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RE: Defining time with atomic fountains
This kind of raises the question as to how the energy of the photon is measured to the precision required to convert it into the desired unit of time.
It feels like the chicken and the egg problem...
The energy of the photon is measured through the frequency of the laser that is adjusted as finely as possible. For photons, energies and frequencies are the same thing, give or take. The precision of the laser of course limits the precision of the full thing.