Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Entanglement of a photon and a collective atomic excitation   We describe a new experimental approach to probabilistic atom-photon (signal)
entanglement. Two qubit states are encoded as orthogonal collective spin
excitations of an unpolarized atomic ensemble. After a programmable delay, the
atomic excitation is converted into a photon (idler). Polarization states of
both the signal and the idler are recorded and are found to be in violation of
the Bell inequality. Atomic coherence times exceeding several microseconds are
achieved by switching off all the trapping fields - including the quadrupole
magnetic field of the magneto-optical trap - and zeroing out the residual
ambient magnetic field.

25
The categories of this paper are: patt-sol
34
The categories of this paper are: chem-ph
16
The categories of this paper are: stat
19
The categories of this paper are: eess
0
The categories of this paper are: hep-ph
28
The categories of this paper are: alg-geom
3
The categories of this paper are: physics
30
The categories of this paper are: q-alg
21
The categories of this paper are: adap-org
15
The categories of this paper are: nucl-ex
7
The categories of this paper are: hep-th
32
The categories of this paper are: atom-ph
22
The categories of this paper are: chao-dyn
8
The categories of this paper are: hep-ex
10
The categories of this paper are: q-bio
24
The categories of this paper are: dg-ga