later on they say “Currently, the value of m_e has an uncer-tainty of 3.0 parts in 10^10, while the uncertainty in the Rydberg constant is 1.9 part in 10^12. These uncertainties are several orders of magnitude larger than the present realizations of the unit of time of the current SI system (few parts in 10^16) and even further away from the capabilities of optical frequency standards (10^−18 or better). Consequently, Option 3 is not practical in the current state of science and technology.” 

 i am imagining these metrologists saying, “oh it would be *so* elegant if we could define it this way! sadly, that would be a million times worse…” 

 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 

 seriously high-quality jargon

 “In addition to continued advances in cavity performance mentioned earlier, there are efforts in parallel to develop novel measurement protocols that mitigate the limitations caused by reference cavity noise, such as”

get this

“zero-dead time interrogation, correlation spectroscopy, and dynamic decoupling of laser phase noise in compound atomic clocks.” 

 something something retro-encabulator