title: Ancillary qubit spectroscopy of vacua in cavity and circuit quantum electrodynamics.
abstract: We investigate theoretically how the spectroscopy of an ancillary qubit can probe cavity (circuit) QED ground states containing photons. We consider three classes of systems (Dicke, Tavis-Cummings, and Hopfield-like models), where nontrivial vacua are the result of ultrastrong coupling between N two-level systems and a single-mode bosonic field. An ancillary qubit detuned with respect to the boson frequency is shown to reveal distinct spectral signatures depending on the type of vacua. In particular, the Lamb shift of the ancilla is sensitive to both ground state photon population and correlations. Backaction of the ancilla on the cavity ground state is investigated, taking into account the dissipation via a consistent master equation for the ultrastrong coupling regime. The conditions for high-fidelity measurements are determined.
authors.affiliation: Université Paris Diderot, Sorbonne Paris Cité, Laboratoire Matériaux et Phénomènes Quantiques, CNRS-UMR 7162, 75013 Paris, France.
authors.forename: Jared
authors.initials: J
authors.lastname: Lolli
authors.affiliation: Université Paris Diderot, Sorbonne Paris Cité, Laboratoire Matériaux et Phénomènes Quantiques, CNRS-UMR 7162, 75013 Paris, France.
authors.forename: Alexandre
authors.initials: A
authors.lastname: Baksic
authors.affiliation: Université Paris Diderot, Sorbonne Paris Cité, Laboratoire Matériaux et Phénomènes Quantiques, CNRS-UMR 7162, 75013 Paris, France.|Institute for Solid State Physics and Optics, Wigner Research Centre, Hungarian Academy of Sciences, P.O. Box 49, H-1525 Budapest, Hungary.
authors.forename: David
authors.initials: D
authors.lastname: Nagy
authors.affiliation: Joint Quantum Institute and Center for Nanophysics and Advanced Materials, University of Maryland, College Park, Maryland 20742, USA.
authors.forename: Vladimir E
authors.initials: VE
authors.lastname: Manucharyan
authors.affiliation: Université Paris Diderot, Sorbonne Paris Cité, Laboratoire Matériaux et Phénomènes Quantiques, CNRS-UMR 7162, 75013 Paris, France.
authors.forename: Cristiano
authors.initials: C
authors.lastname: Ciuti