Ancillary qubit spectroscopy of cavity (circuit) QED vacua

Understand

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.