Breakdown of gauge invariance in ultrastrong-coupling cavity QED

Year: 2018

Authors: De Bernardis D., Pilar P., Jaako T., De Liberato S., Rabl P.

Autors Affiliation: TU Wien, Vienna Ctr Quantum Sci & Technol, Atominst, A-1040 Vienna, Austria; Univ Southampton, Sch Phys & Astron, Southampton SO17 1BJ, Hants, England.

Abstract: 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 multidipole (Dicke) case, and consider the specific examples of dipoles confined by double-well and square-well potentials, and of circuit QED systems with flux qubits coupled to an LC resonator.

Journal/Review: PHYSICAL REVIEW A

Volume: 98 (5)      Pages from: 53819-1  to: 53819-15

More Information: This work was supported by the Austrian Science Fund (FWF) through the SFB FoQuS, Grant No. F40, the DK CoQuS, Grant No. W 1210, and the START Grant No. Y 591-N16. S.D.L. acknowledges financial support from a Royal Society research fellowship.
KeyWords: Phase-transition; Dicke-model; Quantum; Radiation; Field; Molecules; Coherent; Physics
DOI: 10.1103/PhysRevA.98.053819

Citations: 127
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