Microwave atomic clock

Trapped ion spectroscopy does enable tests of fundamental symmetries, such as of the equivalence principle, made for example by comparing the frequency of a Be "clock with the frequency of a hydrogen maser [26]. In another comparison of microwave atomic clocks, based on Cs and Mg, Godone et al. [27] were able to set new experimental limits on the time variation of various fundamental constants. 

At present, there are many accurate experiments comparing different optical and microwave atomic clocks [4,32-39]. These experiments put stringent limits on the time variation of the different combinations of a, p, and gnuc- As we mentioned above, the limit on the a-variation (16.2) follows from the experiment of Ref. [4] and the limit (16.1) from the assumption of a linear time-dependence of all constants. A detailed discussion of the atomic experiments can be found in recent reviews [40,41]. 

Microwave technology is robnst. Highly stable solid state devices are commercially available that operate from 1-18 GHz. These devices can be locked to atomic clocks or global positioning systems to achieve frequency stability that far exceeds the natural linewidth of the measured rotational transitions. The result is a frequency generation system that requires little to no maintenance over long time periods (months to years), a featnre that is currently impossible to achieve with broadly tunable laser systems. These stand-alone microwave systems, which can be remotely controlled, are technically feasible and can be operated with pnsh-bntton control. ... 

Time The SI base unit of time is the second (s). The standard second is defined by the number of oscillations of microwave radiation absorbed by cooled gaseous cesium atoms in an atomic clock precisely 9,192,631,770 of these oscillations are absorbed in 1 second. 

To summarize the above paragraph, trapped ions can serve as exceptional tools for new and functioning atomic clocks, in both the microwave and optical frequency domains. 

In this section, different and varions geometries nsed for atomic clocks will be presented. It is essential that the efficiencies of these devices be sufficient to yield a pure systan under well-controUed environmental conditions. Different routes have been followed in order to achieve the optimum signal-to-noise ratio, S/N. As mentioned above, two classes of atomic clocks exist these classes are either microwave or optical frequency standards, and the constraints imposed by each upon the trap geometry are different. 

In this Section, we will describe briefly the most recent projects of atomic clocks involving/based on ion traps as described above. The first part concerns micro-wave clocks, while the one following will be dedicated to optical frequency clocks. Performances of atomic standards can be evaluated only by comparison (frequency beatings) with another devices. When a new atomic standard can be presumed to out-perform the norm, it can be evaluated only from the comparison with a second system, which must be build in a similar way. It is worth noting that performances of each scheme depend on the local oscillator a quartz (eventually, cryogenic) oscillator for the microwave range, and a laser for the optical one. 

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... 

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). 

Discuss atomic clocks operating at microwave and optical frequencies (brief operating principles). Give examples of applications where a very high time precision is requued. 

Atomic clocks represent one of the basic applications of atom physics, for the precise measurement of time is one of the most important needs of present-day civilization. The most familiar are atomic clocks using a microwave transition in Cs. In 1967, an international standard was introduced for the second 1 second = 9192631770 cycles of the standard Cs transition. Cesium atomic clocks use the magnetic sorting of sublevels in Cs and the method of spatially separated fields (Ramsey 1987) to... 

The next stage in the development of atomic clocks that utilize the control of atoms will be characterized by the use of optical transitions in atoms, specifically laser-cooled trapped atoms or ions, and the laser synthesis of optical and microwave radiation (Chapter 6). 

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 microwave field of frequency 1,420 megacycles per second (Me/s) is sustained in the storage bulb by the constant entry of hydrogen atoms into the bulb from the incident beam. A tiny pickup probe is inserted into the storage bulb and an electrical current is induced in this probe at the same microwave frequency. This signal is fed into a series of electronic circuits that convert the frequency into timing pulses, or the ticks of the hydrogen maser 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. 

Both microwave and optical frequency standards have benefited greatly from the development of the laser and the methods of laser spectroscopy in atomic physics. In particular, the ability to determine both the internal and external (that is, motion) atomic states with laser light - by laser cooling for example - has opened up the prospect of frequency standards with relative uncertainties below lO, for example, the Cs atomic fountain clock. The best atomic theories in some cases at starting to match in accuracy that of measurement, providing thereby refined values of the fundamental, so-called atomic constants. Even quite practical measurements (such as used in GPS navigation and primary standards of length) have advanced in recent years. 

A possible direct link between the UV frequency of the two-photon 15 25 transition in the H atom and the Cs clock in the microwave range with the optical frequency comb is illustrated in Fig. 9.91. 

The atoms then enter another chamber filled with reflecting microwaves. The frequency of the microwaves (9,192, 631,770 cycles per second) is exactly the same frequency required to excite a cesium atom from its ground state to the next higher energy level. These excited cesium atoms then release electromagnetic radiation in a process known as fluorescence. Electronic circuits maintain the microwave frequency at precisely the right level to keep the cesium atoms moving from one level to the next. One second is equal to 9,192,631,770 of these vibrations. The clock is set to this frequency and can keep accurate time for over a million years. 

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. 

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