Photons are superb carriers of information, but they rarely exchange it with one another. In some extreme astrophysical settings, photons interact via electron–positron pairs. But the powers needed to produce the pairs from a vacuum—on the scale of 1029 W/cm2—have never been achieved in a terrestrial lab. So researchers turn instead to atoms as intermediaries.

The conceptually simplest approach is to store one photon’s state in a single atom—or a collection of them—whose response affects the state of another photon. In the past few years, researchers have used, for instance, highly excited (Rydberg) atoms to entangle pairs of photons and build single-photon transistors. In the latter case, a single photon can control the destiny of many, with the optical medium—a cloud of excited atoms—being quickly switched on or off under the control of a gate photon.1 The presence of multiple atoms, however, can be a source...

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