32 research outputs found
Ancillary qubit spectroscopy of cavity (circuit) QED vacua
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
Charge Offsets Fluxonium: Single Cooper-Pair Circuit Free of
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Properties of optimal gauges in multi-mode cavity QED
Multi-mode cavity quantum electrodynamics (QED) describes, for example, the
coupling between an atom and a multi-mode electromagnetic resonator. The gauge
choice is important for practical calculations in truncated Hilbert spaces,
because the exact gauge-invariance is recovered only in the whole space. An
optimal gauge can be defined as the one predicting the most accurate
observables for the same number of atomic levels and modes. Different metrics
quantifying the gauge performance can be introduced depending on the observable
of interest. In this work we demonstrate that the optimal choice is generally
mode-dependent, i.e., a different gauge is needed for each cavity mode. While
the choice of gauge becomes more important for increasing light-matter
interaction, we also show that the optimal gauge does not correspond to the
situation where the entanglement between light and matter is the smallest.Comment: 8 pages, 5 figure