How do Atomic Clocks work?

The U.S. Government needs the accurate time, so they rely on the National Institute of Standards and Technology (NIST). The Time and Frequency Division, located in Boulder, Colorado, maintains the NIST-F1 Cesium Fountain Clock, the nation's standard of time. This clock measures the natural resonance frequency of cesium atoms, which is used to define the second. The NIST-F1 Clock is so accurate, it neither gains nor loses a second in 20 million years! NIST broadcasts the time over the Continental U.S. via its radio station, WWVB. Within 2000 miles of WWVB, the timepieces on these pages receive this signal and synchronize themselves accordingly. They even automatically adjust for daylight saving time.

(tomado del catálogo firstStreet, August 2004, pag. 9)