662 research outputs found
Mixed-state certification of quantum capacities for noisy communication channels
We extend a recent method to detect lower bounds to the quantum capacity of
quantum communication channels by considering realistic scenarios with general
input probe states and arbitrary detection procedures at the output. Realistic
certification relies on a new bound for the coherent information of a quantum
channel that can be applied with arbitrary bipartite mixed input states and
generalized output measurements.Comment: 7 pages, 2 figure
Detection of properties and capacities of quantum channels
We review in a unified way a recently proposed method to detect properties of
unknown quantum channels and lower bounds to quantum capacities, without
resorting to full quantum process tomography. The method is based on the
preparation of a fixed bipartite entangled state at the channel input or,
equivalently, an ensemble of an overcomplete set of single-system states, along
with few local measurements at the channel output.Comment: 8 pages, 1 figure. arXiv admin note: text overlap with
arXiv:1510.0021
Informational power of quantum measurements
We introduce the informational power of a quantum measurement as the maximum
amount of classical information that the measurement can extract from any
ensemble of quantum states. We prove the additivity by showing that the
informational power corresponds to the classical capacity of a
quantum-classical channel. We restate the problem of evaluating the
informational power as the maximization of the accessible information of a
suitable ensemble. We provide a numerical algorithm to find an optimal
ensemble, and quantify the informational power.Comment: 9 pages, 3 figures, added references, published versio
Superbroadcasting of conjugate quantum variables
We consider the problem of broadcasting arbitrary states of radiation modes
from N to M>N copies by a map that preserves the average value of the field and
optimally reduces the total noise in conjugate variables. For N>=2 the
broadcasting can be achieved perfectly, and for sufficiently noisy input states
one can even purify the state while broadcasting--the so-called
superbroadcasting. For purification (i.e. M<=N), the reduction of noise is
independent of M. Similar results are proved for broadcasting with
phase-conjugation. All the optimal maps can be implemented by linear optics and
linear amplification.Comment: 7 pages, 1 eps figures. Accepted for publication on Europhysics
Letter
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