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

Progress towards a two-photon clock based on laser-cooled calcium

Shreyas Potnis, Shira Jackson, Amar C. Vutha

Published Jul 1, 2017• 0 citations
Publisher

Abstract

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].

DOI
10.1109/fcs.2017.8088920

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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].
Optical atomic clocks based on optical transitions have achieved a fractional uncertainty of 10−18, representing a two-order-of-magnitude improvement over caesium clocks. This remarkable progress has prompted ongoing discussions on redefining the SI second using optical standards. Beyond frequency metrology, such unprecedented precision enables new applications in fields ranging from relativistic geodesy to fundamental physics. To advance these possibilities, we have developed a transportable optical lattice clock and demonstrated its practical operation. In this presentation, we will outline the development of transportable optical lattice clocks and highlight their applications.
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.
This thesis presents details on the design, construction, and characterization of a novel compact optical clock based on a two-photon transition in calcium. Optical atomic clocks have reached unprecedented accuracy and precision and have the potential to revolutionize metrology. However, many of the applications of optical clocks, such as the detection of dark matter and gravitational waves, deep space navigation, measurements of variations in fundamental constants, and precision geodetic measurements, require compact portable clocks. Building optical atomic clocks on a scale suitable for most field applications is an ongoing challenge in the precision metrology community. This work describes a clock that can be made smaller and simpler than most existing optical clocks, while still operating with state-of-the-art precision. The basis for these improvements is using a two-photon transition, which eliminates the need for sub-Doppler cooling and tight confinement of the atoms. The design for the calcium two-photon clock is given, and the construction of the clock components is described. Experimental results are presented for the search for the clock transition frequency, which has never before been precisely determined. A detailed uncertainty budget is given, showing that this clock should be able to achieve a fractional frequency uncertainty of ∼10−17\sim 10^{-17}∼10−17. In addition, a novel technique of ``magic polarization'' is presented which can be implemented to mitigate probe laser light shifts, long thought to be a serious impediment to precision two-photon clocks. An experiment in which a two-photon clock based on rubidium was launched and operated on a stratospheric balloon and subsequently recovered is also described. This work represents an advancement in the development of compact portable optical clocks which will further push the boundaries of precision measurement and our understanding of the universe.
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.