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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Spectroscopy of samarium colour centres for a solid-state optical frequency reference

Graham J. A. Edge, Shreyas Potnis, Amar C. Vutha

Published Jul 1, 2017• 0 citations
Publisher

Abstract

Summary form only given. With the impending redefinition of the SI second based on optical clocks, there is a need for new secondary optical frequency standards. Ideally these secondary standards will offer enhanced robustness, portability and high signal-to-noise ratios (SNR), to enable rapid and precise comparisons against primary standards. A clock based on a narrow optical transition, in atomic ions that are naturally trapped in a solid-state host crystal, offers the technological simplicity and large SNR to satisfy these requirements.

DOI
10.1109/fcs.2017.8089027

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Summary form only given. With the impending redefinition of the SI second based on optical clocks, there is a need for new secondary optical frequency standards. Ideally these secondary standards will offer enhanced robustness, portability and high signal-to-noise ratios (SNR), to enable rapid and precise comparisons against primary standards. A clock based on a narrow optical transition, in atomic ions that are naturally trapped in a solid-state host crystal, offers the technological simplicity and large SNR to satisfy these requirements.
The remarkable and steady improvements in the accuracy of optical clocks have greatly advanced the state of the art in time and frequency metrology. In addition to the accuracy of these clocks, the high precision available from them can open up new classes of measurements. In particular, optical clocks with high signal-to-noise ratios (SNR) can be used as components of sensitive antennae for micro-hertz-band gravitational waves [1], [2]. Such clocks could also be useful for field applications such as geodesy [3], and as transportable secondary frequency standards to enable comparisons between distant primary standards [4].
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.
Clocks based on optical transitions in ions and atoms are quickly moving to the forefront of the frequency standards field mainly because of the high spec-tral resolution, and therefore the potential stability and accuracy, which can be achieved. In this thesis a new optical clock based on neutral atoms trapped in an optical lattice is presented, demonstrating spectroscopy of the clock transition free of any lineshape or accuracy degradation due to atomic motion. The system simu-lates a single trapped ion clock, but allows use of thousands of atoms for improved signal to noise ratio, and clock stability. High accuracy can also be achieved as the lattice can be designed to shift the energy of the two atomic clock states equally, such that the transition frequency is unchanged. Strontium is a natural candidate for such a clock as it offers an extremely narrow optical transition (∼1 mHz) based on atomic states which are very insensitive to external fields. The strontium level structure allows efficient laser cooling to 1 µK with diode laser sources, and a con-venient wavelength of 813 nm for the zero-differential Stark shift optical lattice. The strontium lattice clock system has allowed observation of high signal-to-noise
Optical clocks are now the most stable and accurate frequency standards, withperformances in the low 10 −18 , outperforming by two orders of magnitude thebest microwave primary standards based on cesium. This opens the question of apossible redefinition of the SI (Système International) second with an opticaltransition. However, several operational steps have to be demonstrated beforesuch a redefinition can be effective: reliable and interconnected clocks arenecessary to realize the definition. Here, we report on several experimentaladvents in this direction, including the remote comparison of optical latticeclocks via phase compensated optical fiber links between different metrologyinstitutes across Europe.
The microwave caesium (Cs) atomic clock has formed an enduring basis for the second in the International System of Units (SI) over the last few decades. The advent of laser cooling has underpinned the development of cold Cs fountain clocks, which now achieve frequency uncertainties of approximately 5×10(-16). Since 2000, optical atomic clock research has quickened considerably, and now challenges Cs fountain clock performance. This has been suitably shown by recent results for the aluminium Al(+) quantum logic clock, where a fractional frequency inaccuracy below 10(-17) has been reported. A number of optical clock systems now achieve or exceed the performance of the Cs fountain primary standards used to realize the SI second, raising the issues of whether, how and when to redefine it. Optical clocks comprise frequency-stabilized lasers probing very weak absorptions either in a single cold ion confined in an electromagnetic trap or in an ensemble of cold atoms trapped within an optical lattice. In both cases, different species are under consideration as possible redefinition candidates. In this paper, I consider options for redefinition, contrast the performance of various trapped ion and optical lattice systems, and point to potential limiting environmental factors, such as magnetic, electric and light fields, collisions and gravity, together with the challenge of making remote comparisons of optical frequencies between standards laboratories worldwide.
Optical atomic clocks based on laser-cooled trapped ions and atoms have advanced rapidly over the past decade. With fractional frequency uncertainties now surpassing 10 −18 , they are some of the most precise measurement tools ever built. Yet researchers are still pursuing new avenues of research to explore the fundamental limits to their stability, accuracy, and reproducibility. In this mini-review, we provide a survey of the current state of the art by describing the fundamental principles and techniques that underpin this progress, the architectures used to realize optical clocks, and the supporting laser technologies that are essential to their operation. We also examine the progress that has been made toward a redefinition of the second in the International System of Units and the inclusion of optical clocks into the global time and frequency metrology infrastructure. Finally, we discuss emerging applications of optical clocks and look at the prospects for making their precision more readily accessible to end users.