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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Compact and robust repumper light source for 88Sr+ single-ion traps

Thomas Fordell, T. Lindvall, Tuomas Hieta, Mikko Merimaa

Published May 1, 2013• 0 citations
Publisher

Abstract

Optical clocks have accuracies an order of magnitude better than caesium fountain clocks; however, before the second can be re-defined in terms of an optical transition, many different ions and clock configurations must be explored and compared to each other. A clear challenge is then to develop accurate methods to compare clocks. Ideally, transportable optical clocks are needed, and transportability calls for robust light sources.

DOI
10.1109/cleoe-iqec.2013.6801773

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Optical clocks have accuracies an order of magnitude better than caesium fountain clocks; however, before the second can be re-defined in terms of an optical transition, many different ions and clock configurations must be explored and compared to each other. A clear challenge is then to develop accurate methods to compare clocks. Ideally, transportable optical clocks are needed, and transportability calls for robust light sources.
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Transitions between discrete energy levels in well-isolated atomic systems are highly reproducible and therefore make ideal references for highly accurate frequency standards or clocks. In particular, a single laser-cooled ion confined in a radiofrequency ion trap closely approximates the spectroscopic ideal of a particle at rest in a perturbation-free environment. Narrow optical transitions in such systems are therefore very promising for the realization of optical clocks with accuracy around 2 orders of magnitude higher than current microwave primary standards. In this paper the fundamental aspects, current performance and applications of trapped ion optical clocks are reviewed and the potential for future improvements is considered.
For the past 15 years, tremendous progress within the fields of laser stabilization, optical frequency combs and atom cooling and trapping have allowed the realization of optical atomic clocks with unrivaled performances. These instruments can perform frequency comparisons with fractional uncertainties well below 10-17, finding applications in fundamental physics tests, relativistic geodesy and time and frequency metrology. Even though most optical clocks are currently laboratory setups, several proposals for using these clocks for field measurements or within an optical clock network have been published, and most of time and frequency metrology institutes have started to develop transportable optical clocks. For the purpose of this special issue, we chose to focus on trapped-ion optical clocks. Even though their short-term fractional frequency stability is impaired by a lower signal-to-noise ratio, they offer a high potential for compactness: trapped ions demand low optical powers and simple loading schemes, and can be trapped in small vacuum chambers. We review recent advances on the clock key components, including ion trap and ultra-stable optical cavity, as well as existing projects and experiments which draw the picture of what future transportable, single-ion optical clocks may resemble.
The accuracy of state-of-the-art optical atomic clocks has surpassed that of microwave clocks by more than two orders of magnitude. This revolution in accuracy has prompted international dialogue concerning the redefinition of the SI second away from the current cesium standard. Optical clocks employing a single trapped ion are promising candidates for realizing this redefinition as they offer exceptional fractional frequency uncertainty. However, there is still room for improvement: the leading contributions to the uncertainty of most single-ion clocks, such as the 88Sr+ clock operated by the National Research Council of Canada (NRC), are the blackbody radiation shifts and collisional frequency shifts. The uncancelled blackbody radiation shift dominates both the total shift and uncertainty of the existing room temperature strontium ion clocks. To address these sources of inaccuracy, I contributed to developing a strontium ion trap for operation at 4 K. Operating at cryogenic temperatures suppresses the blackbody radiation shift by eight orders of magnitude and collisional shifts by at least two orders of magnitude, enabling substantial improvements to the performance of single-ion clocks. The proposed cryogenic clock is capable of achieving fractional frequency uncertainty below 10−19. This thesis describes the design, construction, and evaluation of the cryogenic ion trap towards realizing a more accurate atomic clock.