Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Optical frequency ratio of a ${}^{171}\mathrm{Yb}^+$ single-ion clock
  and a ${}^{87}\mathrm{Sr}$ lattice clock   We report direct measurements of the frequency ratio of the 642 THz
${}^2S_{1/2} (F=0)$--${}^2F_{7/2} (F=3)$ electric octupole transition in
${}^{171}\mathrm{Yb}^+$ and the 429 THz ${}^1S_0$--${}^3P_0$ transition in
${}^{87}\mathrm{Sr}$. A series of 107 measurements has been performed at the
Physikalisch-Technische Bundesanstalt between December 2012 and October 2019.
Long-term variations of the ratio are larger than expected from the individual
measurement uncertainties of few $10^{-17}$. The cause of these variations
remains unknown. Even taking these into account, we find a fractional
uncertainty of the frequency ratio of $2.5 \times 10^{-17}$, which improves
upon previous knowledge by one order of magnitude. The average frequency ratio
is $\nu_{\mathrm{Yb}^+} / \nu_{\mathrm{Sr}} =
1.495\,991\,618\,544\,900\,537(38)$. This represents one of the most accurate
measurements between two different atomic species to date.

26
The categories of this paper are: bayes-an
18
The categories of this paper are: econ
36
The categories of this paper are: cmp-lg
11
The categories of this paper are: hep-lat
35
The categories of this paper are: mtrl-th
32
The categories of this paper are: atom-ph
19
The categories of this paper are: eess
33
The categories of this paper are: plasm-ph
2
The categories of this paper are: math
29
The categories of this paper are: funct-an
9
The categories of this paper are: nlin
20
The categories of this paper are: acc-phys
22
The categories of this paper are: chao-dyn
15
The categories of this paper are: nucl-ex
16
The categories of this paper are: stat
34
The categories of this paper are: chem-ph