H. Marion

New Limits on the Drift of Fundamental Constants from Laboratory Measurements

We have remeasured the absolute 1S-2S transition frequency nu(H) 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 nu(H) with respect to the ground state hyperfine splitting nu(Cs) in 133Cs. Combining this result with the recently published optical transition frequency in 199Hg+ against nu(Cs) and a microwave 87Rb and 133Cs clock comparison, we deduce separate limits on alpha/alpha=(-0.9+/-2.9) x 10(-15) yr(-1) and the fractional time variation of the ratio of Rb and Cs nuclear magnetic moments mu(Rb)/mu(Cs) equal to (-0.5+/-1.7) x 10(-15) yr(-1). The latter provides information on the temporal behavior of the constant of strong interaction.

Search for variations of fundamental constants using atomic fountain clocks.

Over five years, we have compared the hyperfine frequencies of 133Cs and 87Rb atoms in their electronic ground state using several laser-cooled 133Cs and 87Rb atomic fountains with an accuracy of approximately 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([(nu(Rb))/(nu(Cs))]=(0.2+/-7.0)x 10(-16) yr(-1) (1sigma uncertainty). The same limit applies to a possible variation of the quantity (mu(Rb)/mu(Cs))alpha(-0.44), which involves the ratio of nuclear magnetic moments and the fine structure constant.

Cold atom clocks and applications

This paper describes advances in microwave frequency standards using laser-cooled atoms at BNM-SYRTE. First, recent improvements of the 133Cs and 87Rb 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 87Rb and 133Cs 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.

Controlling the cold collision shift in high precision atomic interferometry.

We present a new method based on a transfer of population by adiabatic passage that allows one to prepare cold atomic samples with a well-defined ratio of atomic density and atom number. This method is used to perform a measurement of the cold collision frequency shift in a laser cooled cesium clock at the percent level, which makes the evaluation of the cesium fountain accuracy at the 10(-16) level realistic. With improvements, the adiabatic passage would allow measurements of density-dependent phase shifts at the 10(-3) level in high precision experiments.

BNM-SYRTE fountains: recent results

This paper describes several recent improvements of the BNM-SYRTE fountain ensemble. A new method for controlling the cold collision shift with improved accuracy has been proposed and demonstrated. A thorough investigation of some cold collision properties of 133Cs is presented, including the observation of molecular Feshbach resonances. Finally, a new microwave synthesis scheme based on a fully operational cryogenic oscillator is presented. With this, a fractional frequency instability below 2 times 10-14tau-frac12 is obtained routinely

Design and realization of a flywheel oscillator for advanced time and frequency metrology

In this article, we describe a new frequency synthesis system that includes a low phase noise cryogenic sapphire oscillator (CSO) and an H-maser to provide metrological low-noise signals to time and frequency experiments. Implementing this system as a local oscillator for a Cs cold atom fountain, a record frequency stability of 1.6×10−14τ−1∕2 is obtained.

Feshbach resonances in cesium at ultralow static magnetic fields

We have observed Feshbach resonances for 133Cs atoms in two different hyperfine states at static magnetic fields of a few milligauss. These resonances are unusual for two main reasons. First, they are the lowest static-field resonances investigated up to now, and we explain their multipeak structure in these ultralow fields. Second, they are robust with respect to temperature effects. We have measured them using an atomic fountain clock and reproduced them using coupled-channels calculations, which are in excellent agreement with our measurements. We show that these are s-wave resonances due to a very weakly bound state of the triplet molecular Hamiltonian. We also describe a model explaining their asymmetric shape in the regime where the kinetic energy dominates over the coupling strength.

Cold atom clocks, precision oscillators and fundamental tests

A new era in fundamental physics began when pulsars were discovered in 1967. Soon it became clear that pulsars were useful tools for a wide variety of physical and astrophysical problems. Further applications became possible with the discovery of the first binary pulsar in 1974 and the discovery of millisecond pulsars in 1982. Ever since pulsars have been used as precise cosmic clocks, taking us beyond the weak-field limit of the solar-system in the study of theories of gravity. Their contribution is crucial as no test can be considered to be complete without probing the strong-field realm of gravitational physics by finding and timing pulsars. This is particularly highlighted by the discovery of the first double pulsar system in 2003. In this review, I will explain some of the most important applications of millisecond pulsar clocks in the study of gravity and fundamental constants.We describe two experimental tests of the Equivalence Principle that are based on frequency measurements between precision oscillators and/or highly accurate atomic frequency standards. Based on comparisons between the hyperfine frequencies of 8 7 Rb and 1 3 3 Cs in atomic fountains, the first experiment constrains the variability of fundamental constant. The second experiment is based on a comparison between a cryogenic sapphire oscillator and a hydrogen maser. It tests Local Lorentz Invariance. In both cases, we report recent results which improve significantly over previous experiments.Applied to three inhomogeneous samples of quasar absorption-line spectra, the many-multiplet method gives a shift in the value of the fine-structure constant of Δα/α = (-5.4 ′ 1.2) x 10 - 6 in the redshift range 0.2 6. Future observations with a new High Accuracy Radial velocity Planet Searcher spectrograph may provide a crucial test for the Δα/α measurements at a level of 10 - 6 .This paper addresses the motivation, technology and recent results in the tests of the general theory of relativity (GR) in the solar system. We specifically discuss Lunar Laser Ranging (LLR), the only technique available to test the Strong Equivalence Principle (SEP) and presently the most accurate method to test for the constancy of the gravitational constant, G. The new Apache Point Observatory Lunar Laser-ranging Operation (APOLLO) will enable tests of the Weak and Strong Equivalence Principles with a sensitivity approaching 10e-14, translating to a test of the SEP violation parameter, \eta, to a precision of ~3x10e-5. In addition, the (v/c)^2 general relativistic effects would be tested to better than 0.1%, and measurements of the relative change in the gravitational constant, \dot{G}/G, would be ~0.1% the inverse age of the universe. We also discuss the Laser Astrometric Test Of Relativity (LATOR) mission that will be able to improve the value of the PPN parameter \gamma to accuracy of 1 part in 10e8 and will measure effects of the next post-Newtonian order (1/c^4) of light deflection resulting from gravitys intrinsic non-linearity, as well as measure a variety of other relativistic effects. LATOR will lead to very robust advances in the tests of fundamental physics: this mission could discover a violation or extension of GR, or reveal the presence of an additional long range interaction in the physical law. There are no analogs to the LATOR experiment; it is unique and is a natural culmination of solar system gravity experiments.

/sup 87/Rb and /sup 133/Cs laser cooled clocks: testing the stability of fundamental constants

Over five years we have compared the hyperfine frequencies of /sup 133/Cs and /sup 87/Rb atoms in their electronic ground state using several laser cooled /sup 133/Cs and /sup 87/Rb atomic fountains with an accuracy of /spl sim/10/sup -15/. These measurements set a stringent upper bound to a possible fractional time variation of the ratio between the two frequencies: d/dt In (/spl nu/Rb//spl nu/Cs)=(0.2/spl plusmn/7.0)/spl times/10/sup -16/ yr/sup -1/ (1/spl sigma/ uncertainty). The same limit applies to a possible variation of the quantity (/spl mu/Rb//spl mu/Cs)/spl alpha//sup -0.44/, which involves the ratio of nuclear magnetic moments and the fine structure constant. To improve this test, one needs more accurate cesium fountain clocks, for which the major limiting factor is the cold collision frequency shift. This effect can now be evaluated with great accuracy using a new method which we also present here. It is based on a transfer of population by adiabatic passage that allows to prepare cold atomic samples with a well defined ratio of atomic density and atom number. This method is used to perform a measurement of the cold collision frequency shift in a laser cooled cesium clock at the percent level. With improvements, the adiabatic passage would allow measurements of density-dependent phase shifts at the 10/sup -3/ level in high precision experiments. With this precision, reaching an accuracy of 10/sup -16/ is possible.

Cs and Rb fountains

We discuss the present performance and limits of our Cs and Rb fountains. BNM-SYRTE operates three cold atom microwave fountains: two Cs fountains and a dual Cs-Rb fountain. By using an ultra-stable cryogenic sapphire oscillator to probe the atoms, the frequency stability reaches 3.5 /spl times/ 10/sup -14//spl tau//sup -1/2/ which will allow an improvement of the accuracy below the present 10/sup -15/. We discuss here the problems to be solved for reaching a 10/sup -16/ accuracy. We also use the fountains to test the stability of the fine structure constant. Measurements of the ratio v/sub Rb//v/sub Cs/ spread over a two year interval show no change of a at the 7 /spl times/ 10/sup -15//year level.