The frequency that is actually counted with a digital counter in this clock is only 500 MHz (i.e. after a frequency divider, because no counter can be used at t

The frequency that is actually counted with a digital counter in this clock is only 500 MHz (i.e. after a frequency divider, because no counter can be used at the hundreds of THz of an optical signal). Nevertheless, in order to measure a frequency difference between two optical clocks you do not need to count their signals. The optical signals can be mixed in a non-linear optical medium, which will provide a signal whose frequency is equal to the difference between the input frequencies. That signal might have a frequency no greater than 1 GHz, so it might be easy to count with a digital counter. Of course, the smaller the frequency difference is, the longer must be the time used for counting, to get enough significant digits. The laser used in this clock has a frequency around 200 THz (like for optical fiber lasers), i.e. about 2E14 Hz. This choice of frequency allows the use of standard optical fibers to compare the frequencies of different optical clocks, even when they are located at great distances. Mixing the light beams of 2 such lasers, in the case of a 1E-17 frequency difference would give a difference signal with a period of many minutes, which might need to be counted for several days to give an acceptable precision. The time can be reduced by a small factor selecting some harmonic, but it would still be of some days.