Cesium atomic frequency standard

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. 

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. 

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)... 

The Rydberg constant is evaluated by comparison of theory and experiment for energy levels in hydrogen. There have been advances in both experiment and theory for the transition frequencies. The most recent experiments calibrate the measured frequency by a chain of comparisons that link to the cesium atomic standard for the second which provides a significant improvement in accuracy compared to earlier methods. Results of these measurements are listed at the end of this section. The main emphasis in the rest of this section... 

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. 

Up to now the hyperfine transition in the ground state of the Cs-atom at 9.192 GHz represents the accepted frequency standard. An alternative to the cesium atomic fountain is the dark resonance of Cs atoms in a cell when a coherent dark state of the hyperfine levels is realized where the optical transition is excited by a frequency modulated laser with a modulation frequency which matches the hyperfine splitting in the Cs ground state. This modulation frequency can be used for the stabilization of the microwave which modulates the laser output. Since the dark resonance is very narrow, the uncertainty of the stable frequency is small. 

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... 

For more developments and technical details, see R.E. Bechler, R.C. Mockler, and J.M. Richardson, Cesium Beam Atomic Time and Frequency Standards, Metrologia, 1 114-131, July 1965. 

Cesium oscillators are primary frequency standards since the SI second is defined using the resonance frequency of the cesium atom ( Cs), which is 9,192,631,770 Hz. A properly working cesium oscillator should be close to its nominal frequency without adjustment, and there should be no change in frequency due to aging. 

The current state-of-the-art in cesium technology is the cesium fountain oscillator, named after its fountain-like movement of cesium atoms. A cesium fountain named NIST-Fl serves as the primary standard of time and frequency for the United States. 

Fig.18.15. Schematic diagram of cesium atomic beam frequency standard.

When this work was carried out, the atomic clocks at the N.P.L. and N.B.S. (Mockler et al. 1960) agreed to within 1 part in 10 and had a substantially higher accuracy than the time interval determined from the mean solar day. Thus in 1964 the International Committee of Weights and Measures adopted the cesium clock as the standard of frequency and time, defining the second as the time interval which contains exactly 9 192 631 770 cycles of the cesium hyperfine frequency in zero magnetic field. 

Time The SI base unit for time is the second (s). The frequency of microwave radiation given off by a cesium-133 atom is the physical standard used to establish the length of a second. Cesium clocks are more reliable than the clocks and stopwatches that you use to measure time. For ordinary tasks, a second is a short amount of time. Many chemical reactions take place in less than a second. To better describe the range of possible measurements, scientists add prefixes to the base units. This task is made easier because the metric system is a decimal system. The prefixes in Table 2-2 are based on multiples, or factors, of ten. These prefixes can be used with all SI units. In Section 2.2, you will learn to express quantities such as 0.000 000 015 s in scientific notation, which also is based on multiples of ten. 

Time The SI base unit for time is the second (s). The physical standard used to define the second is the frequency of the radiation given off by a cesium-133 atom. Cesium-based clocks are used when highly accurate timekeeping is required. For everyday tasks, a second seems like a short amount of time. In chemistry, however, many chemical reactions take place within a fraction of a second. 

The combination of lasers and microwave sources also plays a very important role in metrology. The frequency of 473 THz of the iodine-stabilized HeNe laser at A = 633 nm was measured directly against the cesium standard of time with a chain of lasers and klystrons starting up from a Cs atomic clock. The authors give a total uncertainty of the... 

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