Martin J. Stevens

A superconducting single-photon detector is like a string stretched in the dark. When a single photon hits it, the string momentarily loses its superconductivity, and we hear a click. But until now, the string was always out of tune: the detector performed worse than the material would allow. The reason is the uneven distribution of current across the detector's width.

Two photons fly out of a tiny box, and behave like twins who always wear matching outfits. Turn one, and the other instantly turns the same way, even if they are kilometers apart. This is no magic trick but quantum entanglement: a connection between particles that defies distance. Researchers have built a compact source of such pairs.