Atomic clocks determine the frequency of an atom or molecule’s transition in one of two ways. An active atomic clock induces a group of atoms in an elevated energy state to drop to a lower energy state, measuring the frequency of radiation emitted by the atoms. A passive atomic clock exposes a group of atoms in a lower energy state to electromagnetic radiation with changing frequency. When a majority of atoms jump to the next energy level, it signals that the correct frequency has been achieved.
Most atomic clocks in use today are passive cesium clocks. The United States National Bureau of Standards (NBS, now National Institute of Standards and Technology, or NIST) established the second as the time radiation would take to go through 9,192,631,770 cycles at the frequency emitted by cesium atoms making the transition from one state to another. Cesium clocks are so accurate that they will be off by only one second after running for 300 million years. [Refer to page 3 for a detailed explanation oh how cesium clocks work].
The atomic clock has led to new and more precise techniques for measuring time and distance. Satellite navigation and positioning systems such as the Global Positioning System rely on atomic clocks. Astronomers use atomic clocks to measure the amazingly regular cycles of spinning astronomical objects called millisecond pulsars. Atomic clocks helped support German-American physicist Albert Einstein’s theory of relativity by showing that the passage of time appeared to change with speed. The U.S. National Aeronautics and Space Administration (NASA) uses atomic clocks to time their transmission to space probes.
American physicist Isidor Rabi and his associates at Columbia University built the first apparatus to measure radiation frequencies. The NBS built the first molecular clock, using ammonia gas, in 1949. American physicist Norman Ramsey today built the first model of the cesium clock in use today in 1957.
A beam of cesium atoms emerges from an oven and passes through an inhomogeneous magnet A, which deflects atoms either upward or downward according to their quantum states. After passing through slit S, the atoms continue into a second inhomogeneous magnet B, where they follow the paths indicated by broken lines and are lost to the beam. If an alternating electromagnetic field of frequency v0 is applied to the beam as it traverses the centre region C, transitions between quantum states will occur. The atoms will then follow the solid lines in the diagram and strike a tungsten-wire detector, which gives electric signals in proportion to the number of cesium atoms striking the wire.
Of course, contrary to popular belief, an atomic clock is NOT the most accurate clock on Earth. The most accurate clocks on Earth are nuclear clocks, which are able to detect electromagnetic changes of 1/1014, making them 1000 times more accurate than the best atomic clock.