A method for high-precision time delay measurements based on frequency-resolved Hong-Ou-Mandel (HOM) interference is presented. The scheme is applied to weak coherent states and uses an array of single-photon avalanche diode (SPAD) detectors. Unlike standard HOM, the frequency-resolved approach achieves an error of about 10 ps per coincidence even for delays on the order of 4 ps, significantly exceeding the coherence time where the conventional method is ineffective. The results confirm advanced theoretical models that account for the finite frequency resolution of the detectors. Comparison with classical HOM shows agreement with quantum estimation theory and a substantial reduction in uncertainty. The gain in accuracy is especially large when the measured delay is much greater than the coherence time.
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 [scientist:Roy Glauber]Roy Glauber[/scientist], and experiments with entangled photons by [scientist:Anton Zeilinger]Anton Zeilinger[/scientist] laid the groundwork for ultra-precise measurements.