The official Caltech support site to my local LIGO (Hanford), says down to 1/10,000 the width of the proton charge nucleus. Since that web material gets reviewed by incoming graduate students all the time it is probably correct. The other statistic that amazes me is the vacuum inside the 4km tubes at a pressure lower than interstellar space (and maintained for long periods).
“pressure lower than interstellar space”
I’m intrigued by the idea that we may be creating the profoundest sustained vacuums and lowest temperatures in the entire universe, but there’s bound to be something somewhere that outdoes us humble humans even in those things. Or maybe not.