Clock Synchronization with Correlated Photons

Event synchronization is a ubiquitous task, with applications ranging from 5G technology to indus-trial 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 sig-nificant resource overhead, such as the requirement for ultrastable clocks or additional synchronization lasers. Here, we show how temporal correlations of photons may be harnessed for synchronization in quantum networks. We achieve stable synchronization 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 exter-nal 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.