  Event synchronisation is a ubiquitous task, with applications ranging from 5G
technology to industrial automation and smart power grids. The emergence of
quantum communication networks will further increase the demand for precise
synchronization in the optical and electronic domains, which implies
significant resource overhead, such as the requirement for ultrastable clocks
or additional synchronization lasers. Here we show how temporal correlations of
energy-time entangled photons may be harnessed for synchronisation in quantum
networks. We achieve stable synchronisation jitter <68 ps with as few as 44
correlated detection events per 100-ms data package and demonstrate feasibility
in realistic emulated high-loss link scenarios, including atmospheric
turbulence. In contrast to previous work, this is accomplished without any
external timing reference and only simple crystal oscillators. Our approach
replaces the optical and electronic transmission of timing signals with
classical communication and computer-aided postprocessing. It can be easily
integrated into a wide range of quantum communication networks and could pave
the way to future applications in entanglement-based secure time transmission.
