Distributed cavity phase (DCP) frequency shifts are a leading systematic effect in atomic fountain frequency standards. They originate from the phase variations of the field in the microwave cavity combined with different positions of the atoms in the cavity on the ascent and descent. Here we demonstrate techniques to precisely determine the position of the cloud of atoms in the microwave cavity, using either the approximately linear variation of the transverse components of the microwave field or the quadratic variation of the longitudinal microwave field amplitude in the cavity. We also show that shifting the initial position of the atoms gives a significantly higher sensitivity to DCP variations than the often-used tilting of fountains. A demonstrated centring precision of order 50 micrometres will enable DCP frequency shift uncertainties to be reduced to less than 10^-17 and thereby contribute insignificantly to the accuracy budget of a standard. These techniques to vertically align a fountain are straightforward to automate for routine operation and require a negligible fraction of the standard's averaging time.

中文翻译：

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测量微波腔中的原子位置以评估分布式腔相移

分布式腔相位 (DCP) 频移是原子喷泉频率标准中的主要系统效应。它们源于微波腔内场的相位变化，结合腔内原子在上升和下降时的不同位置。在这里，我们展示了使用微波场横向分量的近似线性变化或腔中纵向微波场幅度的二次变化来精确确定微波腔中原子云位置的技术。我们还表明，与经常使用的喷泉倾斜相比，改变原子的初始位置对 DCP 变化的敏感性要高得多。已证明的 50 微米定心精度将使 DCP 频移不确定性降低到小于 10^-17，从而对标准的精度预算贡献不大。这些垂直对齐喷泉的技术在日常操作中很容易实现自动化，并且所需的标准平均时间可以忽略不计。