Cesium 133 clock

The most important modem system of units is the SI system, which is based around seven primary units time (second, abbreviated s), length (meter, m), temperature (Kelvin, K), mass (kilogram, kg), amount of substance (mole, mol), current (Amperes, A) and luminous intensity (candela, cd). The candela is mainly important for characterizing radiation sources such as light bulbs. Physical artifacts such as the platinum-iridium bar mentioned above no longer define most of the primary units. Instead, most of the definitions rely on fundamental physical properties, which are more readily reproduced. For example, the second is defined in terms of the frequency of microwave radiation that causes atoms of the isotope cesium-133 to absorb energy. This frequency is defined to be 9,192,631,770 cycles per second (Hertz) —in other words, an instrument which counts 9,192,631,770 cycles of this wave will have measured exactly one second. Commercially available cesium clocks use this principle, and are accurate to a few parts in 1014. 

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. 

Realisation of an optical clock requires first of all a narrow and stable resonance as is the case for a trapped indium ion - and, secondly, precise frequency determination in comparison with the cesium clock. We have performed two measurements of the absolute frequency of the n In+ —> 5s5p clock... 

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. 

However, the reign of the cesium-based superclocks may be over very soon. A team led by physicist John C. Berquist at NIST has developed an atomic clock, called the NIST optical clock, that is based on the interactions between ultraviolet light and a single mercury atom. Studies by the Berquist group show that the new optical clock is 10 times more precise than the cesium clock. This means that the optical clock would have an error of only 0.1 second in 70 million years of continuous operation. The search is now underway for an atom that might provide even more precise time measurements than mercury. 

With fast electronic counters, frequencies up to a few gigahertz can be measured directly and compared with a calibrated frequency standard, derived from the cesium clock, which is still the primary frequency standard [1316]. For higher frequencies a heterodyne technique is used, where the unknown frequency Vx is mixed with an appropriate multiple tmvr of the reference frequency vr (m = 1,2, 3,...). The integer m is chosen such that the difference frequency Av = - tmvr at the output... 

The optical frequency chains discussed in the previous section are difficult to build. Many lasers and optical harmonic generators have to be phase-locked and frequency-stabilized, and the whole setup can easily fill a large laboratory. Furthermore, each of these chains is restricted to a single optical frequency, which is linked to the cesium clock, just like the present frequency standard. 

Recently, a new technique has been developed [1323] that allows the direct comparison of widely different reference frequencies and thus considerably simplifies the frequency chain from the cesium clock to optical frequencies by reducing it to a single step. Its basic principle can be understood as follows (Fig. 9.91) The frequency spectrum of a mode-locked continuous laser emitting a regular train of short pulses with repetition rate 1/AT consists of a comb of equally spaced frequency components (the modes of the laser resonator). The spectral width Aw = 2jt/T of this comb spectrum depends on the temporal width T/Ar of the laser pulses (Fourier theorem). Using femtosecond pulses from a Tusapphire Kerr lens mode-locked laser, the comb spectrum extends over more than 30 THz. 

In the field of metrology a big step forward was the use of frequency combs from cw mode-locked femtosecond lasers. It is now possible to directly compare the microwave frequency of the cesium clock with optical frequencies, and it turns out that the stability and the absolute accuracy of frequency measurements in the optical range using frequency-stabilized lasers greatly surpasses that of the cesium clock. Such frequency combs also allow the synchronization of two independent femtosecond lasers. 

The 133 cesium clock is based on the two hyperfine levels of the spin of Its valence electron which may be In the ground state (singlet, opposing the spin direction of the nucleus) or excited by 6.1x10 J (doublet, parallel to the spin of the nucleus). The electromagnetic radiation that can achieve the transition has a frequency of 9,192,631,770 Hz (wavelength of %3.26 cm). The 133 cesium clock Is reproducible to 1-2 ms per year (1 part in lO ). The feedback arrangement that controls a quartz clock to this stability Is shown below (simplified schematic). 

The cesium clock provides our present time or frequency standard that is based on the hfs transition 7% 1/2 (F = 3- F = 4) in the electronic ground state of Cs. The accuracy of the frequency standard depends... 

Recently, a new technique has been developed [14.156] that allows the direct comparison of widely different reference frequencies and thus considerably simplifies the frequency chain from the cesium clock to optical frequencies by reducing it to a single step. Its basic principle can be understood as follows (Fig. 14.60) ... 

The mode spacing Avm is locked to the cesium clock frequency vcs in such a way that vcs = m - Avm- Therefore the frequency difference N Ay between N comb modes is precisely known. The fourth harmonics of a stabilized HeNe laser at k = 3.39p m is now compared with the frequency of a mode of the frequency comb. The beat frequency fc between 4/HeNe nd the mode frequency f is measured by a frequency counter. A dye laser at 486 nm is frequency doubled and excites the two-photon transition 15-25 of the hydrogen atom. Its frequency is locked to the doubled frequency of a diode laser, which is in turn locked to a mode of the frequency comb. The dye laser frequency is not exactly 7 times the HeNe laser frequency /, but differs from 7/ by —2A/. A frequency divider chain (see Fig. 14.59) generates from / and 1 f — lAf the frequency 4/—A/, which is just half of the sum /-f7/ —2A/. This frequency, which corresponds to a wavelength of 851 nm, is compared with that of a mode of the frequency comb. The difference frequency fc2 is measured by a counter. 

Measure The Journal of Measurement Science 2, no. 4 (December, 2007) 74-89. Explains the history and principles behind the NIST frequency standards. Includes references, figures, tables, and color photographs of NIST cesium clocks. 

In 1967 the 13th General Conference on Weights and Measures defined the second on the basis of vibrations of the cesium atom. From that moment the world s timekeeping system no longer had an astronomical basis. A second is instead defined as the duration of9192 631770 cycles of microwave light absorbed or emitted by the hyperfine transition of cesium atoms. The cesium clock is the time standard of our time. 

The standard SI unit of time is the second (s). The second was originally defined as 1/60 of a minute (min), which in turn was defined as 1/60 of an hour (h), which was defined as 1/24 of a day. However, the length of a day varies slightly because the speed of Earth s rotation is not perfectly constant. As a result, a new definition was required. Today, a second is defined by an atomic standard using a cesium clock (Figure 2-4). 

FIGURE 2-4 The standard of time, the second, is defined using a cesium clock. 

Fig. 14.32. Scheme of a frequency chain connecting the I2-stabilized HeNe laser to stabilized CO2 lasers that are controlled by the cesium clock [14.128a]... 

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