Cesium Atoms at Work

"...till like a clock worn out with eating time."
John Dryden (1631-1701)

The 1955 Cesium Atomic Clock at the National Physical Laboratory, UK. It kept time to a second in 300 years.

A "cesium(-beam) atomic clock" (or "cesium-beam frequency standard") is a device that uses as a reference the exact frequency of the microwave spectral line emitted by atoms of the metallic element cesium, in particular its isotope of atomic weight 133 ("Cs-133"). The integral of frequency is time, so this frequency, 9,192,631,770 hertz (Hz = cycles/second), provides the fundamental unit of time, which may thus be measured by cesium clocks.

Today, cesium clocks measure frequency with an accuracy of from 2 to 3 parts in 10 to the 14th, i.e. 0.00000000000002 Hz; this corresponds to a time measurement accuracy of 2 nanoseconds per day or one second in 1,400,000 years. It is the most accurate realization of a unit that mankind has yet achieved. A cesium clock operates by exposing cesium atoms to microwaves until they vibrate at one of their resonant frequencies and then counting the corresponding cycles as a measure of time. The frequency involved is that of the energy absorbed from the incident photons when they excite the outermost electron in a cesium atom to jump ("transition") from a lower to a higher orbit.

According to quantum theory, atoms can only exist in certain discrete ("quantized") energy states depending on what orbits about their nuclei are occupied by their electrons. Different transitions are possible; those in question refer to a change in the electron and nuclear spin ("hyperfine") energy level of the lowest set of orbits called the "ground state." Cesium is the best choice of atom for such a measurement because all of its 55 electrons but the outermost are confined to orbits in stable shells of electromagnetic force. Thus, the outermost electron is not disturbed much by the others. The cesium atoms are kept in a very good vacuum of about 10 trillionths of an atmosphere so that the cesium atoms are little affected by other particles. All this means that they radiate in a narrow spectral line whose wavelength or frequency can be accurately determined.

Kinds of Cesium Clocks

Cesium clocks are of two general kinds: a "laboratory (or primary) standard" about as large as a railroad flatcar and a "commercial (or secondary) standard" about as large as a suitcase. Only a few laboratory standards exist; they are used at research labs for frequency measurements of the highest accuracy. Examples are the NIST-7 standard at the National Institute of Standards and Technology (NIST) in Boulder, CO and the atomic fountains at NIST, Physikalisch-Technische Bundesanstalt (PTB), Germany, the Paris Observatory, France, and USNO. Commercial standards, being industrially produced, are cheaper, but still provide state-of-the-art measurement of precise time and time interval. A timing center maintaining an ensemble of such clocks can average their readings to produce a "mean timescale" for scientific and public use.

The U.S. Naval Observatory operates about 70 such cesium clocks, as well as other precision clocks like hydrogen masers, in 18 vaults whose temperature and, usually, humidity are closely controlled in order to minimize perturbations by their environment. The time measurements are made by devices called time-interval counters that compare each clock's time against that of one "Master Clock," whose frequency is steered to match its time to the average of the other clocks. This time is the Observatory's measure of the atomic time called Coordinated Universal Time (UTC). Some cesium clocks are transported to remote locations in order to synchronize other clocks.


USNO Cesium Clocks

Most of the Observatory's cesium clocks are model HP5071A, made by Agilent Technologies, Inc. of Santa Clara, California. With an improved cesium tube and new microprocessor- controlled servo loops, the 5071A vastly outperforms the earlier 5061 cesium frequency standards. The Naval Observatory 5071A's feature HP's optional high-performance cesium beam tube, with accuracy 1 part in 10E12, frequency stability 8 parts in 10 to the 14th, and a time domain stability of < 2 parts in 10 to the 14th with an averaging time of 5 days. Other companies that produce cesium clocks include Datum, Inc. of Beverly, MA and Frequency Electronics, Inc. of Uniondale, NY.


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).

The Second

In 1967, the 13th General Conference on Weights and Measures first defined the International System (SI) unit of time, the second, in terms of atomic time rather than the motion of the Earth. Specifically, a second was defined as the duration of 9,192,631,770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium-133 atoms in their ground state undisturbed by external fields.

The first commercial atomic frequency standards were built by R. Daly of National Company and J. Holloway of Varian Associates. Their cesium tube design was incorporated by L. Cutler and his coworkers at Hewlett-Packard (now Agilent Technologies), Inc. in what would become the largest selling series of commercial standards.

Recent improvements in cesium clock technology include replacement of the state-selection magnets with laser beams, which can select and detect the required transition with greater efficiency and less motion of, and hence less noise from, the radiating atoms.