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Portable clocks are not better than static caesium clocks, and optical clocks which are better than caesium clocks are not portable.

Eh, not smartphone-like portable, sure, but publicly (https://english.kyodonews.net/news/2025/03/c0945eb14bb6-japa...) it can now be fitted on a ship or submarine (and I won't be surprised if there is classified miniturization out there).