Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Precision spectroscopy and density-dependent frequency shifts in
  ultracold Sr   By varying the density of an ultracold $^{88}$Sr sample from $10^9$ cm$^{-3}$
to $> 10^{12}$ cm$^{-3}$, we make the first definitive measurement of the
density-related frequency shift and linewidth broadening of the $^1S_0$ -
$^3P_1$ optical clock transition in an alkaline earth system. In addition, we
report the most accurate measurement to date of the $^{88}$Sr $^1S_0 - ^3P_1$
optical clock transition frequency. Including a detailed analysis of systematic
errors, the frequency is ($434 829 121 312 334 \pm 20_{stat} \pm 33_{sys}$) Hz.

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The categories of this paper are: gr-qc
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The categories of this paper are: chao-dyn
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The categories of this paper are: eess
29
The categories of this paper are: funct-an
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The categories of this paper are: hep-ph
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
2
The categories of this paper are: math
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: mtrl-th
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The categories of this paper are: solv-int
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The categories of this paper are: econ
27
The categories of this paper are: comp-gas
37
The categories of this paper are: supr-con
25
The categories of this paper are: patt-sol