76 research outputs found
Simple preparation of Bell and GHZ states using ultrastrong-coupling circuit QED
The ability to entangle quantum systems is crucial for many applications in
quantum technology, including quantum communication and quantum computing.
Here, we propose a new, simple, and versatile setup for deterministically
creating Bell and Greenberger-Horne-Zeilinger (GHZ) states between photons of
different frequencies in a two-step protocol. The setup consists of a quantum
bit (qubit) coupled ultrastrongly to three photonic resonator modes. The only
operations needed in our protocol are to put the qubit in a superposition
state, and then tune its frequency in and out of resonance with sums of the
resonator-mode frequencies. By choosing which frequency we tune the qubit to,
we select which entangled state we create. We show that our protocol can be
implemented with high fidelity using feasible experimental parameters in
state-of-the-art circuit quantum electrodynamics. One possible application of
our setup is as a node distributing entanglement in a quantum network.Comment: 15 pages, 7 figure
Non-perturbative Dynamical Casimir Effect in Optomechanical Systems: Vacuum Casimir-Rabi Splittings
We study the dynamical Casimir effect using a fully quantum-mechanical
description of both the cavity field and the oscillating mirror. We do not
linearize the dynamics, nor do we adopt any parametric or perturbative
approximation. By numerically diagonalizing the full optomechanical
Hamiltonian, we show that the resonant generation of photons from the vacuum is
determined by a ladder of mirror-field {\em vacuum Rabi splittings}. We find
that vacuum emission can originate from the free evolution of an initial pure
mechanical excited state, in analogy with the spontaneous emission from excited
atoms. By considering a coherent drive of the mirror, using a master-equation
approach to take losses into account, we are able to study the dynamical
Casimir effect for optomechanical coupling strengths ranging from weak to
ultrastrong. We find that a resonant production of photons out of the vacuum
can be observed even for mechanical frequencies lower than the cavity-mode
frequency. Since high mechanical frequencies, which are hard to achieve
experimentally, were thought to be imperative for realizing the dynamical
Casimir effect, this result removes one of the major obstacles for the
observation of this long-sought effect. We also find that the dynamical Casimir
effect can create entanglement between the oscillating mirror and the radiation
produced by its motion in the vacuum field, and that vacuum Casimir-Rabi
oscillations can occur.Comment: 30 pages, 8 figure
Coherent Resonant Coupling between Atoms and a Mechanical Oscillator Mediated by Cavity-Vacuum Fluctuations
We show that an atom can be coupled to a mechanical oscillator via quantum
vacuum fluctuations of a cavity field enabling energy transfer processes
between them. In a hybrid quantum system consisting of a cavity resonator with
a movable mirror and an atom, these processes are dominated by two
pair-creation mechanisms: the counter-rotating (atom-cavity system) and
dynamical Casimir interaction terms (optomechanical system). Because of these
two pair-creation mechanisms, the resonant atom-mirror coupling is the result
of high-order virtual processes with different transition paths well described
in our theoretical framework. We perform a unitary transformation to the
atom-mirror system Hamiltonian, exhibiting two kinds of multiple-order
transitions of the pair creation. By tuning the frequency of the atom, we show
that photon frequency conversion can be realized within a cavity of multiple
modes. Furthermore, when involving two atoms coupled with the same mechanical
mode, a single vibrating excitation of the mechanical oscillator can be
simultaneously absorbed by the two atoms. Considering recent advances in strong
and ultrastrong coupling for cavity optomechanics and other systems, we believe
our proposals can be implemented using available technology.Comment: 18 pages, 13 figur
Dissipation and Thermal Noise in Hybrid Quantum Systems in the Ultrastrong Coupling Regime
The interaction among the components of a hybrid quantum system is often
neglected when considering the coupling of these components to an environment.
However, if the interaction strength is large, this approximation leads to
unphysical predictions, as has been shown for cavity-QED and optomechanical
systems in the ultrastrong-coupling regime. To deal with these cases, master
equations with dissipators retaining the interaction between these components
have been derived for the quantum Rabi model and for the standard
optomechanical Hamiltonian. In this article, we go beyond these previous
derivations and present a general master equation approach for arbitrary hybrid
quantum systems interacting with thermal reservoirs. Specifically, our approach
can be applied to describe the dynamics of open hybrid systems with harmonic,
quasi-harmonic, and anharmonic transitions. We apply our approach to study the
influence of temperature on multiphoton vacuum Rabi oscillations in circuit
QED. We also analyze the influence of temperature on the conversion of
mechanical energy into photon pairs in an optomechanical system, which has been
recently described at zero temperature. We compare our results with previous
approaches, finding that these sometimes overestimate decoherence rates and
understimate excited-state populations
Existence of solutions to some quasilinear degenerate elliptic systems with right hand side in a Marcinkiewicz space
We prove the existence of a solution to a quasilinear system of degenerate equations, when the datum is in a Marcinkiewicz space. The main assumption asks the off-diagonal coefficients to have support in the union of a geometric progression of squares
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