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Hydrogenic systems for calculable frequency standards: status and options.

Flowers, J L; Klein, H A; Knight, D J E; Margolis, H S (2001) Hydrogenic systems for calculable frequency standards: status and options. NPL Report. CBTLM 11

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The study of hydrogen and hydrogenic (one-electron) ions is an area of rapid progress and one of great potential for future frequency standards. In 1997, the two-photon 1 S-2S transition in the hydrogen atom was included in the list of approved radiations for the practical realisation of the metre, and since then revolutions in optical frequency metrology have reduced the uncertainty in its frequency by more than an order ofmagnitude, to 1.8 parts in 10 to the power 14.
Hydrogenic systems are simple enough that the frequencies of their transitions can be calculated in terms of the Rydberg constant with an accuracy that can approach or exceed the measurement uncertainty. Transitions in such systems can be thought of as forming a natural frequency scale, and offer the prospect of a set of quantum frequency standards which are directly related to the fundamental constants.
The Rydberg constant is currently best detennined from optical frequency measurements in hydrogen. However, to take full advantage of the recent high accuracy 1S-2S frequency measurement requires
- Improved measurements of other transition frequencies in the hydrogen atom;
- Reduced uncertainty in the quantum electrodynamic (QED) contributions to the energy levels, in particular the two-loop binding corrections;
- An improved value for the proton charge radius.
In He+ and one-electron systems of higher atomic number Z, the two-loop binding corrections are a fractionally larger part of the Lamb shift due to their rapid scaling with Z. Measurements of the Lamb shift in medium-Z hydro genic ions can therefore provide tests of these corrections, and feed in to the theoretical understanding of hydrogen itself. Although both theory and experiment are less accurate at higher Z, there is the potential for a new range of X-ray standards, providing that the QED corrections are wellunderstood and new absolute measurement techniques can be developed.
A number of areas are suggested for future investigation:
- Improving the accuracy of hydrogen 2S-nS/D measurements, contributing to an improved Rydberg constant determination;
- Spectroscopy of cooled, trapped hydrogen atoms;
- Improved calculations of the two-loop binding corrections complete to all orders in Za.
- Lamb shift measurements in medium-Z hydrogenic ions by laser excitation, to check the two-loop binding corrections;
- Experiments to resolve discrepancies arising from the proton charge radius;
- Development of a small portable electron beam ion trap (EBIT) capable of producing high yields of hydro genic ions;
- Development of techniques for absolute measurement of X-ray transitions observed in an EBIT.

Item Type: Report/Guide (NPL Report)
NPL Report No.: CBTLM 11
Subjects: Time and Frequency
Time and Frequency > Optical Frequency Standards and Metrology
Last Modified: 02 Feb 2018 13:17
URI: http://eprintspublications.npl.co.uk/id/eprint/1906

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