Principles of the Cesium Clock

Principles of the Cesium Clock

In a cesium clock like these, liquid cesium is heated to a gaseous state in an oven. A hole in the oven allows the atoms to escape at high speed. These particles pass between two electromagnets whose field causes the atoms to separate into two beams, depending on which spin energy state they are in. Those in the lower energy state pass through the ends of a U-shaped cavity in which they are irradiated by microwaves of 3.26-cm wavelength.The absorption of these microwaves excite transitions of many of the atoms from the lower to the higher energy state. The beam continues through another pair of electromagnets, whose field again divides up the beam. Those atoms in the higher energy state strike a hot wire, which ionizes them. Thereafter, a mass spectrometer selects only the cesium atoms from any impurities and directs them onto an electron multiplier.
The frequency of the microwaves is adjusted until the electron multiplier output current is maximized, constituting the measurement of the atoms'resonance frequency. This frequency is electronically divided down and used in a feedback control circuit ("servo-loop") to keep a quartz crystal oscillator locked to a frequency of 5 megahertz (MHz), which is the actual output of the clock, along with a one-pulse-per-second signal. The entire apparatus is shielded from external magnetic fields.
The first method for accurately measuring hyperfine frequencies by molecular beam resonance was developed by I.I. Rabi and his associates in 1937 at Columbia University. The first molecular clock, using ammonia gas, was built by H. Lyons at the National Bureau of Standards in 1949. The first atomic clock, a cesium-beam frequency standard, was built starting in 1949 and was first operated in 1951, resulting in the first direct measurements of cesium hyperfine frequencies. The clock, called NBS-1, was the first in a series that is now up to 7 (NBS is now the National Institute of Standards and Technology, so their latest standard is called NIST-7). Between 1953 and 1955, L. Essen and J.V.L. Parry of the National Physical Laboratory (NPL) in Teddington, England built a cesium atomic clock. These atomic clocks were later refined by others, notably N.F. Ramsey and J.R. Zacharias, and are the primary standards referred to above. The first trapped-ion standard was developed at the NIST in 1985. The first atomic fountain clock was built in 1995 at the Paris Observatory, France.
The first atomic clock timescale was established in 1955 at NPL, though it differed signicantly from the astronomical ephemeris timescale established by Dr. William Markowitz, head of Time Service at the U.S. Naval Observatory. Markowitz and Essen collaborated on the determination of a best value for the cesium hyperfine frequency, and in 1958 they reported a value of 9,192,631,770 hertz (cycles/second).