Quantum protocols for time synchronisation and distribution: a critical assessment

Precise time synchronisation underpins critical infrastructure from telecommunications and financial markets to power grids and scientific metrology. Several families of quantum protocols have been proposed and demonstrated for clock synchronisation and time distribution, exploiting entangled photon pairs, quantum key distribution (QKD) correlations, Hong-Ou-Mandel interference, and entangled clock networks. We critically assess these approaches, reviewing the main quantum time synchronisation (QTS) protocol families, quantifying the gap between theory and experiment, and identifying practical bottlenecks in sources, detectors, and channels. We survey the classical timing landscape from Network Time Protocol (NTP) and GPS to laboratory-grade optical frequency transfer, and compare quantum and classical methods at equivalent maturity. We examine use cases including financial trading, power grids, telecommunications, scientific metrology, and military applications, evaluating whether quantum timing offers a realistic advantage. We show that time transfer, not clock performance, is now the bottleneck for distributed optical timekeeping: the best demonstrated quantum synchronisation uncertainty (2.46 ps, over 20 km of fibre) falls two to three orders of magnitude short of what optical clocks with fractional frequency uncertainties of 10 −18 –10 −19 require. The best deployed link, ELSTAB, calibrates to a few tens of picoseconds in the laboratory and to about 0.2 ns on its 421 km field installation, a further factor of twenty to a hundred behind, while research-grade classical links using optical frequency combs already reach the regime the clocks require, though none is in operational service. Our assessment is that quantum time synchronisation will not replace classical methods for most applications in the near-to-medium future, yet it can complement them. Its near-term value lies in physical-layer security against timing manipulation and integration with quantum communication networks, while closing the synchronisation gap for scientific metrology remains the most critical open challenge.