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Predictions for collisional frequency shifts of ultracold rubidium atomic clocks

S. J. J. M. F. Kokkelmans, BOUDEWIJN J. VERHAAR, Daniel J. Heinzen, Kurt Gibble

Published Apr 1, 1997• 0 citations

Abstract

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.

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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.

Using information from a recent

{}^{85}

Rb two-color photoassociation experiment, we evaluate the merits of fountain clocks based on

{}^{87}

Rb and

{}^{85}

Rb isotopes as alternatives to

{}^{133}

Cs and find that they offer significant advantages. In the case of

{}^{87}

Rb the collisionally induced fractional frequency shift is 15 times smaller than for

{}^{133}

Cs. This small shift is associated with a small difference in the triplet and singlet scattering lengths for

{}^{87}

Rb. For

{}^{85}

Rb, the shift produced by the two

{m}_{f}=0

clock states may have opposite signs allowing the shift to be eliminated by controlling the relative populations of these states. We also present collision quantities relevant to atomic fountain clocks containing multiply launched groups of atoms, and for evaporative cooling of

{}^{85}

Rb atoms.

A technique to cancel the frequency shift due to cold collisions of laser-cooled atoms is proposed. Results for future clocks based on

^{135}\mathrm{Cs}

and

^{137}\mathrm{Cs}

are presented. We find a

^{137}\mathrm{Cs}

fountain clock can be operated with no cold-collision shift by adjusting the population ratios in the fountain, and it may also be possible for

^{135}\mathrm{Cs}

. This technique can also be applied to other phase-sensitive measurements using cold atoms, including atom interferometers.

Abstract—We demonstrate a prototype of a laser-cooled 87Rb fountain clock. We report a preliminary measurement of the frequency shift due to cold collisions and find the shift to be at least a factor of ten less than that in a laser-cooled Cs clock. We also demonstrate the use of the ac Zeeman shift to measure the response of a microwave cavity. Finally, we have demonstrated a juggling atomic fountain and discuss the application of this technique to fountain and microgravity atomic clocks. Index Terms—Atomic clock, microwave resonators, microwave spectroscopy, rubidium materials/devices. I.

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