Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observation of single-photon superradiance and the cooperative Lamb
  shift in an extended sample of cold atoms   We report direct, time-resolved observations of single-photon superradiance
in a highly extended, elliptical sample of cold 87Rb atoms. The observed rapid
decay rate is accompanied by its counterpart, the cooperative Lamb shift. The
rate of the strongly directional decay, and the associated shift, scale
linearly with the number of atoms and show a strong dependence on the sample
shape, demonstrating the collective nature of the observed quantities.

2
The categories of this paper are: math
23
The categories of this paper are: solv-int
18
The categories of this paper are: econ
37
The categories of this paper are: supr-con
1
The categories of this paper are: cs
32
The categories of this paper are: atom-ph
4
The categories of this paper are: cond-mat
20
The categories of this paper are: acc-phys
10
The categories of this paper are: q-bio
33
The categories of this paper are: plasm-ph
22
The categories of this paper are: chao-dyn
29
The categories of this paper are: funct-an
8
The categories of this paper are: hep-ex
25
The categories of this paper are: patt-sol
19
The categories of this paper are: eess
15
The categories of this paper are: nucl-ex