Quantum Clocks Sense Gravitational Time Dilation express
Time flows slower near the ground than on the roof: this was predicted by Einstein in his theory of relativity. To spot the tiny difference, physicists propose using a pair of photons, linked like voices in a duet. The particles are sent into two light traps at different heights, where they wait while time diverges for them. Then they're released and blended in a Hong–Ou–Mandel device. If the photons are in sync, they cancel each other out, and the detector stays silent, like two tuning forks in unison. But gravity shifts the light's frequency (color), breaking the harmony. To amplify the effect, instead of a pair, they use an ensemble of many particles. Then even the slightest off-key note is more noticeable — a choir is louder than a soloist.
This approach brings the test of quantum physics in curved spacetime right into the lab.
Over a year, the clock-time difference between a basement and an attic is less than a billionth of a second — the new method makes it noticeable in just a couple of seconds.
In the movie "Interstellar," time slows dramatically near a black hole; here, it's a modest but real laboratory echo.