7,592 research outputs found
Average output entropy for quantum channels
We study the regularized average Renyi output entropy \bar{S}_{r}^{\reg} of
quantum channels. This quantity gives information about the average noisiness
of the channel output arising from a typical, highly entangled input state in
the limit of infinite dimensions. We find a closed expression for
\beta_{r}^{\reg}, a quantity which we conjecture to be equal to \Srreg. We
find an explicit form for \beta_{r}^{\reg} for some entanglement-breaking
channels, and also for the qubit depolarizing channel as a
function of the parameter . We prove equality of the two quantities in
some cases, in particular we conclude that for both are
non-analytic functions of the variable .Comment: 32 pages, several plots and figures; positivity condition added for
Theorem on entanglement breaking channels; new result for entrywise positive
channel
Standard super-activation for Gaussian channels requires squeezing
The quantum capacity of bosonic Gaussian quantum channels can be non-additive
in a particularly striking way: a pair of such optical-fiber type channels can
individually have zero quantum capacity but super-activate each other such that
the combined channel has strictly positive capacity. This has been shown in
[Nature Photonics 5, 624 (2011)] where it was conjectured that squeezing is a
necessary resource for this phenomenon. We provide a proof of this conjecture
by showing that for gauge covariant channels a Choi matrix with positive
partial transpose implies that the channel is entanglement-breaking. In
addition, we construct an example which shows that this implication fails to
hold for Gaussian channels which arise from passive interactions with a
squeezed environment.Comment: 5 pages, close to published versio
Trade-off coding for universal qudit cloners motivated by the Unruh effect
A "triple trade-off" capacity region of a noisy quantum channel provides a
more complete description of its capabilities than does a single capacity
formula. However, few full descriptions of a channel's ability have been given
due to the difficult nature of the calculation of such regions---it may demand
an optimization of information-theoretic quantities over an infinite number of
channel uses. This work analyzes the d-dimensional Unruh channel, a noisy
quantum channel which emerges in relativistic quantum information theory. We
show that this channel belongs to the class of quantum channels whose capacity
region requires an optimization over a single channel use, and as such is
tractable. We determine two triple-trade off regions, the quantum dynamic
capacity region and the private dynamic capacity region, of the d-dimensional
Unruh channel. Our results show that the set of achievable rate triples using
this coding strategy is larger than the set achieved using a time-sharing
strategy. Furthermore, we prove that the Unruh channel has a distinct structure
made up of universal qudit cloning channels, thus providing a clear
relationship between this relativistic channel and the process of stimulated
emission present in quantum optical amplifiers.Comment: 26 pages, 4 figures; v2 has minor corrections to Definition 2.
Definition 4 and Remark 5 have been adde
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 and consider some
explicitly solvable cases which give simple examples of entanglement-breaking
channels with We also elaborate on the ensemble-observable duality
to show that for the measurement channel is related to the
-quantity for the dual ensemble in the same way as is related to the
accessible information. This provides both accessible information and the
-quantity for the quantum ensembles dual to our examples.Comment: 13 pages. New section and references are added concerning the
ensemble-observable dualit
Probing the quantumness of channels with mixed states
We present an alternative approach to the derivation of benchmarks for
quantum channels, such as memory or teleportation channels. Using the concept
of effective entanglement and the verification thereof, a testing procedure is
derived which demands very few experimental resources. The procedure is
generalized by allowing for mixed test states. By constructing optimized
measure & re-prepare channels, the benchmarks are found to be very tight in the
considered experimental regimes.Comment: 11 Pages, 9 Figures, published versio
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