6,652 research outputs found
Purification-based metric to measure the distance between quantum states and processes
In this work we study the properties of an purification-based entropic metric
for measuring the distance between both quantum states and quantum processes.
This metric is defined as the square root of the entropy of the average of two
purifications of mixed quantum states which maximize the overlap between the
purified states. We analyze this metric and show that it satisfies many
appealing properties, which suggest this metric is an interesting proposal for
theoretical and experimental applications of quantum information.Comment: 11 pages, 2 figures. arXiv admin note: text overlap with
arXiv:quant-ph/0408063, arXiv:1107.1732 by other author
Geometric characterization of separability and entanglement in pure Gaussian states by single-mode unitary operations
We present a geometric approach to the characterization of separability and
entanglement in pure Gaussian states of an arbitrary number of modes. The
analysis is performed adapting to continuous variables a formalism based on
single subsystem unitary transformations that has been recently introduced to
characterize separability and entanglement in pure states of qubits and qutrits
[arXiv:0706.1561]. In analogy with the finite-dimensional case, we demonstrate
that the bipartite entanglement of a multimode pure Gaussian state
can be quantified by the minimum squared Euclidean distance between the state
itself and the set of states obtained by transforming it via suitable local
symplectic (unitary) operations. This minimum distance, corresponding to a,
uniquely determined, extremal local operation, defines a novel entanglement
monotone equivalent to the entropy of entanglement, and amenable to direct
experimental measurement with linear optical schemes.Comment: 7 pages, 1 figure. Discussion expanded, to appear in PR
Optical implementation and entanglement distribution in Gaussian valence bond states
We study Gaussian valence bond states of continuous variable systems,
obtained as the outputs of projection operations from an ancillary space of M
infinitely entangled bonds connecting neighboring sites, applied at each of
sites of an harmonic chain. The entanglement distribution in Gaussian valence
bond states can be controlled by varying the input amount of entanglement
engineered in a (2M+1)-mode Gaussian state known as the building block, which
is isomorphic to the projector applied at a given site. We show how this
mechanism can be interpreted in terms of multiple entanglement swapping from
the chain of ancillary bonds, through the building blocks. We provide optical
schemes to produce bisymmetric three-mode Gaussian building blocks (which
correspond to a single bond, M=1), and study the entanglement structure in the
output Gaussian valence bond states. The usefulness of such states for quantum
communication protocols with continuous variables, like telecloning and
teleportation networks, is finally discussed.Comment: 15 pages, 6 figures. To appear in Optics and Spectroscopy, special
issue for ICQO'2006 (Minsk). This preprint contains extra material with
respect to the journal versio
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