114 research outputs found
On the structure of entanglement witnesses and new class of positive indecomposable maps
We construct a new class of positive indecomposable maps in the algebra of 'd
x d' complex matrices. Each map is uniquely characterized by a cyclic
bistochastic matrix. This class generalizes a Choi map for d=3. It provides a
new reach family of indecomposable entanglement witnesses which define
important tool for investigating quantum entanglement.Comment: 18 page
Constructing new optimal entanglement witnesses
We provide a new class of indecomposable entanglement witnesses. In 4 x 4
case it reproduces the well know Breuer-Hall witness. We prove that these new
witnesses are optimal and atomic, i.e. they are able to detect the "weakest"
quantum entanglement encoded into states with positive partial transposition
(PPT). Equivalently, we provide a new construction of indecomposable atomic
maps in the algebra of 2k x 2k complex matrices. It is shown that their
structural physical approximations give rise to entanglement breaking channels.
This result supports recent conjecture by Korbicz et. al.Comment: 9 page
Optimal entanglement witnesses from generalized reduction and Robertson maps
We provide a generalization of the reduction and Robertson positive maps in
matrix algebras. They give rise to a new class of optimal entanglement
witnesses. Their structural physical approximation is analyzed. As a byproduct
we provide a new examples of PPT (Positive Partial Transpose) entangled states.Comment: 14 page
Geometry of entanglement witnesses for two qutrits
We characterize a convex subset of entanglement witnesses for two qutrits.
Equivalently, we provide a characterization of the set of positive maps in the
matrix algebra of 3 x 3 complex matrices. It turns out that boundary of this
set displays elegant representation in terms of SO(2) rotations. We conjecture
that maps parameterized by rotations are optimal, i.e. they provide the
strongest tool for detecting quantum entanglement. As a byproduct we found a
new class of decomposable entanglement witnesses parameterized by improper
rotations from the orthogonal group O(2).Comment: 9 page
On structural physical approximations and entanglement breaking maps
Very recently a conjecture saying that the so-called structural physical
approximations (SPAa) to optimal positive maps (optimal entanglement witnesses)
give entanglement breaking (EB) maps (separable states) has been posed [J. K.
Korbicz {\it et al.}, Phys. Rev. A {\bf 78}, 062105 (2008)]. The main purpose
of this contribution is to explore this subject. First, we extend the set of
entanglement witnesses (EWs) supporting the conjecture. Then, we ask if SPAs
constructed from other than the depolarizing channel maps also lead to EB maps
and show that in general this is not the case. On the other hand, we prove an
interesting fact that for any positive map there exists an EB channel
such that the SPA of constructed with the aid of is
again an EB channel. Finally, we ask similar questions in the case of
continuous variable systems. We provide a simple way of construction of SPA and
prove that in the case of the transposition map it gives EB channel.Comment: 22 pages, improved version, accepted by Journal of Physics
New tools for investigating positive maps in matrix algebras
We provide a novel tool which may be used to construct new examples of
positive maps in matrix algebras (or, equivalently, entanglement witnesses). It
turns out that this can be used to prove positivity of several well known maps
(such as reduction map, generalized reduction, Robertson map, and many others).
Furthermore, we use it to construct a new family of linear maps and prove that
they are positive, indecomposable and (nd)optimal.Comment: 10 page
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