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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Progress toward a single ytterbium ion optical clock

Paul Taylor, Stephen A. Webster, Matthew Roberts, W. R. C. Rowley, G. P. Barwood, Patrick Robert Gill

Published May 3, 2001• 0 citations
Publisher

Abstract

Forbidden transitions in single trapped ion systems are being considered as references for future optical frequency standards. These standards are expected to have application as highly stable and reproducible optical clocks, realizations of the metre and as optical frequency standards in their own right. This paper prescribes the work carried out at the National Physical Laboratory over recent years to use a highly forbidden 2S1/2 - 2F7/2 467 nm electric octupole transition in a single ion of 171Yb+ as a frequency reference. A review of the previous measurements needed to the locate octupole transition in this 171-isotope is given and a more detailed discussion of recent work is presented. This includes spectroscopy of the octupole transition with kilohertz resolution and a direct measurement of its optical frequency. Measurements of the dynamic Stark and the quadratic Zeeman shifts on the octupole frequency are also discussed.

DOI
10.1117/12.424465

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Forbidden transitions in single trapped ion systems are being considered as references for future optical frequency standards. These standards are expected to have application as highly stable and reproducible optical clocks, realizations of the metre and as optical frequency standards in their own right. This paper prescribes the work carried out at the National Physical Laboratory over recent years to use a highly forbidden 2S1/2 - 2F7/2 467 nm electric octupole transition in a single ion of 171Yb+ as a frequency reference. A review of the previous measurements needed to the locate octupole transition in this 171-isotope is given and a more detailed discussion of recent work is presented. This includes spectroscopy of the octupole transition with kilohertz resolution and a direct measurement of its optical frequency. Measurements of the dynamic Stark and the quadratic Zeeman shifts on the octupole frequency are also discussed.
We experimentally investigate an optical frequency standard based on the 467 nm (642 THz) electric-octupole reference transition (2)S(1/2)(F=0)→(2)F(7/2)(F=3) in a single trapped (171)Yb(+) ion. The extraordinary features of this transition result from the long natural lifetime and from the 4f(13)6s(2) configuration of the upper state. The electric-quadrupole moment of the (2)F(7/2) state is measured as -0.041(5)ea(0)(2), where e is the elementary charge and a(0) the Bohr radius. We also obtain information on the differential scalar and tensorial components of the static polarizability and of the probe-light-induced ac Stark shift of the octupole transition. With a real-time extrapolation scheme that eliminates this shift, the unperturbed transition frequency is realized with a fractional uncertainty of 7.1×10(-17). The frequency is measured as 642 121 496 772 645.15(52) Hz.
Optical frequency standards based on narrow absorptions in laser-cooled single trapped ions have recently begun to demonstrate stabilities that are competitive with cold atom fountain microwave standards. This paper presents a short review of the wider state-of-the-art development of these single cold trapped ion frequency standards, coupled with a more detailed account of recent results achieved at National Physical Laboratory in respect of single ion systems based on 88Sr+, 87Sr+ and 171Yb+. Narrow linewidth data for the optical clock quadrupole and octupole transitions respectively at 674 nm in 88Sr+ and 467 nm in 171Yb+, are presented, together with a discussion of current systematics and future projections. The potential for optical clock operation is outlined.
Spectroscopy has been performed on $\mathrm{the}{}^{2}{S}_{1/2}(F=0,{m}_{F}{=0)\ensuremath{-}}^{2}{F}_{7/2}{(F=3,m}_{F}=0)$ electric octupole transition in 171Yb+{}^{171}{\mathrm{Yb}}^{+}171Yb+ with kilohertz resolution. A quantum jump rate of more than one per second has been achieved, a factor of 100 improvement on previous work. Spectra including the sidebands at the trap secular frequency show the ion to be in the Lambe-Dicke regime and hence the Doppler effect has been eliminated to first order. The dynamic Stark shift is a significant perturbation to the frequency because of the high intensity required to drive the octupole transition. It has been measured as ${42}_{\ensuremath{-}10}^{+12} \ensuremath{\mu}\mathrm{Hz}{\mathrm{W}}^{\ensuremath{-}1}{\mathrm{m}}^{2}$ and with further reduction in the probe laser linewidth, it should be possible to reduce the relative uncertainty in this shift to the parts in 1018{10}^{18}1018 level. The transition is free from the Zeeman effect to first order and has a measured second-order shift of $1.9(3) \mathrm{mHz}(\ensuremath{\mu}\mathrm{T}{)}^{\ensuremath{-}2}.$ It should be possible to reduce the relative uncertainty in this shift to below the part in 1018{10}^{18}1018 level. The electric quadrupole shift may be the limiting systematic effect, but cannot yet be observed with the current resolution on the transition.
We present the setup of our optical frequency standard (OFS) “TOPTICLOCK”. It is based on the 2S1/2 (F=0) – 2D3/2 (F=2) electric quadrupole transition at 435.5 nm of a single laser-cooled and trapped 171Yb+ ion. All required subsystems are contained in two transportable 19” rack compartments. By comparing it with a laboratory-style OFS of the German National Metrology Institute (PTB), which operates on the 467 nm electric octupole transition, using a TOPTICA DFC optical frequency comb, a frequency instability of 5×10-15/√τ with systematic uncertainty below 2×10-17 was demonstrated. The performance of TOPTICLOCK was also benchmarked by directly comparing it to another OFS operating also at the 435.5 nm clock transition. An agreement between both systems with a total uncertainty well below 10-17 was obtained.