Femtosecond Timekeeping: Slip-Free Clockwork for Optical Timescales

Using an optical frequency comb to directly convert a frequency reference to a useable time output will enable tight synchronization of tomorrow's optical-clock networks. Previous frequency-comb systems experienced cycle slips too often to provide true optical timescales. This study combines innovative fiber-comb designs, digital phase locking, and precise phase-determination methods to demonstrate optical clockwork with no phase slips for over a m00exo00exn00ext00exh. Fault-free timekeeping at the femtosecond level over months is a vital step in supplanting the current microwave time standard with an optical standard, and could improve $e.g.$ gravitational geodesy and GPS technology.