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 (Formula presented.) s averaging. The detection scheme proves to be the most resource-efficient possible, saturating (Formula presented.) 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.