Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A strontium optical lattice clock with $1 \times 10^{-17}$ uncertainty
  and measurement of its absolute frequency   We present a measurement of the absolute frequency of the 5s$^2$ $^1$S$_0$ to
5s5p $^3$P$_0$ transition in $^{87}$Sr, which is a secondary representation of
the SI second. We describe the optical lattice clock apparatus used for the
measurement, and we focus in detail on how its systematic frequency shifts are
evaluated with a total fractional uncertainty of $1 \times 10^{-17}$.
Traceability to the International System of Units is provided via comparison to
International Atomic Time (TAI). Gathering data over 5- and 15-day periods,
with the lattice clock operating on average 74$\%$ of the time, we measure the
frequency of the transition to be 429228004229873.1(5) Hz, which corresponds to
a fractional uncertainty of $1 \times 10^{-15}$. We describe in detail how this
uncertainty arises from the intermediate steps linking the optical frequency
standard, through our local time scale UTC(NPL), to an ensemble of primary and
secondary frequency standards which steer TAI. The calculated absolute
frequency of the transition is in good agreement with recent measurements
carried out in other laboratories around the world.

20
The categories of this paper are: acc-phys
24
The categories of this paper are: dg-ga
5
The categories of this paper are: gr-qc
6
The categories of this paper are: astro-ph
7
The categories of this paper are: hep-th
12
The categories of this paper are: nucl-th
4
The categories of this paper are: cond-mat
25
The categories of this paper are: patt-sol
8
The categories of this paper are: hep-ex
34
The categories of this paper are: chem-ph
15
The categories of this paper are: nucl-ex
30
The categories of this paper are: q-alg
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The categories of this paper are: bayes-an
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The categories of this paper are: nlin
2
The categories of this paper are: math
27
The categories of this paper are: comp-gas