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.
Original abstract
as written by the authors arXiv:2601.02470
Gravitational time dilation implies that clocks held at different heights accumulate different proper times. We analyze a memory-assisted quantum clock interferometer in which a frequency-bin photonic clock is stored in two vertically separated quantum memories for a controllable duration, such that the joint state evolves in a quantum superposition of two proper times. After retrieval, the photonic modes interfere in a Hong-Ou-Mandel (HOM) interferometer, for which we derive analytic expressions for the resulting multiphoton detection statistics. Extending this HOM-based scheme from entangled photon pairs to frequency-entangled 2N-photon inputs, we show that the proper-time dependent phase is amplified by a factor N, leading to an N-times faster collapse and revival of the interference signal compared with the two-photon case. Incorporating finite memory efficiency and lifetime, we identify regimes where this modulation remains observable. For parameters compatible with demonstrated Rb and Cs memories and achievable optical frequency separations, the first collapse occurs for height differences in the order of 10-100 m with subsecond to few-second storage times, while suitable rare-earth ion and alkali memory combinations can reduce the required height to the few-metre scale. These results establish near-term laboratory conditions for observing entanglement dynamics driven by gravitational time dilation in a photonic platform.
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.