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Joint Conference of the European Frequency and Time Forum and IEEE International Frequency Control Symposium (EFTF/IFCS)

Rapid Determination of Microwave Cavity Crossing Positions for Atoms in Fountain Clocks

Kathryn Burrows, Richard Hendricks, K. Szymaniec

Published Apr 1, 2019• 0 citations
Publisher

Abstract

We demonstrate a convenient experimental method, that facilitates faster and more accurate evaluation of the distributed cavity phase, a leading systematic effect in atomic fountain clocks. We expand on earlier work by Nemitz et al., which showed that displacement of the atoms from the microwave cavity axis can lead to an asymmetry in resonances observed for hyperfine transitions with ΔmF= 1. By applying an additional small transverse magnetic field, with controlled magnitude and direction, we are able to extract from the measured asymmetry the atom cloud's crossing position to within a fraction of a millimeter.

DOI
10.1109/fcs.2019.8856075

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We demonstrate a convenient experimental method, that facilitates faster and more accurate evaluation of the distributed cavity phase, a leading systematic effect in atomic fountain clocks. We expand on earlier work by Nemitz et al., which showed that displacement of the atoms from the microwave cavity axis can lead to an asymmetry in resonances observed for hyperfine transitions with ΔmF= 1. By applying an additional small transverse magnetic field, with controlled magnitude and direction, we are able to extract from the measured asymmetry the atom cloud's crossing position to within a fraction of a millimeter.
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We give a review of recent advances in the accuracy of atomic fountain clocks. The new features are improved experimental and theoretical treatments of distributed cavity phase shifts and a full modeling of microwave lensing. Here we highlight that state selection is often highly non-uniform when Rabi flopping is used instead of an interrupted adiabatic frequency chirp. We show that detuning the state-selection microwave field as well as changing its amplitude can give a much more homogeneous state selection. A spatially inhomogeneous state selection corrupts the density extrapolation because the density distribution changes, which also mixes effects from distributed cavity phase and microwave lensing. This is expected to be important for the microgravity clock PHARAO on ACES, and can also improve fountain clocks.
In vapor cell atomic clocks the atom-field interaction is typically obtained inside a microwave cavity resonator in which the microwave driving field together with a static magnetic field and an optical field are applied to excite the atoms. These fields are generally well-controlled, mutually aligned to a common quantization axis. Since the exploited atomic transition is sensitive to any potential axis misalignment, the performance of the clock can also be affected. We study the effect of such misalignment for the case of a cylindrical cavity used in vapor-cell atomic clocks, taking into account the misalignments of the optical detection field and the static magnetic field required for the atomic transition. Both the geometry of the cavity and the factors contributing to losses can play role in the degradation of the signal and are taken into account in the misalignment problem discussed.
A preliminary calculation of the frequency shifts resulting from the distributed phase of a rectangular cavity is used to model frequency measurements of the fountain clock NRC-FCs1. The calculation is based on a numerical evaluation of the cavity field made with a finite element EM solver. The frequency shift is obtained by solving the Bloch-equations numerically along the path of the atoms on the free-fall trajectories through the cavity. We study the frequency shifts as a function of the launch direction, the tilt of the physics package and offsets in the position of the MOT relative to the axis defined by the state-selection cavity and the Ramsey cavity.
We report on experimental measurements and theoretical calculations of Rabi oscillation distortion during rubidium clock transition in a compact cold-atom clock. The distortion of the Rabi oscillations of the clock transition caused by the magnetically sensitive transitions between magnetic sublevels is related to the duration and amplitude of the microwave pulse, the density distribution of cold atoms, and the biased magnetic field inside the microwave cavity. The theoretical calculations agreed well with the experimental results. The Rabi oscillation measurements are beneficial for determining the optimal parameters of a cold-atom clock and identifying the uniformity of the microwave field inside the cavity. This measurement can be extended to the atomic fountain clock and the vapor-cell atomic clock.