Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Transverse spatial and frequency properties of two-photon states
  generated by spontaneous parametric down-conversion   We present a detailed account of the two-photon states generated by SPDC in
both type I and type II phase matching, including the effects of anisotropy of
the nonlinear medium and the frequency spread of the down-converted fields.
Accurate as well as simplified expressions are derived for type I and type II
phase matching in the context of Fourier Optics. The main results are compared
with experimental data available in the literature, showing good agreement in
all cases.

34
The categories of this paper are: chem-ph
19
The categories of this paper are: eess
16
The categories of this paper are: stat
28
The categories of this paper are: alg-geom
10
The categories of this paper are: q-bio
5
The categories of this paper are: gr-qc
29
The categories of this paper are: funct-an
2
The categories of this paper are: math
20
The categories of this paper are: acc-phys
7
The categories of this paper are: hep-th
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The categories of this paper are: supr-con
36
The categories of this paper are: cmp-lg
30
The categories of this paper are: q-alg
17
The categories of this paper are: q-fin
32
The categories of this paper are: atom-ph
11
The categories of this paper are: hep-lat