We have remeasured the absolute 1S-2S transition frequency νH in atomic hydrogen. A comparison with the result of the previous measurement performed in 1999 sets a limit of (−29 ± 57) Hz for the drift of νH with respect to the ground state hyperfine splitting νCs in 133 Cs. Combining this result with the recently published optical transition frequency in 199 Hg + against νCs and a microwave 87 Rb and 133 Cs clock comparison, we deduce separate limits onα/α = (−0.9 ± 2.9) × 10 −15 yr −1 and the fractional time variation of the ratio of Rb and Cs nuclear magnetic moments µ Rb /µCs equal to (−0.5 ± 1.7) × 10 −15 yr −1 . The latter provides information on the temporal behavior of the constant of strong interaction. PACS numbers: 06.30.Ft, 06.20.Jr, 32.30.Jc In the era of a rapid development of precision experimental methods, the stability of fundamental constants becomes a question of basic interest. Any drift of non-gravitational constants is forbidden in all metric theories of gravity including general relativity. The basis of these theories is Einstein's Equivalence Principle (EEP) which states that weight is proportional to mass, and that in any local freely falling reference frame, the result of any non-gravitational experiment must be independent of time and space. This hypothesis can be proven only experimentally as no theory predicting the values of fundamental constants exists. In contrast to metric theories, string theory models aiming to unify quantum mechanics and gravitation allow for, or even predict, violations of EEP. Limits on the variation of fundamental constants might therefore provide important constraints on these new theoretical models.A recent analysis of quasar absorption spectra with redshifted UV transition lines indicates a variation of the fine structure constant α = e 2 /4πε 0 c on the level of ∆α/α = (−0.54 ± 0.12) × 10 −5 for a redshift range (0.2 < z < 3.7)[1]. On geological timescales, a limit for the drift of α has been deduced from isotope abundance ratios in the natural fission reactor of Oklo, Gabon, which operated about 2 Gyr ago. Modeling the processes which have changed the isotope ratios of heavy elements gives a limit of ∆α/α = (−0.36 ± 1.44) × 10 −8 [2]. In these measurements, the high sensitivity to the time variation of α is achieved through very long observation times at moderate resolution for ∆α. Therefore, they are vulnerable to systematic effects [3].Laboratory experiments can reach a 10 −15 accuracy within years with better controlled systematics. This type of experiment is typically based on repeated absolute frequency measurements, i.e. comparison of a transition frequency with the reference frequency of the ground state hyperfine transition in Contributions from weak, electromagnetic, and strong interactions can be disentangled by combining several frequency measurements possessing a different sensitivity to the fundamental constants. In this letter, we deduce separate stringent limits for the drifts of the fine structure constant α, µ Cs /µ B and µ Rb /µ Cs ...
This paper describes advances in microwave frequency standards using laser-cooled atoms at BNM-SYRTE. First, recent improvements of the 133 Cs and 87 Rb atomic fountains are described. Thanks to the routine use of a cryogenic sapphire oscillator as an ultra-stable local frequency reference, a fountain frequency instability of 1.6 × 10 −14 τ −1/2 where τ is the measurement time in seconds is measured. The second advance is a powerful method to control the frequency shift due to cold collisions. These two advances lead to a frequency stability of 2 × 10 −16 at 50 000 s for the first time for primary standards. In addition, these clocks realize the SI second with an accuracy of 7 × 10 −16 , one order of magnitude below that of uncooled devices. In a second part, we describe tests of possible variations of fundamental constants using 87 Rb and 133 Cs fountains. Finally we give an update on the cold atom space clock PHARAO developed in collaboration with CNES. This clock is one of the main instruments of the ACES/ESA mission which is scheduled to fly on board the International Space Station in 2008, enabling a new generation of relativity tests.
Over five years we have compared the hyperfine frequencies of 133 Cs and 87 Rb atoms in their electronic ground state using several laser cooled 133 Cs and 87 Rb atomic fountains with an accuracy of ∼ 10 −15 . These measurements set a stringent upper bound to a possible fractional time variation of the ratio between the two frequencies :d dt ln ν Rb ν Cs = (0.2 ± 7.0) × 10 −16 yr −1 (1σ uncertainty).The same limit applies to a possible variation of the quantity (µ Rb /µCs)α −0.44 , which involves the ratio of nuclear magnetic moments and the fine structure constant.PACS numbers: 06.30. Ft, 32.80.Pj, 06.20.Jr Since Dirac's 1937 formulation of his large number hypothesis aiming at tying together the fundamental constants of physics [1], large amount of work has been devoted to test if these constants were indeed constant over time [2,3].In General Relativity and in all metric theories of gravitation, variations with time and space of non gravitational fundamental constants such as the fine structure constant α = e 2 /4πǫ 0 c are forbidden. They would violate Einstein's Equivalence Principle (EEP). EEP imposes the Local Position Invariance stating that in a local freely falling reference frame, the result of any local non gravitational experiment is independent of where and when it is performed. On the other hand, almost all modern theories aiming at unifying gravitation with the three other fundamental interactions predict violation of EEP at levels which are within reach of near-future experiments [4,5]. As the internal energies of atoms or molecules depend on electromagnetic, as well as strong and weak interactions, comparing the frequency of electronic transitions, fine structure transitions and hyperfine transitions as a function of time or gravitational potential provides an interesting test of the validity of EEP.To date, very stringent tests exist on geological and cosmological timescales. The analysis of the Oklo nuclear reactor showed that, 2 × 10 9 years ago, α did not differ from the present value by more than 10 −7 of its value [6]. Light emitted by distant quasars has been used to perform absorption spectroscopy of interstellar clouds. For instance, measurements of the wavelengths of molecular hydrogen transitions test a possible variation of the electron to proton mass ratio m e /m p [7]. Comparisons between the gross structure and the fine structure of neutral atoms and ions would indicate that α for a redshift z ∼ 1.5 (∼ 10 Gyr) differed from the present value: ∆α/α = (−7.2 ± 1.8) × 10 −6 [8]. Today this is the only claim that fundamental constants might change.On much shorter timescales, several tests using frequency standards have been performed [9,10,11]. These laboratory tests have a very high sensitivity to changes in fundamental constants. They are repeatable, systematic errors can be tracked as experimental conditions can be changed.In this letter we present results that place a new stringent limit to the time variation of fundamental constants. By comparing the hyperfine energies of 13...
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