Cesium fountain clock

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. 

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. 

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. 

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. 

NIST F-l cesium fountain clock with developers Steve Jefferts and Dawn Meekhof. 

M. Niering, R. Holzwarth, J. Reichert, P. Pokasov, T. Udem, M. Weitz, T.W. Hansch, P. Lemonde, G. SantarelU, M. AbgraU, P. Laurent, C. Salomon, A. Clairon, Measurement of the hydrogen IS-2S transition frequency by phase coherent comparison with a microwave cesium fountain clock, Phys. Rev. Lett. 84 (2000) 5496-5499. 

M. Niering et al. Measurement of the hydrogen 1S-2S transition frequency by phase coherent comparison with a microwave cesium fountain clock. Phys. Rev. Lett. 84, 5496 (2000)... 

As shown in Fig. 7 we compared the frequency of the cesium Di line at 895 nm with the 4th harmonic of the methane stabilized He-Ne laser operating at 3.4 pm (/ = 88 THz). The laser that creates the frequency comb, the fourth harmonic generation and the HeNe laser are identical with the systems shown in Fig. 4. However, the HeNe laser was stabilized to a methane transition in this experiment and was used as a frequency reference instead of the Cs fountain clock. The frequency of this laser has been calibrated at the Physikalisch Technische Bundesanstalt Braunschweig/Germany (PTB) and in our own laboratory [51] to within a few parts in 1013. 

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. 

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)... 

One of the most accurate clocks in the world is located at the United States National Institute of Standards and Technology (NIST) in Boulder, Colorado. This cesium fountain atomic clock provides the official time for the United States. The dock is based on the natural resonance frequency of the cesium atom (9,192,631,770 Hz.), which defines the second. 

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. 

Standards in Time Measurement. Within the field of time measurement, standards refer to devices or signals that serve as benchmarks for particular measurements, such as time intervals or frequencies. Standards allow other clocks to be precisely adjusted so that they all keep the same time and can be recalibrated according to the same measure if they should happen to gain or lose time. For example, the National Institute of Standards and Technology s cesium fountain atomic clock (known as the NIST-Fl), located in Boulder, Colorado, is the standard atomic... 

Different atom species, for example, ytterbium and barium, were considered. Mercury offered the best capability, such that RF Unear traps with mercury ions are the only competitors of cesium atomic fountains. A project to place an ensemble of atomic clocks in space, including atomic fountains and clocks involving trapped ions, is being undertaken currently. 

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