arXiv:1409.4941

}, doi = {10.1016/j.laa.2015.03.029}, url = {https://doi.org/10.1016/j.laa.2015.03.029}, author = {Charles F. Dunkl and Piotr Gawron and {\L}ukasz Pawela and Zbigniew Pucha{\l}a and Karol {\.Z}yczkowski} } @article {iitisid_0536, title = {Restricted numerical shadow and geometry of quantum entanglement}, journal = {J. Phys. A: Math. Theor.}, volume = {45}, year = {2012}, note = {arXiv:1201.2524}, pages = {415309}, abstract = {The restricted numerical range W\_R(A) of an operator A acting on a D-dimensional Hilbert space is defined as a set of all possible expectation values of this operator among pure states which belong to a certain subset R of the of set of pure quantum states of dimension D. One considers for instance the set of real states, or in the case of composite spaces, the set of product states and the set of maximally entangled states. Combining the operator theory with a probabilistic approach we introduce the restricted numerical shadow of A {\textendash} a normalized probability distribution on the complex plane supported in W\_R(A). Its value at point z in C is equal to the probability that the expectation value is equal to z, where |psi> represents a random quantum state in subset R distributed according to the natural measure on this set, induced by the unitarily invariant Fubini{\textendash}Study measure. Studying restricted shadows of operators of a fixed size D=N\_A N\_B we analyse the geometry of sets of separable and maximally entangled states of the N\_A x N\_B composite quantum system. Investigating trajectories formed by evolving quantum states projected into the plane of the shadow we study the dynamics of quantum entanglement. A similar analysis extended for operators on D=2^3 dimensional Hilbert space allows us to investigate the structure of the orbits of GHZ and W quantum states of a three{\textendash}qubit system.}, author = {Zbigniew Pucha{\l}a and Jaroslaw Miszczak and Piotr Gawron and Charles F. Dunkl and J.A. Holbrook and Karol {\.Z}yczkowski} } @article {iitisid_0509, title = {Numerical shadow and geometry of quantum states}, journal = {J. Phys. A: Math. Theor.}, volume = {44}, number = {33}, year = {2011}, note = {arXiv:1104.2760 IF=1.641(2010);}, pages = {335301}, abstract = {The totality of normalised density matrices of order N forms a convex set Q\_N in R^(N^2-1). Working with the flat geometry induced by the Hilbert-Schmidt distance we consider images of orthogonal projections of Q\_N onto a two-plane and show that they are similar to the numerical ranges of matrices of order N. For a matrix A of a order N one defines its numerical shadow as a probability distribution supported on its numerical range W(A), induced by the unitarily invariant Fubini-Study measure on the complex projective manifold CP^(N-1). We define generalized, mixed-states shadows of A and demonstrate their usefulness to analyse the structure of the set of quantum states and unitary dynamics therein.}, issn = {1751-8113}, author = {Charles F. Dunkl and Piotr Gawron and J.A. Holbrook and Jaroslaw Miszczak and Zbigniew Pucha{\l}a and Karol {\.Z}yczkowski} } @article {iitisid_0504, title = {Numerical shadows: measures and densities on the numerical range}, journal = {Linear Algebra Appl.}, volume = {434}, year = {2011}, note = {arXiv:1010.4189 IF=1.005(2010);}, pages = {2042{\textendash}2080}, abstract = {For any operator M acting on an N-dimensional Hilbert space H\_N we introduce its numerical shadow, which is a probability measure on the complex plane supported by the numerical range of M. The shadow of M at point z is defined as the probability that the inner product (Mu,u) is equal to z, where u stands for a random complex vector from H\_N, satisfying ||u||=1. In the case of N=2 the numerical shadow of a non-normal operator can be interpreted as a shadow of a hollow sphere projected on a plane. A similar interpretation is provided also for higher dimensions. For a hermitian M its numerical shadow forms a probability distribution on the real axis which is shown to be a one dimensional B-spline. In the case of a normal M the numerical shadow corresponds to a shadow of a transparent solid simplex in R^{N-1} onto the complex plane. Numerical shadow is found explicitly for Jordan matrices J\_N, direct sums of matrices and in all cases where the shadow is rotation invariant. Results concerning the moments of shadow measures play an important role. A general technique to study numerical shadow via the Cartesian decomposition is described, and a link of the numerical shadow of an operator to its higher-rank numerical range is emphasized.}, issn = {0024-3795}, author = {Charles F. Dunkl and Piotr Gawron and J.A. Holbrook and Zbigniew Pucha{\l}a and Karol {\.Z}yczkowski} }