other atoms from being excited—an effect called Rydberg blockade.

Honer et al. consider $N$ atoms in a trap, which behave as a sort of superatom. The superatom has $N$ excited states, with one Rydberg excitation collectively shared among the $N$ atoms. Only one of these excited states interacts with light like a two-level system, while $N-1$ states remain dark. By introducing a second light field that causes dephasing, Honer et al. show that with large fidelity the superatom ends up in one of these $N-1$ dark states, and consequently greatly enhances the chance of photon absorption. At the same time the Rydberg blockade prevents the absorption of multiple photons within one such atom trap.

A series of these atom trap devices could, according to Horner et al., be used to count the photons in a few-photon light stream. – Jessica Thomas

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