Light from a single source can be made to 'meet' itself, and then fringes appear on the screen, like ripples from two pebbles dropped into water. Physicists have long known how to do this with two photons, but making four particles of light merge at once was tricky: single-photon sources lost too many particles along the way. Now the team used a quantum dot, a tiny crystal that emits photons one at a time, and a clever scheme that accumulates and releases them at the right moment. As a result, four photons hit the beam splitter simultaneously and created a pattern of fringes.

The pattern was not just a pretty picture. Among the fringes, some 'deep' ones were found: their dark spots remain dark even if the photons differ slightly from each other. This is unexpected: usually any differences blur the interference. But such fringes are very sensitive to extra photons that sometimes come out of the source in pairs. So by measuring the depth of the dark fringes, one can precisely check how 'clean' the source is.

Such sensitivity is useful in quantum metrology — the science of ultra-precise measurements. The analysis showed that the fringes from this source allow measuring the phase of light more accurately than the standard quantum limit. Simply put, we get a ruler with finer divisions than thought possible. And the more photons we can make interfere, the more precise the measurements will be — up to creating a new generation of quantum sensors.