Atomic clocks, cesium

Abstract. We present a frequency comparison and an absolute frequency measurement of two independent -stabilized frequency-doubled Nd YAG lasers at 532 nm, one set up at the Institute of Laser Physics, Novosibirsk, Russia, the other at the Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. The absolute frequency of the l2-stabilized lasers was determined using a CH4-stabilized He-Ne laser as a reference. This laser had been calibrated prior to the measurement by an atomic cesium fountain clock. The frequency chain linking phase-coherently the two frequencies made use of the frequency comb of a Kerr-lens mode-locked Ti sapphire femtosecond laser where the comb mode separation was controlled by a local cesium atomic clock. A new value for the R.(56)32-0 aio component, recommended by the Comite International des Poids et Mesures (CIPM) for the realization of the metre [1], was obtained with reduced uncertainty. Absolute frequencies of the R(56)32-0 and P(54)32-0 iodine absorp tion lines together with the hyperfine line separations were measured. 

The frequency chain works as follows to the second harmonic of the He-Ne laser at 3.39 jum a NaCl OH color center laser at 1.70 pm is phase locked. To the second harmonic of the color center laser a laser diode at 848 nm is then phase locked. This is accomplished by first locking the laser diode to a selected mode of the frequency comb of a Kerr-lens mode-locked Ti sapphire femtosecond laser (Coherent model Mira 900), frequency-broadened in a standard single-mode silica fiber (Newport FS-F), and then controlling the position of the comb in frequency space [21,11]. At the same time the combs mode separation of 76 MHz is controlled by a local cesium atomic clock [22]. With one mode locked to the 4th harmonic of the CH4 standard and at the same time the pulse repetition rate (i.e. the mode separation) fixed [22], the femtosecond frequency comb provides a dense grid of reference frequencies known with the same fractional precision as the He-Ne S tandard [23,21,11]. With this tool a frequency interval of about 37 THz is bridged to lock a laser diode at 946 nm to the frequency comb, positioned n = 482 285 modes to lower frequencies from the initial mode at 848 nm. 

To celebrate the turn of the century the National Institute of Standards and Technology (NIST) in Boulder, Colorado, gave the world a new precision timepiece—a clock so accurate that it will neither gain nor lose a second in 20 million years Called NIST F-l, the new cesium atomic clock is classified as a fountain clock because it tosses spheres of cesium atoms upward inside the device. 

Since the first Cesium-atomic clock based on an atomic beam, different approaches have been developed, justified by the need to reduce the global uncertainty of the system. The search for an appropriate atomic transition is guided by seeking its potentiality to have the narrowest spectral transition possible. Atomic calculations have assisted in making this choice. At the beginning of the atomic time era. 

A transition between two levels in the hyperfine structure of the cesium isotope Cs corresponds to a frequency of 9192 631770 Hz (Hz = Hertz = cycles/second). A cesium atomic clock is a device that measures this frequency with an accuracy of 0.0002 Hz, corresponding to a time measurement accuracy of two nanoseconds per day or one second in 1400000 years. It is the most accurate realization of a unit that mankind has yet achieved. 

Atomic clocks represent one of the basic applications of atom physics, for the precise measurement of time is one of the most important needs of present-day civilization. The most familiar are atomic clocks using a microwave transition in Cs. In 1967, an international standard was introduced for the second 1 second = 9192631770 cycles of the standard Cs transition. Cesium atomic clocks use the magnetic sorting of sublevels in Cs and the method of spatially separated fields (Ramsey 1987) to... 

The metal has recently found application in ion propulsion systems. Cesium is used in atomic clocks, which are accurate to 5 s in 300 years. Its chief compounds are the chloride and the nitrate. 

In 1960 the International Committee of Weights and Measures selected radioactive cesium-137 (with a half-life of about 33 years) as the standard for measuring time. They equated the second with the radiation emitted by a Cs-137 atom that is excited by a small energy source. Thus, the second is now defined as 9,192,631,770 vibrations of the radiation emitted by an atom of Cs-137. There are about 200 atomic clocks around the world that collaborate their efforts to maintain this extremely accurate clock that never needs winding or batteries. 

Cesium-137 is a highly useful radioisotope that emits its radiation at a very steady and controllable rate. This makes it useful as an atomic clock because it is extremely accurate and never needs winding or a new battery. It is also useful as a radiation source for treatment of malignant cancers. Cs-137 has replaced the much more dangerous cobalt-60 as a source of radiation in industry and medicine. 

Cesium is used as a getter in electron tubes. Other applications are in photoelectric cells ion propulsion systems heat transfer fluid in power generators and atomic clocks. The radioactive Cs-37 has prospective applications in sterilization of wheat, flour, and potatoes. 

Abstract. A suitable femtosecond (fs) laser system can provide a broad band comb of stable optical frequencies and thus can serve as an rf/optical coherent link. In this way we have performed a direct comparison of the IS — 2S transition in atomic hydrogen at 121 nm with a cesium fountain clock, built at the LPTF/Paris, to reach an accuracy of 1.9 x 10-14. The same comb-line counting technique was exploited to determine and recalibrate several important optical frequency standards. In particular, the improved measurement of the Cesium Di line is necessary for a more precise determination of the fine structure constant. In addition, several of the best-known optical frequency standards have been recalibrated via the fs method. By creating an octave-spanning frequency comb a single-laser frequency chain has been realized and tested. 

We present a frequency comparison of two independent -stabilized Nd YAG lasers at 532 nm and an absolute frequency measurement of the laser frequencies which were locked to different HFS components of the R(56)32-0 and P (54)32-0 iodine absorption line. The absolute frequencies have been determinded using a phase-coherent frequency chain which links the 12-stabilized laser frequency to a CH4-stabilized He-Ne laser at 3.39 pm. This laser had been calibrated before the measurements against an atomic cesium fountain clock. 

The atom exhibits very regular, hyperfine energy-level transitions and it is possible to count these cycles of energy. In 1967 the General Conference accepted 9,192,631,770 cycles of cesium-133 as the measurement of one second, making the atomic clock the true international timekeeper. The cesium clock is maintained in Boulder, Colorado, in the offices of the National Institute of Standards and Technology (formally the National Bureau of Standards). Its accuracy is one part in 1,000,000,000,000 (10 12). It will not gain or lose a second in 6000 years. 

About forty years after Zacharias s attempts, the atomic cesium fountain clock became a reality. Contemporary cesium clocks keep time with great accuracy. The best cesium clocks are fountain clocks and are accurate to about one second in 20 million years. These clocks keep better time than either the daily rotation of the Earth or the annual revolution of the Earth around the Sun. For this reason, a new definition of the basic unit of time, the second, was adopted in 1967. The second, once defined as 1/ 86,400 of a day, is now defined as 9,192,631,770 periods of the resonance frequency of the Cs atom. Cesium clocks are commercially available and widely used. 

Atomic clocks were invented in the late 1940. Cesium is used in some atomic clocks, the most precise instrument of time-keeping. ROYAL GREENWICH OBSERVATORY, NATIONAL AUDUBON SOCIETY COLLECTION/PHOTO RESEARCHERS, INC. 

An important use of cesium is in an atomic clock. An atomic clock is the most precise method now available for measuring time. Here is how an atomic clock works ... 

Cesium is used in photovoltaic cells, vacuum tubes, scintillation counters, and atomic clocks. 

Cesium was discovered in 1860 by Robert Bunsen and Gustaff Kirchoff. It is used in the most accurate atomic clocks. Cesium melts at 28.4°C (just below body temperature) and occurs in Earth s crust at 2.6 ppm. 

The name comes from the Latin caesius, meaning sky blue. Cesium was discovered by Robert Wilhelm Bunsen (1811-1899) and Gustav Robert Kirchhoff (1824-1887) in 1860. They used a spectroscope on a drop of mineral water and saw previously unnoted blue lines in the spectra. Cesium is rare, but it is used in photoelectric cells and as a hydrogenation catalyst. It is also used in some atomic clocks. 

Chapter 4 examines the heavier alkali metals—rubidium, cesium, and francium. Francium is a radioactive, rare element its longest-lived isotope has a half-life of only 22 minutes. The relative abundances of rubidium and cesium are much less than the abundances of lithium, sodium, or potassium, yet rubidium and cesium find important applications in atomic clocks and laser technology. 

In this chapter the reader will learn about the syntheses of rubidium, cesium, and francium in stars, the chemistry of rubidium and cesium, the use of cesium in atomic clocks, and other applications of rubidium and cesium. Too little francium exists for it to have any practical uses. 

NIST-F1, the nation s primary time and frequency standard, is a cesium fountain atomic clock developed at the NIST laboratories in Boulder, Colorado. (Geoffrey Wheeler Photography/NIST)... 

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