primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency, primaarsagedusetalon, milles väljundsageduse reguleerimiseks kasutatakse footonite neeldumist tseesium-133 aatomite kahe hüperfiinse põhioleku vaheliste üleminekute teel
