458 research outputs found
Distribution of entanglement in networks of bi-partite full-rank mixed states
We study quantum entanglement distribution on networks with full-rank
bi-partite mixed states linking qubits on nodes. In particular, we use
entanglement swapping and purification to partially entangle widely separated
nodes. The simplest method consists of performing entanglement swappings along
the shortest chain of links connecting the two nodes. However, we show that
this method may be improved upon by choosing a protocol with a specific
ordering of swappings and purifications. A priori, the design that produces
optimal improvement is not clear. However, we parametrize the choices and find
that the optimal values depend strongly on the desired measure of improvement.
As an initial application, we apply the new improved protocols to the
Erd\"os--R\'enyi network and obtain results including low density limits and an
exact calculation of the average entanglement gained at the critical point.Comment: 15 pages, 19 figures. New version includes improvements suggested in
referee repor
Tensor network representations from the geometry of entangled states
Tensor network states provide successful descriptions of strongly correlated
quantum systems with applications ranging from condensed matter physics to
cosmology. Any family of tensor network states possesses an underlying
entanglement structure given by a graph of maximally entangled states along the
edges that identify the indices of the tensors to be contracted. Recently, more
general tensor networks have been considered, where the maximally entangled
states on edges are replaced by multipartite entangled states on plaquettes.
Both the structure of the underlying graph and the dimensionality of the
entangled states influence the computational cost of contracting these
networks. Using the geometrical properties of entangled states, we provide a
method to construct tensor network representations with smaller effective bond
dimension. We illustrate our method with the resonating valence bond state on
the kagome lattice.Comment: 35 pages, 9 figure
Quantum entanglement
All our former experience with application of quantum theory seems to say:
{\it what is predicted by quantum formalism must occur in laboratory}. But the
essence of quantum formalism - entanglement, recognized by Einstein, Podolsky,
Rosen and Schr\"odinger - waited over 70 years to enter to laboratories as a
new resource as real as energy.
This holistic property of compound quantum systems, which involves
nonclassical correlations between subsystems, is a potential for many quantum
processes, including ``canonical'' ones: quantum cryptography, quantum
teleportation and dense coding. However, it appeared that this new resource is
very complex and difficult to detect. Being usually fragile to environment, it
is robust against conceptual and mathematical tools, the task of which is to
decipher its rich structure.
This article reviews basic aspects of entanglement including its
characterization, detection, distillation and quantifying. In particular, the
authors discuss various manifestations of entanglement via Bell inequalities,
entropic inequalities, entanglement witnesses, quantum cryptography and point
out some interrelations. They also discuss a basic role of entanglement in
quantum communication within distant labs paradigm and stress some
peculiarities such as irreversibility of entanglement manipulations including
its extremal form - bound entanglement phenomenon. A basic role of entanglement
witnesses in detection of entanglement is emphasized.Comment: 110 pages, 3 figures, ReVTex4, Improved (slightly extended)
presentation, updated references, minor changes, submitted to Rev. Mod. Phys
Random graph states, maximal flow and Fuss-Catalan distributions
For any graph consisting of vertices and edges we construct an
ensemble of random pure quantum states which describe a system composed of
subsystems. Each edge of the graph represents a bi-partite, maximally entangled
state. Each vertex represents a random unitary matrix generated according to
the Haar measure, which describes the coupling between subsystems. Dividing all
subsystems into two parts, one may study entanglement with respect to this
partition. A general technique to derive an expression for the average
entanglement entropy of random pure states associated to a given graph is
presented. Our technique relies on Weingarten calculus and flow problems. We
analyze statistical properties of spectra of such random density matrices and
show for which cases they are described by the free Poissonian
(Marchenko-Pastur) distribution. We derive a discrete family of generalized,
Fuss-Catalan distributions and explicitly construct graphs which lead to
ensembles of random states characterized by these novel distributions of
eigenvalues.Comment: 37 pages, 24 figure
Area law for random graph states
Random pure states of multi-partite quantum systems, associated with
arbitrary graphs, are investigated. Each vertex of the graph represents a
generic interaction between subsystems, described by a random unitary matrix
distributed according to the Haar measure, while each edge of the graph
represents a bi-partite, maximally entangled state. For any splitting of the
graph into two parts we consider the corresponding partition of the quantum
system and compute the average entropy of entanglement. First, in the special
case where the partition does not "cross" any vertex of the graph, we show that
the area law is satisfied exactly. In the general case, we show that the
entropy of entanglement obeys an area law on average, this time with a
correction term that depends on the topologies of the graph and of the
partition. The results obtained are applied to the problem of distribution of
quantum entanglement in a quantum network with prescribed topology.Comment: v2: minor typos correcte
Direct estimation of functionals of density operators by local operations and classical communication
We present a method of direct estimation of important properties of a shared bipartite quantum state, within the "distant laboratories" paradigm, using only local operations and classical communication. We apply this procedure to spectrum estimation of shared states, and locally implementable structural physical approximations to incompletely positive maps. This procedure can also be applied to the estimation of channel capacity and measures of entanglement
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