Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Coherent control of collective atom phase for ultralong, inversion-free
  photon echoes   To overcome fundamental limitations of the \pi optical pulse-induced
population inversion and optical decay-caused short storage time in
conventional photon echoes, a coherent control of collective atoms is studied
for inversion-free, optical decay-halted photon echoes, where the constraint of
photon storage time is now replaced by a spin population decay process. Using
phase-controlled double rephasing, an inversion-free photon echo scheme is
obtained, where no spontaneous or stimulated emission-driven quantum noise
exists. Thus, the present method can be applied for ultralong quantum memories
in quantum repeaters for long-distance quantum communications.

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The categories of this paper are: cond-mat
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The categories of this paper are: acc-phys
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The categories of this paper are: gr-qc
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The categories of this paper are: dg-ga
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The categories of this paper are: alg-geom
32
The categories of this paper are: atom-ph
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The categories of this paper are: adap-org
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The categories of this paper are: q-fin
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The categories of this paper are: chem-ph
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The categories of this paper are: comp-gas
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The categories of this paper are: patt-sol
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The categories of this paper are: eess
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The categories of this paper are: q-alg
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The categories of this paper are: cmp-lg
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The categories of this paper are: nucl-th
33
The categories of this paper are: plasm-ph