Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Purity and Covariance Matrix   Basing on the simplest single-mode field source, we investigate the role of
the various covariance matrices for reconstructing the field state and
describing its quantum statistical properties. In spite of the fact that the
intracavity field is a single-mode field, we take into account the natural
multimode structure arising in the field, when it leaves the cavity for the
free-space propagation. We show how the purity of the field state can be
calculated using the different covariance matrices.

1
The categories of this paper are: cs
36
The categories of this paper are: cmp-lg
4
The categories of this paper are: cond-mat
10
The categories of this paper are: q-bio
6
The categories of this paper are: astro-ph
30
The categories of this paper are: q-alg
3
The categories of this paper are: physics
5
The categories of this paper are: gr-qc
17
The categories of this paper are: q-fin
20
The categories of this paper are: acc-phys
19
The categories of this paper are: eess
37
The categories of this paper are: supr-con
31
The categories of this paper are: ao-sci
26
The categories of this paper are: bayes-an
25
The categories of this paper are: patt-sol
2
The categories of this paper are: math