Nous decrivons le travail realise au LNE-SYRTE (Observatoire de Paris) ces dernieres annees, en vue de l'amelioration et de l'utilisation d'etalons de frequence micro-onde fondes sur l'utilisation d'atomes refroidis par laser. Nous exposons les ameliorations recentes des fontaines atomiques a 133 Cs et 87 Rb. La precision atteinte sur la realisation de la seconde avec de tels systemes (une exactitude relative de 7.10 -16 ) permet de mener des experiences de physique fondamentale poussees et ouvre un champ d'applications dans les domaines de la navigation et des telecommunications.

At SYRTE, we have performed a direct local comparison between two atomic fountains, FO1 and F02 using as the interrogation oscillator a cryogenic sapphire resonator oscillator weakly phase-locked to a hydrogen maser. The best short term stabilities measured against this interrogation oscillator are 2.9 and 1.6 × 10 -14 at 1 s, respectively. When accounting for the real time measurement of the cold collision shift as well as correction for all the other systematic shifts, we measure combined stability of 5.0 x 10 -14 , going down to 2.2 x 10 -16 at 50000 s. The measured systematic frequency difference is (4 ± 2.2) x 10 -16 . It is fully consistent with the clock accuracies of 7.5 x 10 -16 for FO1 and 6.5 x 10 -16 for FO2.

This paper describes the work performed at BNM-SYRTE (Observatoire de Paris) over the past few years toward the improvement and the use of microwave frequency standards using laser-cooled atoms. First, recent improvements of the Cs-133 and Rb-87 atomic fountains are described. An important advance is the achievement of a fractional frequency instability of 1.6 x 10(-14)tau(-1/2) where tau is the measurement time in seconds, thanks to the routine use of a cryogenic sapphire oscillator as an ultra-stable local frequency reference. 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 x 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 x 10(-16), one order of magnitude below that of uncooled devices.

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.

This paper describes advances in microwave frequency standards using laser-cooled atoms at BNM-SYRTE. First, recent improvements of the Cs-133 and Rb-87 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 x 10(-14) iota(-1/2) where iota 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 x 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 x 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 (CS)-C-133 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.

We present a comparison between SYRTE's Cs fountain FO1 and Cs-Rb fountain FO2 using a cryogenic sapphire oscillator (CSO) to generate an ultra-stable interrogation signal. A description of the experimental setup is given and the clock accuracies are discussed. For the first time, a frequency resolution in the low 10-16 is achieved in the comparison between two primary standards. The paper also summarizes recent contributions of these fountains to research activities such as test of the stability of fundamental constants, timekeeping and test of sub-systems of the PHARAO space clock.

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

We present a comparison between SYRTE's Cs fountain FO1 and Cs-Rb fountain FO2 using a cryogenic sapphire oscillator (CSO) to generate an ultra-stable interrogation signal. A description of the experimental setup is given and the clock accuracies are discussed. For the first time, a frequency resolution in the low 10/sup -16/ is achieved in the comparison between two primary standards. The paper also summarizes recent contributions of these fountains to research activities such as test of the stability of fundamental constants, timekeeping and test of sub-systems of the PHARAO space clock.

We report a test of the stability of fundamental constants based on laboratory experiments. This test relies on high-precision comparisons of atomic frequencies. The ground state hyperfine frequencies of Rb-87 and Cs-133 are compared over five years using atomic fountains. These measurements lead to the following constraint d/dt In ((g(Cs)/g(Rb))alpha(0.44)) = (0.2 +/- 7.0) x 10(-16) yr(-1) (1 sigma uncertainty) where g is the nuclear g-factor and alpha the fine structure constant.

Cet article presente les derniers resultats des horloges en fontaine du BNM-SYRTE avec leurs stabilites et exactitudes respectives (FOM: 1,7.10 -13 τ -1/2 0,8.10 -15 ; DF Cs : 3,5.10 -14 τ -1/2 , 0,8.10 -15 ; DF Rb : 1,4.10 -14 τ -1/2 , 0,7.10 -15 ). Pour la determination du deplacement collisionel une nouvelle methode de mesure differentielle des densites a ete mis en place. La methode de passage adiabatique permet la preparation des atomes avec 50% ou 100% de la densite maximale. Les comparaisons entre les fontaines ont abouti a un test de la variation eventuelle de la constante de structure fine α. En supposant que seule a varie la variation relative est (-0.4±16).10 -16 an -1 .

To date atomic fountain clocks using laser cooled Cs atoms prove to be the most accurate frequency standards. Among the three set-ups at BNM-SYRTE, the historically first Cs fountain clock, FO1, has recently undergone substantial changes. The cold atom preparation and selection have been rebuilt. Selection of the F = 3, mF = 0 state can be carried out by adiabatic passage allowing the preparation of 100% or 50% of the atom density with a precision of 10-3. Thereby, we can quantify the collisional frequency shift at a level of 10-16. Furthermore, a cryogenic sapphire oscillator is used for the synthesis of the microwave interrogation signal. With these improvements FO1 is currently operating at a stability of 2.8 x 10-14s-1/2 and an accuracy of a few 10-16.

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 87Rb and 133Cs in atomic fountains, the first experiment constrains the stability of fundamental constants. 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.

This article describes the work performed at BNM-SYRTE (Observatoire de Paris) in the past few years, toward the improvement and the use of microwave frequency standards using laser-cooled atoms. First, recent improvements of the Cs-133 and Rb-87 atomic fountains are described. An important advance is the achievement of a fractional frequency instability of 1.6 x 10(-14)tau(-1/2) where tau is the measurement time in seconds, thanks to the routine use of a cryogenic sapphire oscillator as an ultra-stable local frequency reference. The second advance is a powerful method to control the frequency shift due to cold collisions. These two advances lead to a fractional frequency in stability of 2 x 10(-16) at 50000 s between two independent primary standards. In addition, these clocks realize the SI second with an accuracy of 7 x 10(-16), one order of magnitude below that of uncooled devices. Tests of fundamental physical laws constitute an important field of application for highly accurate atomic clocks. In a second part, we describe tests of possible variations of fundamental constants using Rb-87 and Cs-133 fountains. The third part is 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 will fly on board the International Space Station in 2007-2008, enabling a new generation of relativity tests. (c) 2004 Academie des sciences. Published by Elsevier SAS. All rights reserved.

Over five years we have compared the hyperfine frequencies of Cs-113 and Rb-87 atoms in their electronic ground state using several laser cooled (CS)-C-133 and Rb-87 atomic fountains with an accuracy of similar to 10(-15). These measurements set a stringent upper bound to a possible fractional time variation of the ratio between the two frequencies : d/dt In ( nuRb/nuCs) = (0.2 +/- 7.0) x 10(-16) yr(-1) (1sigma- uncertainity). The same limit applies to a possible variation of the quantity (muRb/muCs)alpha(-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(-3) level in high precision experiments. With this precision, reaching an accuracy of 10(-16) is possible.