Clock, atomic

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. 

N. F. Ramsey (Harvard) invention of the separated oscillatory fields method and its use in the hydrogen maser and other atomic clocks. 

Charles Hard Townes operates an atomic clock. (Corbis-Bettmann)... 

There are also some important differences in the technology required to realise bistatic radar. In monostatic radar synchronisation between transmission and reception is done via a stable source, usually a local oscillator. In bistatic radar the separation of transmitter and receiver makes this much more difficult. An equivalent situation has to be achieved and this is done either via synchronised atomic clocks, a signal such as GPS or by reception of a reference signal received directly from the transmitter. The latter technique is typically used in PCL systems and we shall return to this later. 

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. 

The black body shifts are not confined to Rydberg atoms, but also alter the frequency of atomic clock transitions. Itano et al. have shown that the Cs 9 GHz ground state hyperfine interval, the definition of the second, is increased by one part in 1014 when the temperature is raised from 0 K to 300 K.29. 

The first frequency measurement of the 15 — 25 resonance made use of a transportable ClU-stabilized HeNe infrared frequency standard at 88 THz [24], built at the Institute of Laser Physics in Novosibirsk/Russia. For calibration it was transported repeatedly to the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig/Germany where it could be compared with a Cs atomic clock using the PTB frequency chain [25]. 

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. 

International System of Weights and Measures defined the second to be 1/ 31,556,925.9747 for the tropical year 1900 January 0 at 12 hours ephemeris time. 4 In 1960 this standard was accepted by the General Conference with the caveat that work continue toward development of an atomic clock for the accurate measurement of time. 

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. 

This particular quantum transition later rose to stardom in the world of physics. It was the same transition that Rabi and his students, shortly after the war, found to be at odds with Dirac theory, and it was this transition that led to a new value for the electron s magnetic moment. By 1947, this transition had gained prominence from an experiment on earthbound hydrogen atoms soon this same transition would assume galactic significance. Also, this same transition would become the basis for the most accurate atomic clock, which was developed in the late 1950s (discussed in Chapter 18). 

Atomic clocks have their roots in Rabi s magnetic resonance... 

When an atom makes a transition from a high-energy quantum state to a lower energy state, electromagnetic radiation with a definite frequency and a definite period is emitted. When properly detected, this frequency, or period, becomes the ticking of an atomic clock, just as the crystal vibration frequency and the swinging frequency are the inaudible ticks of a quartz clock and a pendulum clock. The frequency emanating from the atom, however, is much less influenced by environmental factors such as temperature, pressure, humidity, and acceleration than are the frequencies from quartz crystals or pendula. Thus, atomic clocks hold inherently the potential for reproducibility, stability, and accuracy. 

Other additional alkali metal vapor applications include Cs and Rb for atomic clocks (, ) and gettering ( ), Na for solar tower heat transfer ( ) and K for a boiler topping cycle (32). 

The Paul trap is a device of considerable importance for high-resolution laser spectroscopy. Moreover, the Paul trap has been proposed as the centrepiece of a new generation of ultra-stable atomic clocks (see, e.g., Itano et al. (1983), Wineland et al. (1984)). Related trap designs have very recently come into prominence from their use in first observation of gaseous Bose-Einstein condensation (Anderson et al. (1995), Collins (1995)). 

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