TITLE: Smallest measurable time increment
QUESTION [4 upvotes]: What is the smallest increment of time which can be measured using modern methods, and is not theoretical?

REPLY [3 votes]: I think it's $10^{-21}$ seconds, a zeptosecond, reference: https://www.ph.tum.de/latest/news/zeptosekunden/?language=en 
" When light strikes electrons in atoms, their state can change unimaginably quickly. Laser physicists in Munich have measured such a phenomenon – namely that of photoionization, in which an electron exits a helium atom after excitation by light – for the first time with zeptosecond precision. A zeptosecond is a trillionth of a billionth of a of a second (10-21 seconds). This is the greatest accuracy of time determination of an event in the microcosm ever achieved, as well as the first absolute determination of the timescale of photoionization...."
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