Zeptosecond‐Scale Single‐Photon Gyroscope

This work presents an all‐fiber telecom‐range optical gyroscope employing a spontaneous parametric down conversion crystal to produce ultra‐low intensity thermal light by tracing‐out one of the heralded photons. The prototype exhibits a detection limit on photon delay measurements of 249 zs over a 72 s averaging time and 26 zs in differential delay measurements at t=104$t=10^4$ s averaging. The detection scheme proves to be the most resource‐efficient possible, saturating >99.5%$>99.5\%$ of the Cramér–Rao bound. These results are groundbreaking in the context of low‐photon regime quantum metrology, paving the way to novel experimental configurations to bridge quantum optics with special or general relativity.