there’s a table of candidate optical standards, ranging from 0.4 to 1.1 THz, using Hg or Al or Yb or Sr or Ca, in trapped ion optical clocks or neutral atom optical lattice clocks

it says 4 labs have Yb lattices and 3 have Sr lattices in production contributing to TAI

then one lab using oldskool rubidium, i’m pretty sure that’s the USNO’s rubidium fountains, which are unusual in (a) using rubidium instead of caesium and (b) operating continuously rather than for calibrating other clocks