Ancillary qubit spectroscopy of cavity (circuit) QED vacua

TL;DR

This paper proposes a theoretical method using an ancillary qubit to probe and distinguish different cavity QED ground states with photons, revealing their properties through spectral signatures and Lamb shifts.

Contribution

It introduces a novel spectroscopy technique employing an ancillary qubit to analyze ground states in ultrastrong coupling cavity QED models, including Dicke, Tavis-Cummings, and Hopfield-like systems.

Findings

Ancillary qubit spectral signatures depend on the type of vacuum state.

Lamb shift of the ancilla reveals photon population and correlations.

Conditions for high-fidelity measurements are established.

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 non-trivial 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. Back-action 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.