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 133Cs and 87Rb atomic fountains are described. An important advance is the achievement of a fractional frequency instability of 1.6 × 10−14τ−1/2 where τ 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 × 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.
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 133Cs and 87Rb atomic fountains are described. An important advance is the achievement of a fractional frequency instability of 1.6 × 10−14τ−1/2 where τ 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 × 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.
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 of133Cs 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-frac12is obtained routinely
The well-known MOT technique is used to trap and cool 133 Cs atoms to reproduce the SI second in an atomic fountain. About 10 8 cesium atoms are caught in a magnetooptical trap from a vapor cell at room temperature. The fountain has a (1,1,1)-geometry for the laser cooling and launching operation. The three upper beams have the same polarization, opposite to the three lower ones. The interrogation and preparation cavities have also been constructed, as well as characterized, concerning the temperature tunability in vacuum. The microwave synthesizer has been constructed in collaboration with BNM-SYRTE, time-and-frequency group. The synthesizer has been compared to a very stable oscillator at BNM-SYRTE resulting in a short-term stability of 9.7 × 10 -14 τ -1/2 .
It was anticipated that laser cooling of atoms to 2 {mu}K would improve the accuracy and resolution of laser and microwave spectroscopies by 2 to 3 orders of magnitude. In state-of-the-art room temperature Cs frequency standards, the 9 GHz ground state hyperfine transition is measured with a fractional accuracy of 3 x 10{sup {minus}14}. Since all of the formerly known systematic effects that limit the accuracy increase either linearly or as some high power of the atom`s velocity, the accuracy of a laser cooled Cs standard should be {approx} 10{sup {minus}16}. However, at {mu}K temperatures, the de Broglie wavelengths of the atoms are much larger than the scale of the interatomic potential so that collision cross sections can be as large as {lambda}{sub dB}{sup 2}/2{pi} {approx} 10{sup {minus}10} cm{sup 2}. Here, collisional frequency shifts in a laser-cooled Cs fountain are measured to be {minus}1.4 x 10{sup {minus}12} corresponding to a cross section of 0.5 x 10{sup {minus}10} cm{sup 2}. The author also expects to report experimental results showing a potentially strong magnetic field dependence of these cross sections for fields smaller than 0.2 G. The field of ultra-cold collisions will also be reviewed.
A few years ago atomic fountains using cold ^133Cs atoms led to a breakthrough in the field of atomic frequency standards(A. Clairon, C. Salomon, S. Guellati, and W. D. Phillips, Europhys. Lett. 16), 165 (1991); K. Gibble and S. Chu, Phys. Rev. Lett. 70, 1771 (1993).. It was soon found that the frequency shifts induced by collisions between atoms during their fountain orbit stand in the way to drawing the full benefits from this development. A possible way out is to switch to another atomic species(K. Gibble and B.J. Verhaar, Phys. Rev. A 52), 3370 (1995).. Recent experiments have made it possible to determine cold collision parameters for pairs of rubidium atoms with unprecedented accuracy(J.M. Vogels, C.C. Tsai, R.S. Freeland, S.J.J.M.F. Kokkelmans, B.J. Verhaar, and D.J. Heinzen (submitted).). Making use of these parameters we predict the collisional frequency shifts for a ^87Rb and a ^85Rb laser-cooled clock. Our results show the prospects for new atomic clocks based on ultracold rubidium to be promising.
The accelerated development of laser methods for manipulating the internal state and motion of atoms, and the recent primary evaluation in France of a cesium frequency standard using cold atoms in an atomic fountain configuration, allow the frontiers of time metrology, accuracy, and stability to be pushed near parts in the 1016 level, a factor of 100 over the present primary laboratory clocks. These positive results revived the development of more accurate cesium atomic clocks, and the Institute of National Measurement Standards (INMS), of the National Research Council of Canada (NRC) is undertaking a cesium-fountain project aimed at reaching these new levels of accuracy and stability. This paper reviews its present status. To appreciate the considerable impact of cold atoms in time and frequency metrology, we begin by recalling the background principles of traditional cesium frequency standards, their typical performance and main limitations. After introducing the important theoretical results essential to understanding the techniques of laser cooling and magneto-optical trapping, we focus on describing the characterization and control of our magneto-optical trap (MOT) and report the parameters that allow us to fill the trap with a few million atoms in less than a second and cool them to a temperature lower than 5 muK using optical molasses. We will describe the specifics of our cesium-fountain prototype, which operates with a 2-dimensional moving molasses. This special configuration is applicable for multipulse operation of the fountain, which can greatly relax the local oscillator requirements for time keeping. Finally, progress toward our objective of launching cooled atoms, at a rate of one ensemble per second, at speeds of 7 m/s, for a fountain with an interrogation time of about 1 s, will be analysed.PACS Nos.: 06.20, 06.30, and 32.80P
One of the main limitations of cesium atomic fountains has been the cold collision frequency shift. By using a method based on a transfer of population by adiabatic passage allowing to prepare cold atomic samples with a well defined ratio of atomic density as well as atom number the collisional shift is controlled at the 10E-3 of its value. A calibration of Zeeman sub-states contribution to the clock shift as a function of the field has been performed. Feshbach resonances have been observed for the first time at very low magnetic field and with a very good resolution. A Monte Carlo simulation has been performed and could fit properly some of experimental data. This constrains some parameters of the theory of collisions.