[fanf2]

The term of art you want for searchengineering is “time transfer”.

There are a variety of mechanisms:

* fibre links when the labs are close enough

* two-way satellite time transfer, when they are further apart

* in the past, literally carrying an atomic clock from A to B (they had to ask the pilot for precise details of the flight so that they could integrate relativistic effects of the speed and height)

* there’s an example in the talk, of how Essen and Markowitz compared their measurements by using a shared reference, the WWV time signal.