Optical frequency standard trapped ions

A state-of-the-art example [28] for trapped-ion optical frequency standards is the case of a laser with a line-width of less than 25 Hz locked to a electric quadrupole transition at 282 nm in a single laser-cooled Hg ion. The inherent stability of this trap based on the radiative lifetime of the metastable upper level of this transition is calculated to be about 1,5 x 10" x[29]. This is an exceptional example - in most cases as yet, the lasers used as oscillators for optical frequency standards based on ion traps often do not have a stability which matches the spectral sharpness of the trapped ion reference resonance. 

As shown in figure 2, examples of a stable optical frequency standard of similar performance to the best microwave oscillators mentioned above are those with Ba [30] and Hg [31] trapped in Paul and Penning traps. In many cases, uncertainties due to coherent ion motion present limitations similar to those of the microwave standards discussed above. 

The most promising apphcation of trapped ions is then-use in optical frequency standards. These devices use ion traps that resonate at optical, rather than microwave frequencies. The resonance frequency of these devices is about 10 Hz for example, the Hg+ ion has an optical wavelength of just 282 nm. Although long observation times are difflcult with this approach, experiments have shown that a resonance width of 1 Hz might eventually be possible. This means that the g of an optical fiequency standard could reach 10, several orders of magnitude higher than the best microwave experiments. 

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. 

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