Optical-Clock-Based Time Scale

Highly accurate and precise timekeeping supports many technologies for $e.g.$ navigation and telecommunication, as well as fundamental science. Nowadays optical clocks are far more stable than older microwave clocks, yet cannot be operated continuously. This study presents a hybrid microwave-optical time scale, in which the optical clock runs intermittently, and importantly explores improving this system aside from its optical clock. The hybrid system offers world-record accuracy, maintaining a fractional frequency uncertainty of 1.45×{}10^-16 at 30 days and reaching the 10^-17 decade at 50 days. This major improvement in accuracy could profoundly influence future architectures.