33 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
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
Charge Offsets Fluxonium: Single Cooper-Pair Circuit Free of
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Superconducting Nanowires as Nonlinear Inductive Elements for Qubits
We report microwave transmission measurements of superconducting Fabry-Perot
resonators (SFPR), having a superconducting nanowire placed at a supercurrent
antinode. As the plasma oscillation is excited, the supercurrent is forced to
flow through the nanowire. The microwave transmission of the resonator-nanowire
device shows a nonlinear resonance behavior, significantly dependent on the
amplitude of the supercurrent oscillation. We show that such
amplitude-dependent response is due to the nonlinearity of the current-phase
relationship (CPR) of the nanowire. The results are explained within a
nonlinear oscillator model of the Duffing oscillator, in which the nanowire
acts as a purely inductive element, in the limit of low temperatures and low
amplitudes. The low quality factor sample exhibits a "crater" at the resonance
peak at higher driving power, which is due to dissipation. We observe a
hysteretic bifurcation behavior of the transmission response to frequency sweep
in a sample with a higher quality factor. The Duffing model is used to explain
the Duffing bistability diagram. We also propose a concept of a nanowire-based
qubit that relies on the current dependence of the kinetic inductance of a
superconducting nanowire.Comment: 28 pages, 7 figure