161 research outputs found
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
Genuine Multipartite Entanglement in the -Photon Decay of Positronium
The electron-positron annihilation into two photons is a standard technology
in medicine to observe e.g. metabolic processes in human bodies. A new
tomograph will provide the possibility to observe not only direct
annihilations but also the photons from the decay of ortho-positronium
atoms formed in the body. We show in this contribution that the three-photon
state with respect to polarisation degrees of freedom depends on the angles
between the photons and exhibits various specific entanglement features. In
particular genuine multipartite entanglement, a type of entanglement involving
all degrees of freedoms, is subsistent if the positronium was in a definite
spin eigenstate. Remarkably, when all spin eigenstates are mixed equally,
entanglement --and even stronger genuine multipartite entanglement-- survives.
Due to a "\textit{symmetrization}" process, however, -type of
entanglement remains whereas -type of entanglement vanishes. The survival
of particular entanglement properties in the mixing scenario may make it
possible to extract quantum information in form of distinct entanglement
features, e.g., from metabolic processes in human bodies.Comment: 9 pages, 5 figure
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