Measuring Time: Frequency measurements and related developments in physics by Masatoshi Kajita

Author:Masatoshi Kajita
Language: eng
Format: epub
ISBN: 9780750321242
Publisher: IOP Publishing
Published: 2018-09-17T16:00:00+00:00

The Stark shift induced by, for example, the electric field produced by the probe laser and blackbody radiation, is much less than that for neutral atoms because energy gaps between the different electron states are much larger than those for neutral atoms. Moreover, the second-order Zeeman shift is less than 10−11 G−2; therefore, the Zeeman shift is negligibly small for sub-components where the first-order Zeeman shift is zero (M = 0).

There is a technical difficulty in controlling highly charged ions because the initial kinetic energy is of order 1 MK. Laser cooling of a highly charged ion is not realistic because the electron transition cannot be induced by laser light operating in the optical region. However, Ar13+ ions were cooled in interactions with laser cooled Be+ ions and trapped in a crystal [30].

Precise measurements of nuclear transition frequencies are also of interest. The energy gap between different nuclear states is much larger than that for atomic states because of the strong repulsive force between protons bounded in an area of femtometer size (figure 3.22). Most of the nuclear transition frequencies are in the γ-ray region, where frequency measurements are difficult. Nevertheless, the thorium-299 nucleus (299Th) has a transition in the vacuum ultra-violet (VUV) region (1.89 PHz, 159 nm). Such frequencies can be measured using a frequency comb [31]. With a suitably chosen electron state, this nuclear transition is free from the first-order Zeeman shift and the second-order Stark shift. Therefore, the systematic uncertainty is much lower than that for atomic transition frequencies.

Figure 3.22. Schematic of the nuclear structure. The strong repulsive forces between protons are bounded in an area five orders smaller than atoms.

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