Synchronization of an optical frequency comb and a microwave oscillator with 53 zs/Hz^(1/2) resolution and 10^(-20) -level stability

Precise and stable synchronization between an optical frequency comb(femtosecond mode-locked laser oscillator or microresonator-based comb) and a microwave oscillator is important for various fields including telecommunication, radio astronomy, metrology, and ultrafast X-ray and electron science. Timing detection and synchronization using electro-optic sampling with an interferometer has been actively used for low-noise microwave generation, long-distance timing transfer, comb stabilization, time-of-flight sensing, and laser-microwave synchronization for ultrafast science facilities. Despite its outstanding performance, there has been a discrepancy in synchronization performance of more than 10 dB between the projected shot-noise-limited noise floor and the measured residual noise floor. In this work, we demonstrate the shot-noise-limited performance of an electro-optic timing detector-based comb-microwave synchronization, which enabled an unprecedented residual phase noise floor of -174.5 dBc∕Hz at 8 GHz carrier frequency(i.e., 53 zs∕Hz^(1/2)timing noise floor), integrated rms timing jitter of 88 as(1 Hz to 1 MHz), rms timing drift of 319 as over 12 h, and frequency instability of 3.6 × 10^(-20) over 10,000 s averaging time. We identified that bandpass filtering of the microwave signal and optical pulse repetition-rate multiplication are critical for achieving this performance.