Clock synchronization with correlated photons.

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 demands for synchronisation in optical and electronic domains, thus incurring a significant resource overhead, e.g. through the use of ultra-stable clocks or additional synchronisation 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 <50 ps with as few as 36 correlated detection events per 100 ms and demonstrate feasibility in realistic high-loss link scenarios. 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 post-processing. 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.