NIST-F1 Cesium Fountain Atomic Clock The Primary Time and Frequency Standard for the United States f = 9,192,631,770 Hz T ~ K The base unit of time is officially defined as "the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium- 133 atom".

Cesium 133 energy diagram

Commercial standard: 1 in accuracy

Cesium Beam ClockCesium Fountain Clock » Hot atoms, heated in oven to ~350K » Cold atoms, laser cooled to ~2 x 10-6K » Horizontal launch at ~250m/s (velocity of gaseous cesium atoms at 350K) » Vertical launch at 2 to 5 m/s (by laser beams) » Using magnets, atoms in two atomic ground states are separated. Atoms in the desired state enter the microwave cavity; the other atoms are discarded. » The atoms are all pumped (using laser pumping) into the desired state. All atoms enter the microwave cavity. » The atoms undergo Ramsey interrogation as they pass through the two parts of the double cavity. Interrogation time: s » The atoms pass through the same microwave cavity on their way up and way down, undergoing Ramsey interrogation. Interrogation time: 0.5 s » Continuous flow of atoms » Pulsed operation