Cesium laser cooling

Ultracold neutral plasmas may be produced by laser cooling and trapping of different types of neutral atoms [105] such as calcium, strontium, rubidium, cesium etc., by photoionizing Bose condensates [106] and also by spontaneous ionization of dense Rydberg atoms [107,108]. A review on ultracold neutral plasmas due to Killan et al. [61] gives an excellent disposition on the subject. 

Over two hundred years ago the work of Charles and Gay-Lussac led to the suspicion that an absolute low temperature exists for matter. In recent years scientists have come very close to cooling matter to 0 K. The latest low-temperature record was achieved at the University of Colorado in Boulder when a team of scientists led by Carl Wieman reported that they had cooled a sample containing 2 X 107 cesium atoms to 1.1 X 10-6 K, about one-millionth of a degree above absolute zero. This record-low temperature was achieved by a technique known as laser cooling, in which a laser beam is directed against a beam of individual atoms, dramatically slowing the movement of the... 

Other Earth-based research into laser cooling of cesium may result in higher and higher accuracies of timekeeping standards. Such precision could lead to more rigorous tests of general relativity and help understand variations in pulsars. New alkaline earth-based materials will also continue to be explored and developed. 

C. Salomon, J. Dalibard, W.D. Phillips, A. Clairon, S. GueUati, Laser cooling of cesium atoms below 3 pK. Europhys. Lett. 12, 683 (1990)... 

Time The si base unit of time is the second (s), which is now based on an atomic standard. The most recent version of the atomic clock is accnrate to within 1 second in 20 million years The atomic clock measures the oscillations of microwave radiation absorbed by gaseous cesium atoms cooled to around 10 K I second is defined as 9,192,631,770 of these oscillations. Chemists now nse lasers to measure the speed of extremely fast reactions that occur in a few picoseconds (10 s) or femtoseconds (10-15 s). 

Ghezah S, Laurent P, Lea SN, Clairon A. (1996) An experimental study of the spin-exchange frequency shift in a laser-cooled cesium fountain frequency standard. Europhys. Lett. 36 25-30. 

The 0 of a cesium fountain is about 10, or about 100 times higherthanatraditional cesiumbeam. Although the resonance frequency is the same, the resonance width is much narrower (< 1 Hz), due to the longer observation times made possible by the combination of laser cooling and the fountain design. The combined frequency uncertainty of NIST-Fl is estimated at <2 x 10 . ... 

An ensemble of cold Rydberg atoms is easily obtained after laser excitation of a cold atomic cloud, as those performed in a Gs or lib vapor-cell magnetooptical trap, at a temperature of 135 /jK or 300 fiK respectively. In the case of cesium (for the experiments performed at Laboratoire Aime Cotton) or rubidium (for the experiment performed at the University of Virginia), the atoms p-excited by the cooling lasers are Rydberg-excited by using a laser pulse provided by a dye laser pumped by the third harmonic of a Nd YAG laser... 

In 1995, the first caesium fountain atomic clock was constructed at the Paris Observatory in France. A fountain clock, NIST-Fl, was introduced in 1999 in the US to function as the country s primary time and frequency standard NIST-Fl is accurate to within one second in 20 x 10 years. While earlier caesium clocks observed Cs atoms at ambient temperatures, caesium fountain clocks use lasers to slow down and cool the atoms to temperatures approaching 0 K. For an on-line demonstration of how NIST-Fl works, go to the website http //tf.nist.gov/cesium/fountain.htm. Current atomic clock research is focusing on instruments based on optical transitions of neutral atoms or of a single ion (e.g. Sr ). Progress in this area became viable after 1999 when optical counters based on femtosecond lasers (see Box 26.2) became available. 

A favorable alternative to dye lasers is a GaAs diode laser, which can cool rubidium or cesium atoms [1127-1129] and also metastable noble gas atoms, such as He or Ar [1130]. The experimental expenditures are greatly reduced since the GaAs laser is much less expensive than an argon laser plus dye-laser combination. Furthermore, the frequency modulation is more readily realized with a diode laser than with a dye laser. 

A cesium fountain works by releasing a gas of cesium atoms into a vacuum chamber. Six infrared laser beams are directed at right angles to each other at the center of the chamber. The lasers gently push the cesium atoms together into a ball. In the process of creating this ball, the lasers slow down the movement of the atoms and cool them to temperatures a few thousandths of a degree above absolute zero. This reduces their thermal velocity to a few centimeters per second. 

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