2,774 research outputs found
"Partial" Fidelities
For pairs, omega, rho, of density operators on a finite dimensional Hilbert
space of dimension d I call k-fidelity the d - k smallest eigenvalues of |
omega^1/2 rho^1/2 |. k-fidelities are jointly concave in omega, rho. This
follows by representing them as infima over linear functions. For k = 0 known
properties of fidelity and transition probability are reproduced. Partial
fidelities characterize equivalence classes which are partially ordered in a
natural way.Comment: LATEX2e, 14 page
Realistic continuous-variable quantum teleportation with non-Gaussian resources
We present a comprehensive investigation of nonideal continuous-variable
quantum teleportation implemented with entangled non-Gaussian resources. We
discuss in a unified framework the main decoherence mechanisms, including
imperfect Bell measurements and propagation of optical fields in lossy fibers,
applying the formalism of the characteristic function. By exploiting
appropriate displacement strategies, we compute analytically the success
probability of teleportation for input coherent states, and two classes of
non-Gaussian entangled resources: Two-mode squeezed Bell-like states (that
include as particular cases photon-added and photon-subtracted de-Gaussified
states), and two-mode squeezed cat-like states. We discuss the optimization
procedure on the free parameters of the non-Gaussian resources at fixed values
of the squeezing and of the experimental quantities determining the
inefficiencies of the non-ideal protocol. It is found that non-Gaussian
resources enhance significantly the efficiency of teleportation and are more
robust against decoherence than the corresponding Gaussian ones. Partial
information on the alphabet of input states allows further significant
improvement in the performance of the non-ideal teleportation protocol.Comment: 14 pages, 6 figure
Matrices of fidelities for ensembles of quantum states and the Holevo quantity
The entropy of the Gram matrix of a joint purification of an ensemble of K
mixed states yields an upper bound for the Holevo information Chi of the
ensemble. In this work we combine geometrical and probabilistic aspects of the
ensemble in order to obtain useful bounds for Chi. This is done by constructing
various correlation matrices involving fidelities between every pair of states
from the ensemble. For K=3 quantum states we design a matrix of root fidelities
that is positive and the entropy of which is conjectured to upper bound Chi.
Slightly weaker bounds are established for arbitrary ensembles. Finally, we
investigate correlation matrices involving multi-state fidelities in relation
to the Holevo quantity.Comment: 24 pages, 3 figure
On multipartite invariant states I. Unitary symmetry
We propose a natural generalization of bipartite Werner and isotropic states
to multipartite systems consisting of an arbitrary even number of d-dimensional
subsystems (qudits). These generalized states are invariant under the action of
local unitary operations. We study basic properties of multipartite invariant
states: separability criteria and multi-PPT conditions.Comment: 9 pages; slight correction
Continuity and Stability of Partial Entropic Sums
Extensions of Fannes' inequality with partial sums of the Tsallis entropy are
obtained for both the classical and quantum cases. The definition of kth
partial sum under the prescribed order of terms is given. Basic properties of
introduced entropic measures and some applications are discussed. The derived
estimates provide a complete characterization of the continuity and stability
properties in the refined scale. The results are also reformulated in terms of
Uhlmann's partial fidelities.Comment: 9 pages, no figures. Some explanatory and technical improvements are
made. The bibliography is extended. Detected errors and typos are correcte
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