1,796 research outputs found

### Correlations in local measurements on a quantum state, and complementarity as an explanation of nonclassicality

We consider the classical correlations that two observers can extract by
measurements on a bipartite quantum state, and we discuss how they are related
to the quantum mutual information of the state. We show with several examples
how complementarity gives rise to a gap between the quantum and the classical
correlations, and we relate our quantitative finding to the so-called classical
correlation locked in a quantum state. We derive upper bounds for the sum of
classical correlation obtained by measurements in different mutually unbiased
bases and we show that the complementarity gap is also present in the
deterministic quantum computation with one quantum bit.Comment: 15 pages, 4 figures, references adde

### Remarks on entanglement assisted classical capacity

The property of the optimal signal ensembles of entanglement assisted channel
capacity is studied. A relationship between entanglement assisted channel
capacity and one-shot capacity of unassisted channel is obtained. The data
processing inequalities, convexity and additivity of the entanglement assisted
channel capacity are reformulated by simple methods.Comment: Revtex, 5 page

### Information capacity of quantum observable

In this paper we consider the classical capacities of quantum-classical
channels corresponding to measurement of observables. Special attention is paid
to the case of continuous observables. We give the formulas for unassisted and
entanglement-assisted classical capacities $C,C_{ea}$ and consider some
explicitly solvable cases which give simple examples of entanglement-breaking
channels with $C<C_{ea}.$ We also elaborate on the ensemble-observable duality
to show that $C_{ea}$ for the measurement channel is related to the
$\chi$-quantity for the dual ensemble in the same way as $C$ is related to the
accessible information. This provides both accessible information and the
$\chi$-quantity for the quantum ensembles dual to our examples.Comment: 13 pages. New section and references are added concerning the
ensemble-observable dualit

### Pure state estimation and the characterization of entanglement

A connection between the state estimation problem and the separability
problem is noticed and exploited to find efficient numerical algorithms to
solve the first one. Based on these ideas, we also derive a systematic method
to obtain upper bounds on the maximum local fidelity when the states are
distributed among several distant parties.Comment: Closer to published versio

### Reversibility conditions for quantum channels and their applications

A necessary condition for reversibility (sufficiency) of a quantum channel
with respect to complete families of states with bounded rank is obtained. A
full description (up to isometrical equivalence) of all quantum channels
reversible with respect to orthogonal and nonorthogonal complete families of
pure states is given. Some applications in quantum information theory are
considered.
The main results can be formulated in terms of the operator algebras theory
(as conditions for reversibility of channels between algebras of all bounded
operators).Comment: 28 pages, this version contains strengthened results of the previous
one and of arXiv:1106.3297; to appear in Sbornik: Mathematics, 204:7 (2013

### Transition probabilities between quasifree states

We obtain a general formula for the transition probabilities between any
state of the algebra of the canonical commutation relations (CCR-algebra) and a
squeezed quasifree state. Applications of this formula are made for the case of
multimode thermal squeezed states of quantum optics using a general canonical
decomposition of the correlation matrix valid for any quasifree state. In the
particular case of a one mode CCR-algebra we show that the transition
probability between two quasifree squeezed states is a decreasing function of
the geodesic distance between the points of the upper half plane representing
these states. In the special case of the purification map it is shown that the
transition probability between the state of the enlarged system and the product
state of real and fictitious subsystems can be a measure for the entanglement.Comment: 13 pages, REVTeX, no figure

### Complete quantum measurements break entanglement

Complete measurement of a quantum observable (POVM) is a measurement of the
maximally refined version of the POVM. Complete measurements give information
on multiplicities of measurement outcomes and can be viewed as state
preparation procedures. Moreover, any observable can be measured completely. In
this Letter, we show that a complete measurement breaks entanglement completely
between the system, ancilla and their environment. Finally, consequences to the
quantum Zeno effect and complete position measurements are discussed.Comment: 5 page

### Generalized minimal output entropy conjecture for one-mode Gaussian channels: definitions and some exact results

A formulation of the generalized minimal output entropy conjecture for
Gaussian channels is presented. It asserts that, for states with fixed input
entropy, the minimal value of the output entropy of the channel (i.e. the
minimal output entropy increment for fixed input entropy) is achieved by
Gaussian states. In the case of centered channels (i.e. channels which do not
add squeezing to the input state) this implies that the minimum is obtained by
thermal (Gibbs) inputs. The conjecture is proved to be valid in some special
cases.Comment: 7 pages, updated version minor typos correcte

### On entanglement-assisted classical capacity

This paper is essentially a lecture from the author's course on quantum
information theory, which is devoted to the result of C. H. Bennett, P. W.
Shor, J. A. Smolin and A. V. Thapliyal (quant-ph/0106052) concerning
entanglement-assisted classical capacity of a quantum channel. A modified proof
of this result is given and relation between entanglement-assisted and
unassisted classical capacities is discussed.Comment: 10 pages, LATE

- …