We describe the operation of a laser cooled cesium fountain clock in the quantum limited regime. An ultrastable cryogenic sapphire oscillator is used to measure the short-term frequency stability of the fountain as a function of the number of detected atoms N at. For N at varying from 4 3 10 4 to 6 3 10 5 the Allan standard deviation of the frequency fluctuations is in excellent agreement with the N 21͞2 at law of atomic projection noise. With 6 3 10 5 atoms, the relative frequency stability is 4 3 10 214 t 21͞2 , where t is the integration time in seconds. This is the best short-term stability ever reported for primary frequency standards, a factor of 5 improvement over previous results. [S0031-9007(99)09299-6]

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 ...

We present two ultra-stable lasers based on two vibration insensitive cavity designs, one with vertical optical axis geometry, the other horizontal. Ultra-stable cavities are constructed with fused silica mirror substrates, shown to decrease the thermal noise limit, in order to improve the frequency stability over previous designs. Vibration sensitivity components measured are equal to or better than 1.5e-11 per m.s^-2 for each spatial direction, which shows significant improvement over previous studies. We have tested the very low dependence on the position of the cavity support points, in order to establish that our designs eliminate the need for fine tuning to achieve extremely low vibration sensitivity. Relative frequency measurements show that at least one of the stabilized lasers has a stability better than 5.6e-16 at 1 second, which is the best result obtained for this length of cavity.Comment: 8 pages 12 figure