21 research outputs found
Entanglement and Disentanglement in Circuit QED Architectures
We propose a protocol for creating entanglement within a dissipative circuit
QED network architecture that consists of two electromagnetic circuits
(cavities) and two superconducting qubits. The system interacts with a quantum
environment, giving rise to decoherence and dissipation. We discuss the
preparation of two separate entangled cavity-qubit states via Landau-Zener
sweeps, after which the cavities interact via a tunable "quantum switch" which
is realized with an ancilla qubit. Moreover, we discuss the decay of the
resulting entangled two-cavity state due to the influence of the environment,
where we focus on the entanglement decay.Comment: 7 pages, 5 figure
Nonequilibrium phases in hybrid arrays with flux qubits and NV centers
We propose a startling hybrid quantum architecture for simulating a
localization-delocalization transition. The concept is based on an array of
superconducting flux qubits which are coupled to a diamond crystal containing
nitrogen-vacancy (NV) centers. The underlying description is a
Jaynes-Cummings-lattice in the strong-coupling regime. However, in contrast to
well-studied coupled cavity arrays the interaction between lattice sites is
mediated here by the qubit rather than by the oscillator degrees of freedom.
Nevertheless, we point out that a transition between a localized and a
delocalized phase occurs in this system as well. We demonstrate the possibility
of monitoring this transition in a non-equilibrium scenario, including
decoherence effects. The proposed scheme allows the monitoring of
localization-delocalization transitions in Jaynes-Cummings-lattices by use of
currently available experimental technology. Contrary to cavity-coupled
lattices, our proposed recourse to stylized qubit networks facilitates (i) to
investigate localization-delocalization transitions in arbitrary dimensions and
(ii) to tune the inter-site coupling in-situ.Comment: Version to be published in Phys. Rev.
Time-Resolved Measurement of a Charge Qubit
We propose a scheme for monitoring coherent quantum dynamics with good
time-resolution and low backaction, which relies on the response of the
considered quantum system to high-frequency ac driving. An approximate
analytical solution of the corresponding quantum master equation reveals that
the phase of an outgoing signal, which can directly be measured in an
experiment with lock-in technique, is proportional to the expectation value of
a particular system observable. This result is corroborated by the numerical
solution of the master equation for a charge qubit realized with a Cooper-pair
box, where we focus on monitoring coherent oscillations.Comment: 4 pages, 3 figure
Monitoring Entanglement Evolution and Collective Quantum Dynamics
We generalize a recently developed scheme for monitoring coherent quantum
dynamics with good time-resolution and low backaction [Reuther et al., Phys.
Rev. Lett. 102, 033602 (2009)] to the case of more complex quantum dynamics of
one or several qubits. The underlying idea is to measure with lock-in
techniques the response of the quantum system to a high-frequency ac field. We
demonstrate that this scheme also allows one to observe quantum dynamics with
many frequency scales, such as that of a qubit undergoing Landau-Zener
transitions. Moreover, we propose how to measure the entanglement between two
qubits as well as the collective dynamics of qubit arrays.Comment: 11 pages, 5 figure
Two-resonator circuit QED: Dissipative Theory
We present a theoretical treatment for the dissipative two-resonator circuit
quantum electrodynamics setup referred to as quantum switch. There, switchable
coupling between two superconducting resonators is mediated by a
superconducting qubit operating in the dispersive regime, where the qubit
transition frequency is far detuned from those of the resonators. We derive an
effective Hamiltonian for the quantum switch beyond the rotating wave
approximation and study the dissipative dynamics within a Bloch-Redfield
quantum master equation approach. We derive analytically how the qubit affects
the quantum switch even if the qubit has no dynamics, and we estimate the
strength of this influence. The analytical results are corroborated by
numerical calculations, where coherent oscillations between the resonators, the
decay of coherent and Fock states, and the decay of resonator-resonator
entanglement are studied. Finally, we suggest an experimental protocol for
extracting the damping constants of qubit and resonators by measuring the
quadratures of the resonator fields.Comment: 17 pages, 9 figure
Non-Markovian qubit decoherence during dispersive readout
We study qubit decoherence under generalized dispersive readout, i.e., we
investigate a qubit coupled to a resonantly driven dissipative harmonic
oscillator. We provide a complete picture by allowing for arbitrarily large
qubit-oscillator detuning and by considering also a coupling to the square of
the oscillator coordinate, which is relevant for flux qubits. Analytical
results for the decoherence time are obtained by a transformation of the
qubit-oscillator Hamiltonian to the dispersive frame and a subsequent master
equation treatment beyond the Markov limit. We predict a crossover from
Markovian decay to a decay with Gaussian shape. Our results are corroborated by
the numerical solution of the full qubit-oscillator master equation in the
original frame.Comment: 8 pages, 3 figure
Qubit-oscillator dynamics in the dispersive regime: Analytical theory beyond the rotating-wave approximation
We generalize the dispersive theory of the Jaynes-Cummings model beyond the
frequently employed rotating-wave approximation (RWA) in the coupling between
the two-level system and the resonator. For a detuning sufficiently larger than
the qubit-oscillator coupling, we diagonalize the non-RWA Hamiltonian and
discuss the differences to the known RWA results. Our results extend the regime
in which dispersive qubit readout is possible. If several qubits are coupled to
one resonator, an effective qubit-qubit interaction of Ising type emerges,
whereas RWA leads to isotropic interaction. This impacts on the entanglement
characteristics of the qubits.Comment: 6 pages, 1 figur