A ruler made of light: that's one way to describe a new method for measuring tiny time delays. In 1987, Chong Ki Hong, Zhe-Yu Ou, and Leonard Mandel noticed that when two indistinguishable photons hit a beam splitter simultaneously, they always exit together from the same port. This Hong-Ou-Mandel interference became a razor-sharp tool for measuring time. But it has a limit: the photons must arrive almost simultaneously, within their coherence time (here, just 0.44 picoseconds). If the delay is larger, the pattern blurs, and the ruler stops working.

Physicists in Mehul Malik's group found a way to extend this ruler. They directed pairs of photons onto a diffraction grating and an array of eight avalanche photodiodes, which record not just the arrival of a photon but also its frequency. Frequency is the color, or energy, of a photon. If photons arrive with different frequencies, they become distinguishable, and interference disappears. But if their frequencies overlap, quantum interference emerges even at large delays. In the experiment, they measured delays up to 4 picoseconds with an uncertainty of about 10 picoseconds per coincidence, which is tens of times larger than the coherence time.

The key idea is that frequency resolution erases the information about which photon arrived first. A conventional detector that doesn't distinguish frequencies sees only a blurred picture. But a spectrally sensitive array splits the light into separate frequency channels, and in each channel, interference is restored. It's like breaking a noisy hall into separate rooms, where you can hear your conversation partner clearly in each one.

Until now, it was believed that measuring such delays required nonclassical light, such as pairs of entangled photons. The new work shows that an ordinary laser, attenuated to the single-photon level, works just as well. This removes the brightness and bandwidth limitations inherent in single-photon sources, making the method practical. It can already be used in quantum ranging, synchronization of remote clocks, and optical coherence tomography, anywhere you need to precisely measure the time light takes to travel.

The future of this ruler of light lies in increasing the number of pixels and improving their temporal resolution. Then we can approach the quantum limit of precision and measure even smaller delays. Perhaps we will learn to measure transverse displacements and other parameters where frequency resolution also lifts degeneracy. In this way, the fundamental physics of interference of indistinguishable photons becomes a practical tool for precise measurements.