Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: A modern review of the two-level approximation   The paradigm of the two-level atom is revisited and its perturbative analysis
is discussed in view of the principle of duality in perturbation theory. The
models we consider are a two-level atom and an ensemble of two-level atoms both
interacting with a single radiation mode. The aim is to see how the latter can
be actually used as an amplifier of quantum fluctuations to the classical level
through the thermodynamic limit of a very large ensemble of two-level atoms [M.
Frasca, Phys. Lett. A {\bf 283}, 271 (2001)] and how can remove Schr\"odinger
cat states. The thermodynamic limit can be very effective for producing both
classical states and decoherence on a quantum system that evolves without
dissipation. Decoherence without dissipation is indeed an effect of a single
two-level atom interacting with an ensemble of two-level atoms, a situation
that proves to be useful to understand recent experiments on nanoscale devices
showing unexpected disappearance of quantum coherence at very low temperatures.

33
The categories of this paper are: plasm-ph
23
The categories of this paper are: solv-int
8
The categories of this paper are: hep-ex
31
The categories of this paper are: ao-sci
30
The categories of this paper are: q-alg
28
The categories of this paper are: alg-geom
11
The categories of this paper are: hep-lat
5
The categories of this paper are: gr-qc
20
The categories of this paper are: acc-phys
36
The categories of this paper are: cmp-lg
35
The categories of this paper are: mtrl-th
25
The categories of this paper are: patt-sol
27
The categories of this paper are: comp-gas
19
The categories of this paper are: eess
10
The categories of this paper are: q-bio
7
The categories of this paper are: hep-th