NISTF 1 Cesium Fountain Atomic Clock The Primary

NIST-F 1 Cesium Fountain Atomic Clock The Primary Time and Frequency Standard for the United States f = 9, 192, 631, 770 Hz T ~ 10 -6 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". http: //tf. nist. gov/cesium/fountain. htm

Cesium 133 energy diagram http: //inms-ienm. nrc-cnrc. gc. ca/research/cesium_clock_e. html

Commercial standard: 1 in 1012 accuracy

Cesium Beam Clock » Hot atoms, heated in oven to ~350 K Cesium Fountain Clock » Cold atoms, laser cooled to ~2 x 10 -6 K » Horizontal launch at ~250 m/s » Vertical launch at 2 to 5 m/s (by (velocity of gaseous cesium atoms at laser beams) 350 K) » Using magnets, atoms in two » The atoms are all pumped (using atomic ground states are separated. laser pumping) into the desired state. Atoms in the desired state enter the All atoms enter the microwave cavity; the other atoms are discarded. » The atoms undergo Ramsey interrogation as they pass through the two parts of the double cavity. Interrogation time: 0. 005 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