Spiess, ChristopherChristopherSpiessPaciorek, KarolinaKarolinaPaciorekDöll, Nico LennartNico LennartDöllKrzic, AndrejAndrejKrzicSteinlechner, Fabian OliverFabian OliverSteinlechner2024-04-112024-04-112023https://publica.fraunhofer.de/handle/publica/46599010.1109/CLEO/EUROPE-EQEC57999.2023.102325962-s2.0-85175692598Quantum communication networks exploit single photons to generate secure encryption keys and can have very tight timing requirements [1]. These synchronization requirements are met through highly stable references, but there is ongoing research on using only single photons to enable synchronization [2-4]. The single photons come naturally with the quantum communication protocol and motivate its use for both data and time transfer. As point-to-point quantum communication scenarios rapidly expand to large networks, there is great demand for robust and resource-saving synchronization methods. In this contribution, we show that single photons can be indeed a solid carrier of timing information. We complement other publications [2,5] by describing the performance of the synchronization approach and evaluating it on a 1.7 km free space and a 70 km deployed intercity fiber link. We demonstrate time deviations of 100 ps (1.7 km free-space link) and 174 ps (70 km fiber-link) at integration times of 1 second with only crystal oscillators, which is slightly larger than 54 ps in a lab environment (Fig. 1(a)). Our results provide advances towards a strong, independent, and secure quantum communication network without external hardware and a reliable operation under versatile link scenarios.enconference paper