Measurement and Cancellation of the Cold Collision Frequency Shift in an87RbFountain Clock

Abstract

We measure a cold collision frequency shift in an 87Rb fountain clock that is fractionally 30 times smaller than that for Cs. The shift is -0.38(8) mHz for a density of 1.0(6)x10(9) cm(-3). We study the cavity pulling of the atomic transition and use it to cancel the cold collision shift. We also measure the partial frequency shifts of each clock state finding 2(lambda(10)-lambda(20))/(lambda(10)+lambda(20)) = 0.1(6).

Cold atom clocks and applications

“…As pointed out in the first paper observing the cold collision shift in 133 Cs fountains [12], selecting atoms in the clock levels using microwaves may lead to distortions of the position or velocity distribution. Methods to cope with these effects have been proposed [8], yet the linear extrapolations have proved to be valid only at the 5% to 10% level.…”

“…A frequency comparison between two fountains exhibits a stability of 2 10 −16 at 50 000 second averaging time, for the first time for atomic standards. This frequency resolution sets the stage for clock accuracy at the 10 −16 level for cesium, almost one order of magnitude potential gain, and even better for rubidium with its far reduced collision shift [7,8]. We begin by recalling the basic operation of fountain atomic clocks and introduce several new techniques which demonstrate frequency measurements with a frequency resolution at the 10 −16 level.…”

“…Since the collisional shift is proportional to the atomic density, it can be extrapolated to zero density with this accuracy. In addition, with this method, the cavity frequency pulling [7,8,15] is also accounted for. The collisional shift is measured in real-time with the following differential method.…”

“…The second advance deals with a new technique to measure and cancel with high precision the collisional shift in the clock. This shift is a major plague in cesium clocks and is much reduced (two orders of magnitude) in rubidium devices [7,8]. The method uses interrupted adiabatic population transfer to prepare precise ratios of atomic densities.…”

“…This represents a one order of magnitude improvement over uncooled cesium devices. In the future, we anticipate that the extensive use of the methods described above will enable us to bring the accuracy of 133 Cs fountains below 2 parts in 10 16 and the accuracy of 87 Rb to a even lower value thanks to its 100-fold lower collision shift [7,8].…”