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1.
Phys Rev Lett ; 113(21): 210801, 2014 Nov 21.
Article in English | MEDLINE | ID: mdl-25479482

ABSTRACT

Singly ionized ytterbium, with ultranarrow optical clock transitions at 467 and 436 nm, is a convenient system for the realization of optical atomic clocks and tests of present-day variation of fundamental constants. We present the first direct measurement of the frequency ratio of these two clock transitions, without reference to a cesium primary standard, and using the same single ion of 171Yb+. The absolute frequencies of both transitions are also presented, each with a relative standard uncertainty of 6×10(-16). Combining our results with those from other experiments, we report a threefold improvement in the constraint on the time variation of the proton-to-electron mass ratio, µ/µ=0.2(1.1)×10(-16) yr(-1), along with an improved constraint on time variation of the fine structure constant, α/α=-0.7(2.1)×10(-17) yr(-1).

2.
Article in English | MEDLINE | ID: mdl-17941377

ABSTRACT

We describe a situation where a varying collisional frequency shift in cesium fountain primary frequency standards can be misinterpreted as a power-dependent shift. This misinterpretation may affect analyses of fountains test-operated at multiple pi/2 microwave pulse areas. Such tests are typically performed in the search for microwave- and cavity-related systematic frequency biases.


Subject(s)
Cesium/chemistry , Energy Transfer , Microwaves , Models, Theoretical , Oscillometry/methods , Time Factors , Computer Simulation , Half-Life
3.
Phys Rev Lett ; 98(15): 153002, 2007 Apr 13.
Article in English | MEDLINE | ID: mdl-17501343

ABSTRACT

We have observed that the collisional frequency shift in primary caesium fountain clocks varies with the clock state population composition and, in particular, is zero for a given fraction of the |F=4,mF=0) atoms, depending on the initial cloud parameters. We present a theoretical model explaining our observations. The possibility of the collisional shift cancellation implies an improvement in the performance of caesium fountain standards and a simplification in their operation.

4.
Science ; 306(5700): 1355-8, 2004 Nov 19.
Article in English | MEDLINE | ID: mdl-15550666

ABSTRACT

The frequency of the 5s 2S(1/2)-4d 2D(5/2) electric quadrupole clock transition in a single, trapped, laser-cooled 88Sr+ ion has been measured by using an optical frequency comb referenced to a cesium fountain primary frequency standard. The frequency of the transition is measured as 444,779,044,095,484.6 (1.5) hertz, with a fractional uncertainty within a factor of 3 of that of the cesium standard. Improvements required to obtain a cesium-limited frequency measurement are described and are expected to lead to a 88Sr+ optical clock with stability and reproducibility exceeding that of the primary cesium standard.

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