Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Counting Photons in Static Electric and Magnetic Fields   We describe the electromagnetic field by the massless limit of a massive
vector field in the presence of a Coulomb gauge fixing term. The gauge fixing
term ensures that, in the massless limit, the longitudinal mode is removed from
the spectrum and only the two transverse modes survive. The system, coupled to
a classical conserved current, is quantized in the canonical formalism. The
classical field configurations due to time-independent electric charges and
currents are represented by coherent states of longitudinal and transverse
photons, respectively. The occupation number in these states is finite. In
particular, the number of longitudinal photons bound by an electric charge q is
given by N=q^2/(16\pi\hbar).

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The categories of this paper are: nucl-ex
3
The categories of this paper are: physics
11
The categories of this paper are: hep-lat
37
The categories of this paper are: supr-con
25
The categories of this paper are: patt-sol
19
The categories of this paper are: eess
6
The categories of this paper are: astro-ph
16
The categories of this paper are: stat
35
The categories of this paper are: mtrl-th
28
The categories of this paper are: alg-geom
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The categories of this paper are: quant-ph
30
The categories of this paper are: q-alg
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The categories of this paper are: chem-ph
0
The categories of this paper are: hep-ph
4
The categories of this paper are: cond-mat
18
The categories of this paper are: econ