Abstract : The grant as in support of a research program to explore the possibility of a microwave atomic clock based on an atomic fountain of cesium atoms. During the two year granting period, the following work was done: A large magneto-optic trap in a vapor cell was constructed where over 4x 10(exp 10) atoms were trapped at a density on the order of 10(exp 11) atoms/cu cm. This work increased the number of atoms optically trapped by a factor of -400 compared to previous work. The optical density of the atoms in this trap has a peak attenuation of e(-175). In an invited review article we examined the prospects for slow atom frequency standards. Most of the systematic effects that limit the current accuracy today's frequency standards decrease as the velocity of the atoms is reduced. We identified the one possible exception: collisions between ultra-cold atom in an atomic fountain could induce a phase shift that would ultimately limit the accuracy of the fountain clock. We brought the problem to the attention of B. Verhaar, who began a series of calculations on the effects of low energy collisions. jg p.4
Abstract : The grant as in support of a research program to explore the possibility of a microwave atomic clock based on an atomic fountain of cesium atoms. During the two year granting period, the following work was done: A large magneto-optic trap in a vapor cell was constructed where over 4x 10(exp 10) atoms were trapped at a density on the order of 10(exp 11) atoms/cu cm. This work increased the number of atoms optically trapped by a factor of -400 compared to previous work. The optical density of the atoms in this trap has a peak attenuation of e(-175). In an invited review article we examined the prospects for slow atom frequency standards. Most of the systematic effects that limit the current accuracy today's frequency standards decrease as the velocity of the atoms is reduced. We identified the one possible exception: collisions between ultra-cold atom in an atomic fountain could induce a phase shift that would ultimately limit the accuracy of the fountain clock. We brought the problem to the attention of B. Verhaar, who began a series of calculations on the effects of low energy collisions. jg p.4
Abstract : With the funding provided under this grant, we have procured equipments for creating a sample of 87Rb atoms with ultracold temperature (35 nano-Kelvin) and high density (10(exp 12 /sq cm), using Raman cooling via velocity selective coherent population trapping. We have already been able to create a magneto-optic trap (MOT) of 87Rb atoms, loaded from background rubidium vapor inside the trap chamber. We have achieved a density close to 10(exp 12) /sq cm, at a temperature of 80 mu-K, which is typical of such a trap. The next step is to load the trap from a chirp-slowed atomic beam, and use a repumping beam which is dark at the center. This is expected to increase the density to about 10(exp 12) /sq cm. We will then Raman- cool the sample to about 35 nano-Kelvin, thus yielding a sample for a rubidium fountain clock. The fountain clock loaded from this source is expected to have a short term accuracy that is more than two orders of magnitude better than existing cesium fountain, which are already a factor of two better than existing cesium beam clocks used as primary frequency standards.
Cesium atomic fountain clock is the quantum frequency standard with very high accuracy. Research on laser cooling and trapping of cesium atom at Peking University began in 1993. A cesium atomic fountain frequency standard is being developed. A magneto-optical trap with some 108 trapped atoms and an optical molasses with the cold atoms of the temperature of 10/spl mu/K have been achieved previously. The device has been carefully designed and is under construction. It is composed of two main parts: vacuum system and laser system. The article describes the apparatus and presents a brief introduction to some considerations on the atomic clock.
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
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.
In our Cs atomic fountain, we laser cooled and trapped 10(exp 10) Cs atoms in 0.4 s. By shifting the frequencies of the laser beams, the atoms were launched upwards at 2.5 m/s and a fraction of the atoms were optically pumped into the F=3 ground state. The unwanted atoms in the F=4 ground state were removed from the fountain with radiation pressure from a laser beam tuned to excite only those atoms. The Cs atoms in the F=3 state traveled ballistically upwards, were excited by the microwave cavity, and then returned back through the same cavity in the atomic fountain configuration.
A scheme for a microwave atomic clock is proposed for Cs or Rb atoms trapped in a blue detuned optical lattice. The ac Stark shift of the clock transition due to a trapping laser is calculated. We analyze it at some specific laser wavelength. Compared with the case of the fountain clock, the cavity related shifts, the collision shift and the Doppler effect are eliminated or suppressed dramatically in an atomic lattice clock. By analyzing various sources of clock uncertainty, a microwave atomic lattice clock with a high accuracy and small volume is feasible.