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Measurement Science and Technology

Trapped ion optical frequency standards

Patrick Robert Gill, G. P. Barwood, Hugh A. Klein, Guozheng Huang, Stephen A. Webster, Peter Blythe, Kazumoto Hosaka, S.N. Lea, Helen S. Margolis

Published Jul 16, 2003• 44 citations
Publisher

Abstract

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.

DOI
10.1088/0957-0233/14/8/302

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Aug 29, 2007•8 citations
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.
An optical clock or frequency standard based on the interrogation of an atomic transition in the optical domain, operates at ~1014Hz is capable of providing orders of magnitude better accuracy than the present primary standard of frequency or time operating in the microwave range. Optical transitions of an ions, confined within a radio frequency ion trap and laser cooled to ~mK temperature, provides better long term stability since they are free from Coulomb and intra-atomic interactions. Optical frequency standards have been realized with several ions, e.g.,199Hg+,171Yb+,115In+,88Sr+,40Ca+,27Al+and among those171Y+has multiple ultra-narrow optical transitions suitable for serving as frequency standards [1-2].
Weak transitions in single cold trapped ions are highly reproducible as optical frequency standards on account of the high quality factors and stabilities achievable. In recent years, there has been a quickening pace in the development of these optical frequency standards, particularly due to the emergence of femtosecond laser combs for relating the optical frequency directly to the Cs standard. This paper reports progress on optical frequency standards based on the quadrupole transition in a single cold ion of 88 Sr + and the octupole transition in a single cold ion of 171 Yb + .
A number of laboratories are working towards developing optical frequency standards based on single trapped ions. The options are briefly reviewed. Recent progress using the singly ionised strontium system at the National Physical Laboratory (NPL) is outlined. A comparison of the frequency of the 2 S 1/2 – 2 D 5/2 transition at 674 nm in 88 Sr + , observed simultaneously in two traps, showed agreement at the 3 parts in 10 13 level. The possibility of cooling and probing 87 Sr + has recently been proposed because it offers a 674 nm transition between m F = 0 levels which is independent of magnetic field to first order. Trapping of 87 Sr + ions prepared using an oven containing an isotopically enriched strontium compound has been demonstrated at NPL.
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.
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.
The present decade has seen great advances in optical frequency standards following the development of femtosecond laser frequency combs. With this technology, combined with a stable and narrow probe laser source (50-100 Hz linewidth), we made a systematic study of the 5s 2S1/2-4d 2D5/2 (S-D) transition frequency of the 88Sr+ ion as a function of the quantization axis in the trap. This study revealed the presence of systematic shifts caused by residual micromotion in our rf Paul trap and led to an evaluation of those shifts. Also, a method was developed based on the measurement of the Zeeman spectrum of the clock transition to cancel the electric quadrupole shift caused by patch potentials on the trap electrodes. From these results, we also determined the absolute frequency of the S-D clock transition frequency at 674 nm with a fractional frequency uncertainty of 3.4 × 10-14. We have since made improvements to our probe laser system and have observed 5 Hz Fourier transform limited linewidths on a Zeeman component of the S-D transition of the 88Sr+ ion. To observe such a narrow transition it is essential to have a known and well behaved drift of the laser frequency during the few minutes it takes to measure the ion resonance using the quantum jump signal. Our approach to minimize the drift rate and its sensitivity to temperature fluctuations has been to stabilize the cavity at the temperature where its coefficient of thermal expansion is zero. We will present an overview of our recent progress made on the frequency standard based on the laser-cooled and trapped single ion of 88Sr+.