151 research outputs found
Bringing entanglement to the high temperature limit
We show the existence of an entangled nonequilibrium state at very high
temperatures when two linearly coupled harmonic oscillators are parametrically
driven and dissipate into two independent heat baths. This result has a twofold
meaning: first, it fundamentally shifts the classical-quantum border to
temperatures as high as our experimental ability allows us, and second, it can
help increase by at least one order of magnitude the temperature at which
current experimental setups are operated.Comment: accepted in Phys. Rev. Let
Information sharing in Quantum Complex Networks
We introduce the use of entanglement entropy as a tool for studying the
amount of information shared between the nodes of quantum complex networks. By
considering the ground state of a network of coupled quantum harmonic
oscillators, we compute the information that each node has on the rest of the
system. We show that the nodes storing the largest amount of information are
not the ones with the highest connectivity, but those with intermediate
connectivity thus breaking down the usual hierarchical picture of classical
networks. We show both numerically and analytically that the mutual information
characterizes the network topology. As a byproduct, our results point out that
the amount of information available for an external node connecting to a
quantum network allows to determine the network topology.Comment: text and title updated, published version [Phys. Rev. A 87, 052312
(2013)
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
Quantum Navigation and Ranking in Complex Networks
Complex networks are formal frameworks capturing the interdependencies
between the elements of large systems and databases. This formalism allows to
use network navigation methods to rank the importance that each constituent has
on the global organization of the system. A key example is Pagerank navigation
which is at the core of the most used search engine of the World Wide Web.
Inspired in this classical algorithm, we define a quantum navigation method
providing a unique ranking of the elements of a network. We analyze the
convergence of quantum navigation to the stationary rank of networks and show
that quantumness decreases the number of navigation steps before convergence.
In addition, we show that quantum navigation allows to solve degeneracies found
in classical ranks. By implementing the quantum algorithm in real networks, we
confirm these improvements and show that quantum coherence unveils new
hierarchical features about the global organization of complex systems.Comment: title changed, more real networks analyzed, version published in
scientific report
Influence of Non-Markovian Dynamics in Thermal-Equilibrium Uncertainty-Relations
Contrary to the conventional wisdom that deviations from standard
thermodynamics originate from the strong coupling to the bath, it is shown that
in quantum mechanics, these deviations originate from the uncertainty principle
and are supported by the non-Markovian character of the dynamics. Specifically,
it is shown that the lower bound of the dispersion of the total energy of the
system, imposed by the uncertainty principle, is dominated by the bath power
spectrum and therefore, quantum mechanics inhibits the system
thermal-equilibrium-state from being described by the canonical Boltzmann's
distribution. We show that for a wide class of systems, systems interacting via
central forces with pairwise-self-interacting environments, this general
observation is in sharp contrast to the classical case, for which the thermal
equilibrium distribution, irrespective of the interaction strength, is
\emph{exactly} characterized by the canonical Boltzmann distribution and
therefore, no dependence on the bath power spectrum is present. We define an
\emph{effective coupling} to the environment that depends on all energy scales
in the system and reservoir interaction. Sample computations in regimes
predicted by this effective coupling are demonstrated. For example, for the
case of strong effective coupling, deviations from standard thermodynamics are
present and, for the case of weak effective coupling, quantum features such as
stationary entanglement are possible at high temperatures.Comment: 9 pages, 3 figure
Stationary discrete solitons in circuit QED
We demonstrate that stationary localized solutions (discrete solitons) exist
in a one dimensional Bose-Hubbard lattices with gain and loss in the
semiclassical regime. Stationary solutions, by defi- nition, are robust and do
not demand for state preparation. Losses, unavoidable in experiments, are not a
drawback, but a necessary ingredient for these modes to exist. The
semiclassical calculations are complemented with their classical limit and
dynamics based on a Gutzwiller Ansatz. We argue that circuit QED architectures
are ideal platforms for realizing the physics developed here. Finally, within
the input-output formalism, we explain how to experimentally access the
different phases, including the solitons, of the chain.Comment: 10 pages including appendix, 7 figure
Nonlinear quantum optics in the (ultra)strong light-matter coupling
The propagation of photons in one dimensional waveguides coupled to
qubits is discussed, both in the strong and ultrastrong qubit-waveguide
coupling. Special emphasis is placed on the characterisation of the nonlinear
response and its linear limit for the scattered photons as a function of ,
, qubit inter distance and light-matter coupling. The quantum evolution is
numerically solved via the Matrix Product States technique. Both the time
evolution for the field and qubits is computed. The nonlinear character (as a
function of ) depends on the computed observable. While perfect reflection
is obtained for , photon-photon correlations are still resolved
for ratios . Inter-qubit distance enhances the nonlinear response.
Moving to the ultrastrong coupling regime, we observe that inelastic processes
are \emph{robust} against the number of qubits and that the qubit-qubit
interaction mediated by the photons is qualitatively modified. The theory
developed in this work modelises experiments in circuit QED, photonic crystals
and dielectric waveguides.Comment: Comments are wellcom
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.
Single photons by quenching the vacuum
Heisenberg's uncertainty principle implies that the quantum vacuum is not
empty but fluctuates. These fluctuations can be converted into radiation
through nonadiabatic changes in the Hamiltonian. Here, we discuss how to
control this vacuum radiation, engineering a single-photon emitter out of a
two-level system (2LS) ultrastrongly coupled to a finite-band waveguide in a
vacuum state. More precisely, we show the 2LS nonlinearity shapes the vacuum
radiation into a nonGaussian superposition of even and odd cat states. When the
2LS bare frequency lays within the band gaps, this emission can be well
approximated by individual photons. This picture is confirmed by a
characterization of the ground and bound states, and a study of the dynamics
with matrix product states and polaron Hamiltonian methods.Comment: 9 pages, 10 figure
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