Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A naturally trapped rare-earth doped solid-state superradiant laser
  clock   We propose a solid-state based superradiance laser which is almost
insensitive to the cavity mirror vibration. Therefore, it can compete with the
best frequency-stable local oscillators. The long coherence time and the large
optical density of rare-earth-ions (REIs) doped solids are employed to find a
regime to demonstrate a steady-state laser emission with linewidth smaller than
the atomic decay rate. The experimental parameters are discussed and
intracavity photon number and laser linewidth are calculated based on the
mean-field theory. A procedure for measuring absolute laser linewidth is
proposed.

3
The categories of this paper are: physics
11
The categories of this paper are: hep-lat
24
The categories of this paper are: dg-ga
27
The categories of this paper are: comp-gas
21
The categories of this paper are: adap-org
5
The categories of this paper are: gr-qc
15
The categories of this paper are: nucl-ex
6
The categories of this paper are: astro-ph
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The categories of this paper are: cond-mat
14
The categories of this paper are: math-ph
12
The categories of this paper are: nucl-th
7
The categories of this paper are: hep-th
8
The categories of this paper are: hep-ex
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The categories of this paper are: nlin
22
The categories of this paper are: chao-dyn
25
The categories of this paper are: patt-sol