Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Breakdown of gauge invariance in ultrastrong-coupling cavity QED   We revisit the derivation of Rabi- and Dicke-type models, which are commonly
used for the study of quantum light-matter interactions in cavity and circuit
QED. We demonstrate that the validity of the two-level approximation, which is
an essential step in this derivation, depends explicitly on the choice of gauge
once the system enters the ultrastrong coupling regime. In particular, while in
the electric dipole gauge the two-level approximation can be performed as long
as the Rabi frequency remains much smaller than the energies of all
higher-lying levels, it can dramatically fail in the Coulomb gauge, even for
systems with an extremely anharmonic spectrum. We extensively investigate this
phenomenon both in the single-dipole (Rabi) and multi-dipole (Dicke) case, and
considering the specific examples of dipoles confined by double-well and by
square-well potentials, and of circuit QED systems with flux qubits coupled to
an LC resonator.

33
The categories of this paper are: plasm-ph
3
The categories of this paper are: physics
25
The categories of this paper are: patt-sol
21
The categories of this paper are: adap-org
34
The categories of this paper are: chem-ph
23
The categories of this paper are: solv-int
14
The categories of this paper are: math-ph
36
The categories of this paper are: cmp-lg
12
The categories of this paper are: nucl-th
1
The categories of this paper are: cs
26
The categories of this paper are: bayes-an
19
The categories of this paper are: eess
6
The categories of this paper are: astro-ph
31
The categories of this paper are: ao-sci
17
The categories of this paper are: q-fin
15
The categories of this paper are: nucl-ex