18,736 research outputs found
Revival of quantum correlations without system-environment back-action
Revivals of quantum correlations have often been explained in terms of
back-action on quantum systems by their quantum environment(s). Here we
consider a system of two independently evolving qubits, each locally
interacting with a classical random external field. The environments of the
qubits are also independent, and there is no back-action on the qubits.
Nevertheless, entanglement, quantum discord and classical correlations between
the two qubits may revive in this model. We explain the revivals in terms of
correlations in a classical-quantum state of the environments and the qubits.
Although classical states cannot store entanglement on their own, they can play
a role in storing and reviving entanglement. It is important to know how the
absence of back-action, or modelling an environment as classical, affects the
kind of system time evolutions one is able to describe. We find a class of
global time evolutions where back-action is absent and for which there is no
loss of generality in modelling the environment as classical. Finally, we show
that the revivals can be connected with the increase of a parameter used to
quantify non-Markovianity of the single-qubit dynamics.Comment: 8 pages, 4 figures; this version to appear in Phys. Rev.
D-brane Instantons as Gauge Instantons in Orientifolds of Chiral Quiver Theories
Systems of D3-branes at orientifold singularities can receive
non-perturbative D-brane instanton corrections, inducing field theory operators
in the 4d effective theory. In certain non-chiral examples, these systems have
been realized as the infrared endpoint of a Seiberg duality cascade, in which
the D-brane instanton effects arise from strong gauge theory dynamics. We
present the first UV duality cascade completion of chiral D3-brane theories, in
which the D-brane instantons arise from gauge theory dynamics. Chiral examples
are interesting because the instanton fermion zero mode sector is topologically
protected, and therefore lead to more robust setups. As an application of our
results, we provide a UV completion of certain D-brane orientifold systems
recently claimed to produce conformal field theories with conformal invariance
broken only by D-brane instantons.Comment: 50 pages, 32 figures. v2: version published in JHEP with references
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On Dimer Models and Closed String Theories
We study some aspects of the recently discovered connection between dimer
models and D-brane gauge theories. We argue that dimer models are also
naturally related to closed string theories on non compact orbifolds of \BC^2
and \BC^3, via their twisted sector R charges, and show that perfect
matchings in dimer models correspond to twisted sector states in the closed
string theory. We also use this formalism to study the combinatorics of some
unstable orbifolds of \BC^2.Comment: 1 + 25 pages, LaTeX, 11 epsf figure
Dynamics of Entanglement and Bell-nonlocality for Two Stochastic Qubits with Dipole-Dipole Interaction
We have studied the analytical dynamics of Bell nonlocality as measured by
CHSH inequality and entanglement as measured by concurrence for two noisy
qubits that have dipole-dipole interaction. The nonlocal entanglement created
by the dipole-dipole interaction is found to be protected from sudden death for
certain initial states
Entanglement Dynamics of Two Independent Cavity-Embedded Quantum Dots
We investigate the dynamical behavior of entanglement in a system made by two
solid-state emitters, as two quantum dots, embedded in two separated
micro-cavities. In these solid-state systems, in addition to the coupling with
the cavity mode, the emitter is coupled to a continuum of leaky modes providing
additional losses and it is also subject to a phonon-induced pure dephasing
mechanism. We model this physical configuration as a multipartite system
composed by two independent parts each containing a qubit embedded in a
single-mode cavity, exposed to cavity losses, spontaneous emission and pure
dephasing. We study the time evolution of entanglement of this multipartite
open system finally applying this theoretical framework to the case of
currently available solid-state quantum dots in micro-cavities.Comment: 10 pages, 4 figures, to appear in Topical Issue of Physica Scripta on
proceedings of CEWQO 201
Mimicking the probability distribution of a two-dimensional Grover walk with a single-qubit coin
Multi-dimensional quantum walks usually require large coin spaces. Here we
show that the non-localized case of the spatial density probability of the
two-dimensional Grover walk can be obtained using only a two-dimensional coin
space and a quantum walk in alternate directions. We present a formal proof of
this correspondence and analyze the behavior of the coin-position entanglement
as well as the x-y spatial entanglement in our scheme with respect to the
Grover one. We show that our experimentally simpler scheme allows to entangle
the two orthogonal directions of the walk more efficiently.Comment: 5 pages, 2 figures, RevTeX
Quivers, Tilings, Branes and Rhombi
We describe a simple algorithm that computes the recently discovered brane
tilings for a given generic toric singular Calabi-Yau threefold. This therefore
gives AdS/CFT dual quiver gauge theories for D3-branes probing the given
non-compact manifold. The algorithm solves a longstanding problem by computing
superpotentials for these theories directly from the toric diagram of the
singularity. We study the parameter space of a-maximization; this study is made
possible by identifying the R-charges of bifundamental fields as angles in the
brane tiling. We also study Seiberg duality from a new perspective.Comment: 36 pages, 40 figures, JHEP
Tripartite entanglement dynamics in a system of strongly driven qubits
We study the dynamics of tripartite entanglement in a system of two strongly
driven qubits individually coupled to a dissipative cavity. We aim at
explanation of the previously noted entanglement revival between two qubits in
this system. We show that the periods of entanglement loss correspond to the
strong tripartite entanglement between the qubits and the cavity and the
recovery has to do with an inverse process. We demonstrate that the overall
process of qubit-qubit entanglement loss is due to the second order coupling to
the external continuum which explains the exp[-g^2 t/2+g^2 k t^3/6+\cdot] for
of the entanglement loss reported previously.Comment: 9 pages, 5 figure
Maximal Commutative Subalgebras Invariant for CP-Maps: (Counter-)Examples
We solve, mainly by counterexamples, many natural questions regarding maximal
commutative subalgebras invariant under CP-maps or semigroups of CP-maps on a
von Neumann algebra. In particular, we discuss the structure of the generators
of norm continuous semigroups on B(G) leaving a maximal commutative subalgebra
invariant and show that there exists Markov CP-semigroups on M_d without
invariant maximal commutative subalgebras for any d>2.Comment: After the elemenitation in Version 2 of a false class of examples in
Version 1, we now provide also correct examples for unital CP-maps and Markov
semigroups on M_d for d>2 without invariant masa
Entanglement dynamics of two independent qubits in environments with and without memory
A procedure to obtain the dynamics of independent qudits (-level
systems) each interacting with its own reservoir, for any arbitrary initial
state, is presented. This is then applied to study the dynamics of the
entanglement of two qubits, initially in an extended Werner-like mixed state
with each of them in a zero temperature non-Markovian environment. The
dependence of the entanglement dynamics on the purity and degree of
entanglement of the initial states and on the amount of non-Markovianity is
also given. This extends the previous work about non-Markovian effects on the
two-qubit entanglement dynamics for initial Bell-like states [B. Bellomo
\textit{et al.}, Phys. Rev. Lett. \textbf{99}, 160502 (2007)]. The effect of
temperature on the two-qubit entanglement dynamics in a Markovian environment
is finally obtained.Comment: 10 pages, 6 figure
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