📄 Showing the "Simple" version — "Popular" is not ready yet. Add it to favorites to help prioritize it. Ask for the full read

Two photons meet at a half-silvered mirror. If they arrive at exactly the same time, they always leave together — side by side, like twins who refuse to be parted. This effect, discovered in 1987, became the basis of a quantum ruler for measuring tiny delays. But the ruler had a flaw: as soon as the delay exceeded the lifetime of a single photon, all precision vanished like ripples on water. The instrument saw only coincidences, not the reason for their disappearance.

The authors of the new study made the instrument see more. Instead of merely counting coincidences, they split the light by color, and found distinct bands in the spectrum, reminiscent of the rainbow rings in a soap film. The spacing between the bands revealed the delay between the photons with remarkable precision. Thus, the old ruler was transformed into a spectral comb: it now measures not the mere fact of meeting, but its color.

The method proved surprisingly resilient. Photon losses, mirror vibrations, imperfect optics — the spectral bands merely dimmed slightly but did not shift. It's like determining the distance between mountain peaks by counting lunar craters rather than by the Moon's brightness: clouds may obscure the light, but they won't change the count. Thanks to this robustness, the need for constant calibration disappeared — a rarity for quantum devices.

In a test, they measured the thickness of a transparent plate of 300 micrometers with nanometer precision. The light ruler, which previously worked only over distances of a few microns, now spans hundreds of microns, and promises to go even further. Where bulky laser systems were once required, a pair of entangled photons and a spectrometer now suffice.