We report on an absolute frequency measurement of the hydrogen 1S-2S two-photon transition in a cold atomic beam with an accuracy of 1.8 parts in 10(14). Our experimental result of 2 466 061 413 187 103(46) Hz has been obtained by phase coherent comparison of the hydrogen transition frequency with an atomic cesium fountain clock. Both frequencies are linked with a comb of laser frequencies emitted by a mode locked laser.

We give an overview of the work done with the Laboratoire National de Métrologie et d'Essais-Systèmes de Référence Temps-Espace (LNE-SYRTE) fountain ensemble during the last five years. After a description of the clock ensemble, comprising three fountains, FO1, FO2, and FOM, and the newest developments, we review recent studies of several systematic frequency shifts. This includes the distributed cavity phase shift, which we evaluate for the FO1 and FOM fountains, applying the techniques of our recent work on FO2. We also report calculations of the microwave lensing frequency shift for the three fountains, review the status of the blackbody radiation shift, and summarize recent experimental work to control microwave leakage and spurious phase perturbations. We give current accuracy budgets. We also describe several applications in time and frequency metrology: fountain comparisons, calibrations of the international atomic time, secondary representation of the SI second based on the (87)Rb hyperfine frequency, absolute measurements of optical frequencies, tests of the T2L2 satellite laser link, and review fundamental physics applications of the LNE-SYRTE fountain ensemble. Finally, we give a summary of the tests of the PHARAO cold atom space clock performed using the FOM transportable fountain.

We have measured the 1S-2S transition frequency in atomic hydrogen via two-photon spectroscopy on a 5.8 K atomic beam. We obtain f(1S-2S) = 2,466,061,413,187,035 (10)  Hz for the hyperfine centroid, in agreement with, but 3.3 times better than the previous result [M. Fischer et al., Phys. Rev. Lett. 92, 230802 (2004)]. The improvement to a fractional frequency uncertainty of 4.2 × 10(-15) arises mainly from an improved stability of the spectroscopy laser, and a better determination of the main systematic uncertainties, namely, the second order Doppler and ac and dc Stark shifts. The probe laser frequency was phase coherently linked to the mobile cesium fountain clock FOM via a frequency comb.

Leveraging the unrivalled performance of optical clocks as key tools for geo-science, for astronomy and for fundamental physics beyond the standard model requires comparing the frequency of distant optical clocks faithfully. Here, we report on the comparison and agreement of two strontium optical clocks at an uncertainty of 5 × 10−17 via a newly established phase-coherent frequency link connecting Paris and Braunschweig using 1,415 km of telecom fibre. The remote comparison is limited only by the instability and uncertainty of the strontium lattice clocks themselves, with negligible contributions from the optical frequency transfer. A fractional precision of 3 × 10−17 is reached after only 1,000 s averaging time, which is already 10 times better and more than four orders of magnitude faster than any previous long-distance clock comparison. The capability of performing high resolution international clock comparisons paves the way for a redefinition of the unit of time and an all-optical dissemination of the SI-second.

Progress in realizing the SI second had multiple technological impacts and enabled further constraint of theoretical models in fundamental physics. Caesium microwave fountains, realizing best the second according to its current definition with a relative uncertainty of 2-4 Â 10 À 16 , have already been overtaken by atomic clocks referenced to an optical transition, which are both more stable and more accurate. Here we present an important step in the direction of a possible new definition of the second. Our system of five clocks connects with an unprecedented consistency the optical and the microwave worlds. For the first time, two state-of-the-art strontium optical lattice clocks are proven to agree within their accuracy budget, with a total uncertainty of 1.5 Â 10 À 16 . Their comparison with three independent caesium fountains shows a degree of accuracy now only limited by the best realizations of the microwave-defined second, at the level of 3.1 Â 10 À 16 .

We use six years of accurate hyperfine frequency comparison data of the dual rubidium and caesium cold atom fountain FO2 at LNE-SYRTE to search for a massive scalar dark matter candidate. Such a scalar field can induce harmonic variations of the fine structure constant, of the mass of fermions and of the quantum chromodynamic mass scale, which will directly impact the rubidium/caesium hyperfine transition frequency ratio. We find no signal consistent with a scalar dark matter candidate but provide improved constraints on the coupling of the putative scalar field to standard matter. Our limits are complementary to previous results that were only sensitive to the fine structure constant, and improve them by more than an order of magnitude when only a coupling to electromagnetism is assumed.PACS numbers: 04.50. Kd,04.80.Cc,06.20.Jr,95.35.+d While thoroughly tested [1], the theory of General Relativity (GR) is currently challenged by theoretical considerations and by galactic and cosmological observations. Indeed, the development of a quantum theory of gravitation or of a theory that would unify gravitation with the other fundamental interactions leads to deviations from GR. These modifications are usually characterized by the introduction of new fields in addition to the space-time metric to model the gravitational interaction. For example, string theory generically predicts the existence of new scalar fields (dilaton, moduli, axions). In addition, in the current cosmological paradigm, some galactic and cosmological observations are explained by the introduction of cold Dark Matter (DM) and of Dark Energy. Little is currently known about these two components that constitute the major part of our Universe. They can be interpreted as new types of matter (although they have not been directly detected so far), as a modification of the theory of gravitation or even as a combination of the two.The introduction of nonminimally coupled scalar fields additionally to GR (tensor-scalar theories) generally leads to a space-time dependence of fundamental constants, which can then be searched for by experiments that test the Einstein equivalence principle (EEP) like weak equivalence principle (WEP) tests or tests of local position or Lorentz invariance (LPI and LLI) [1]. In the past, spectroscopy of different atomic transitions has been widely used to carry out such searches, and has set the tightest limits so far on a possible present-day spacetime variation of fundamental constants [2][3][4][5][6][7][8][9][10][11][12][13][14].Such scalar fields could be a candidate for DM and/or dark energy. Different cosmological evolutions of the scalar fields are possible (see e.g. [15,16]). In several scenarios (in particular in the one defined by the action below), a massive scalar field will oscillate at a frequency related to its mass, leading to a corresponding oscillation of fundamental constants (see e.g. [17,18]). Recently atomic spectroscopy of Dy has been used to constrain such oscillations [2] of the fine structure constant α....

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 a new measurement of the 1S-3S two-photon transition frequency of hydrogen, realized with a continuous-wave excitation laser at 205 nm on a room-temperature atomic beam, with a relative uncertainty of 9×10^{-13}. The proton charge radius deduced from this measurement, r_{p}=0.877(13)  fm, is in very good agreement with the current CODATA-recommended value. This result contributes to the ongoing search to solve the proton charge radius puzzle, which arose from a discrepancy between the CODATA value and a more precise determination of r_{p} from muonic hydrogen spectroscopy.