NIST F1 Clock

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© NIST.

This apparatus, found in the NIST physics laboratory in Boulder, Colorado, houses the primary time and frequency standard of the United States. It is an atomic fountain clock that makes use of two hyperfine levels in the ground state of cesium atoms to define the second. The fountain rises and falls within the vertical cylinder at the top of the image. The optical table and electronics rack to the right of the table contain the laser beams and microwave electronics needed to operate the clock. The uncertainty in the clock frequency is about 5 x 10-16, which corresponds to a gain or loss of about 1 second in 60 million years! (Unit: 5)