Windell H. Oskay

Frequency Ratio of Al+ and Hg+ Single-Ion Optical Clocks; Metrology at the 17th Decimal Place

Time has always had a special status in physics because of its fundamental role in specifying the regularities of nature and because of the extraordinary precision with which it can be measured. This precision enables tests of fundamental physics and cosmology, as well as practical applications such as satellite navigation. Recently, a regime of operation for atomic clocks based on optical transitions has become possible, promising even higher performance. We report the frequency ratio of two optical atomic clocks with a fractional uncertainty of 5.2 × 10–17. The ratio of aluminum and mercury single-ion optical clock frequencies νAl+/νHg+ is 1.052871833148990438(55), where the uncertainty comprises a statistical measurement uncertainty of 4.3 × 10–17, and systematic uncertainties of 1.9 × 10–17 and 2.3 × 10–17 in the mercury and aluminum frequency standards, respectively. Repeated measurements during the past year yield a preliminary constraint on the temporal variation of the fine-structure constant α of \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \({\dot{{\alpha}}}{/}{\alpha}=(-1.6{\pm}2.3){\times}10^{-17}{/}\mathrm{year}\) \end{document}.

Testing the stability of fundamental constants with the 199Hg+ single-ion optical clock

Over a two-year duration, we have compared the frequency of the 199Hg+ 5d(10)6s (2)S(1/2)(F=0)<-->5d(9)6s(2) (2)D(5/2)(F=2) electric-quadrupole transition at 282 nm with the frequency of the ground-state hyperfine splitting in neutral 133Cs. These measurements show that any fractional time variation of the ratio nu(Cs)/nu(Hg) between the two frequencies is smaller than +/-7 x 10(-15) yr(-1) (1sigma uncertainty). According to recent atomic structure calculations, this sets an upper limit to a possible fractional time variation of g(Cs)(m(e)/m(p))alpha(6.0) at the same level.

Femtosecond-laser-based synthesis of ultrastable microwave signals from optical frequency references

We use femtosecond laser frequency combs to convert optical frequency references to the microwave domain, where we demonstrate the synthesis of 10-GHz signals having a fractional frequency instability of < or =3.5 x 10(-15) at a 1-s averaging time, limited by the optical reference. The residual instability and phase noise of the femtosecond-laser-based frequency synthesizers are 6.5 x 10(-16) at 1 s and -98 dBc/Hz at a 1-Hz offset from the 10-GHz carrier, respectively. The timing jitter of the microwave signals is 3.3 fs.

Optical frequency/wavelength references

For more than 100 years, optical atomic/molecular frequency references have played important roles in science and technology, and provide standards enabling precision measurements. Frequency-stable optical sources have been central to experimental tests of Einsteins relativity, and also serve to realize our base unit of length. The technology has evolved from atomic discharge lamps and interferometry, to narrow atomic resonances in laser-cooled atoms that are probed by frequency-stabilized cw lasers that in turn control optical frequency synthesizers (combs) based on ultra-fast mode-locked lasers. Recent technological advances have improved the performance of optical frequency references by almost four orders of magnitude in the last eight years. This has stimulated new enthusiasm for the development of optical atomic clocks, and allows new probes into nature, such as searches for time variation of fundamental constants and precision spectroscopy.

Ballistic peaks at quantum resonance

We report studies of the motion of cold atoms in a time-dependent optical potential. The dynamics of our system are that of the quantum kicked rotor, and exhibit a wide variety of phenomena. One purely quantum effect is the quantum resonance, which occurs for well-chosen initial conditions and specific values of the period between kicks. Distinctly nonclassical behavior, such as ballistic growth in momentum, is possible at a quantum resonance. Previous experimental studies have observed these resonances, but have not clearly resolved the expected ballistic motion. We now observe ballistic motion at quantum resonances and compare our momentum distributions with theory and numerical simulations.

Quantum chaos with cesium atoms: pushing the boundaries

Atomic motion in pulsed, periodic optical potentials provides a unique experimental testing ground for quantum chaos. In the first generation of experiments with sodium atoms we observed dynamical localization, a quantum suppression of chaotic diffusion. To go beyond this work we have constructed an experiment with cold cesium atoms, and report our first results from this system. The larger mass and longer wavelength push out the momentum boundary in phase space that arises from the nonzero duration of the pulses. This feature should enable the study of noise effects and dimensionality on dynamical localization. We propose a new method of quantum state preparation based on stimulated Raman transitions for studies of mixed phase space dynamics. c 1999 Elsevier Science B.V. All rights reserved.

Timing noise effects on dynamical localization

The quantum kicked rotor is studied experimentally in an atom-optics setting, where we observe the center-of-mass motion of cold cesium atoms. Dynamical localization in this system typically suppresses classical diffusive motion, but is susceptible to the addition of various forms of noise. We study in detail the effects of timing noise, where variations are introduced to the times at which the kicks occur. This noise is particularly interesting because it does not directly induce momentum diffusion. However, it is found that the addition of timing noise efficiently destroys both the classical correlations that give rise to fluctuations in the classical diffusion rate as well as quantum coherences that lead to dynamical localization.

Ion optical clocks and quantum information processing

Techniques developed for quantum-information processing using trapped ions may be useful in future atomic clocks. Here, we summarize experiments at NIST that (1) use quantum gates to entangle ions and thereby improve the measurement signal-to-noise ratio in spectroscopy and (2) implement sympathetic cooling and quantum state transfer techniques, which might be used to increase the number of choices of ions used for clocks.

Ultra-high stability optical frequency standard based on laser-cooled neutral calcium

A beatnote between the Ca and Hg/sup +/ optical frequency standards via a mode-locked fs-laser frequency comb demonstrates the highest frequency stability measured to date. The high stability accelerates evaluation of the Ca standards systematic shifts.

The mercury-ion optical clock and the search for temporal variation of fundamental constants

The repeated comparison of atomic frequency standards based upon different transitions enables the search for time variation of the fundamental constants that determine the transition frequencies. Over the course of two years we compared the frequency of the /sup 199/Hg/sup +/ 5d/sup 10/6s/sup 2/S/sub 1/2 /(F=0)/spl harr/5d/sup 9/6s/sup 2/ /sup 2/D/sub 5/2/(F=2) electric-quadrupole transition at 282 nm with the frequency of the ground-state hyperfine splitting in neutral /sup 133/Cs that defines the SI second. These measurements constrain any fractional time variation of the ratio/spl nu//sub Cs///spl nu//sub Hg/ between the two frequencies to be less than /spl plusmn/7/spl times/10/sup -15/yr/sup -1/ (1/spl sigma/ uncertainty). According to recent atomic structure calculations, this sets an upper limit to a possible fractional time variation of the product gCs(m/sub e//m/sub p/)/spl alpha//sup 6.0/ at the same level.