Instruct: Identify the main and secondary category of Arxiv papers based on the titles
Query: Generation of two-color polarization-entangled optical beams with a
  self-phase-locked two-crystal Optical Parametric Oscillator
17
The categories of this paper are: q-fin
8
The categories of this paper are: hep-ex
4
The categories of this paper are: cond-mat
5
The categories of this paper are: gr-qc
28
The categories of this paper are: alg-geom
18
The categories of this paper are: econ
27
The categories of this paper are: comp-gas
2
The categories of this paper are: math
35
The categories of this paper are: mtrl-th
12
The categories of this paper are: nucl-th
6
The categories of this paper are: astro-ph
23
The categories of this paper are: solv-int
26
The categories of this paper are: bayes-an
20
The categories of this paper are: acc-phys
25
The categories of this paper are: patt-sol
7
The categories of this paper are: hep-th