Quantum interference helps measure minuscule light delays. When two identical photons hit a semi-transparent mirror at the same time, they always exit together — like two musicians playing a note in perfect unison. But if one is delayed by picoseconds, the synchrony vanishes, and old detectors stop noticing it.
Physicists added spectroscopy to the detectors: now each one records not just a photon’s arrival, but also its frequency. It’s like your ear distinguishing not only loudness but also pitch, letting you hear dissonance even with a large time gap. Using an array of such detectors, scientists achieved photometric precision down to 10 picoseconds, even when the delay between photons exceeded their coherence time.
The idea of quantum interference of photons was first described by Roy Glauber, and experiments with entangled photons by Anton Zeilinger laid the groundwork for ultra-precise measurements.