This paper 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 s averaging. The detection scheme proves to be the most resource-efficient possible, saturating >99.5% of the Cram\'er-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.
@article{arxiv.2402.15385,
title = {Zeptosecond-scale single-photon gyroscope},
author = {Fabrizio Sgobba and Danilo Triggiani and Vincenzo Tamma and Paolo De Natale and Gianluca Gagliardi and Saverio Avino and Luigi Santamaria Amato},
journal= {arXiv preprint arXiv:2402.15385},
year = {2024}
}