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Breakdown of gauge invariance in ultrastrong-coupling cavity QED

Breakdown of gauge invariance in ultrastrong-coupling cavity QED
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 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.
2469-9926
De Bernardis, Daniele
993732bd-d305-4575-86d8-08ae5d204c0d
Pilar, Philipp
f011b46d-4a3a-4f17-ab16-da2497a9fc54
Jaako, Tuomas
6c3486a7-1e6a-4596-acf7-31f6d0a7ec01
De Liberato, Simone
5942e45f-3115-4027-8653-a82667ed8473
Rabl, Peter
feae4b02-759a-40e8-8745-bbf82501c656
De Bernardis, Daniele
993732bd-d305-4575-86d8-08ae5d204c0d
Pilar, Philipp
f011b46d-4a3a-4f17-ab16-da2497a9fc54
Jaako, Tuomas
6c3486a7-1e6a-4596-acf7-31f6d0a7ec01
De Liberato, Simone
5942e45f-3115-4027-8653-a82667ed8473
Rabl, Peter
feae4b02-759a-40e8-8745-bbf82501c656

De Bernardis, Daniele, Pilar, Philipp, Jaako, Tuomas, De Liberato, Simone and Rabl, Peter (2018) Breakdown of gauge invariance in ultrastrong-coupling cavity QED. Physical Review A, 98. (doi:10.1103/PhysRevA.98.053819).

Record type: Article

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.

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PhysRevA.98.053819 - Version of Record
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e-pub ahead of print date: 13 November 2018
Published date: 13 November 2018

Identifiers

Local EPrints ID: 426427
URI: https://eprints.soton.ac.uk/id/eprint/426427
ISSN: 2469-9926
PURE UUID: de039bcc-a264-40ba-bccf-31c568b0a647
ORCID for Simone De Liberato: ORCID iD orcid.org/0000-0002-4851-2633

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Date deposited: 27 Nov 2018 17:30
Last modified: 24 May 2019 00:31

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