178 research outputs found
Electromagnetic channel capacity for practical purposes
We give analytic upper bounds to the channel capacity C for transmission of
classical information in electromagnetic channels (bosonic channels with
thermal noise). In the practically relevant regimes of high noise and low
transmissivity, by comparison with know lower bounds on C, our inequalities
determine the value of the capacity up to corrections which are irrelevant for
all practical purposes. Examples of such channels are radio communication,
infrared or visible-wavelength free space channels. We also provide bounds to
active channels that include amplification.Comment: 6 pages, 3 figures. NB: the capacity bounds are constructed by
generalizing to the multi-mode case the minimum-output entropy bounds of
arXiv:quant-ph/0404005 [Phys. Rev. A 70, 032315 (2004)
Quantum parallel dense coding of optical images
We propose quantum dense coding protocol for optical images. This protocol
extends the earlier proposed dense coding scheme for continuous variables
[S.L.Braunstein and H.J.Kimble, Phys.Rev.A 61, 042302 (2000)] to an essentially
multimode in space and time optical quantum communication channel. This new
scheme allows, in particular, for parallel dense coding of non-stationary
optical images. Similar to some other quantum dense coding protocols, our
scheme exploits the possibility of sending a classical message through only one
of the two entangled spatially-multimode beams, using the other one as a
reference system. We evaluate the Shannon mutual information for our protocol
and find that it is superior to the standard quantum limit. Finally, we show
how to optimize the performance of our scheme as a function of the
spatio-temporal parameters of the multimode entangled light and of the input
images.Comment: 15 pages, 4 figures, RevTeX4. Submitted to the Special Issue on
Quantum Imaging in Journal of Modern Optic
A generalization of the Entropy Power Inequality to Bosonic Quantum Systems
In most communication schemes information is transmitted via travelling modes
of electromagnetic radiation. These modes are unavoidably subject to
environmental noise along any physical transmission medium and the quality of
the communication channel strongly depends on the minimum noise achievable at
the output. For classical signals such noise can be rigorously quantified in
terms of the associated Shannon entropy and it is subject to a fundamental
lower bound called entropy power inequality. Electromagnetic fields are however
quantum mechanical systems and then, especially in low intensity signals, the
quantum nature of the information carrier cannot be neglected and many
important results derived within classical information theory require
non-trivial extensions to the quantum regime. Here we prove one possible
generalization of the Entropy Power Inequality to quantum bosonic systems. The
impact of this inequality in quantum information theory is potentially large
and some relevant implications are considered in this work
Entropy on Spin Factors
Recently it has been demonstrated that the Shannon entropy or the von Neuman
entropy are the only entropy functions that generate a local Bregman
divergences as long as the state space has rank 3 or higher. In this paper we
will study the properties of Bregman divergences for convex bodies of rank 2.
The two most important convex bodies of rank 2 can be identified with the bit
and the qubit. We demonstrate that if a convex body of rank 2 has a Bregman
divergence that satisfies sufficiency then the convex body is spectral and if
the Bregman divergence is monotone then the convex body has the shape of a
ball. A ball can be represented as the state space of a spin factor, which is
the most simple type of Jordan algebra. We also study the existence of recovery
maps for Bregman divergences on spin factors. In general the convex bodies of
rank 2 appear as faces of state spaces of higher rank. Therefore our results
give strong restrictions on which convex bodies could be the state space of a
physical system with a well-behaved entropy function.Comment: 30 pages, 6 figure
Random qubit-states and how best to measure them
We consider the problem of measuring a single qubit, known to have been prepared in either a randomly selected pure state or a randomly selected real pure state. We seek the measurements that provide either the best estimate of the state prepared or maximise the accessible information. Surprisingly, any sensible measurement turns out to be optimal. We discuss the application of these ideas to multiple qubits and higher-dimensional systems
Classical communication and non-classical fidelity of quantum teleportation
In quantum teleportation, the role of entanglement has been much discussed.
It is known that entanglement is necessary for achieving non-classical
teleportation fidelity. Here we focus on the amount of classical communication
that is necessary to obtain non-classical fidelity in teleportation. We
quantify the amount of classical communication that is sufficient for achieving
non-classical fidelity for two independent 1-bit and single 2-bits noisy
classical channels. It is shown that on average 0.208 bits of classical
communication is sufficient to get non-classical fidelity. We also find the
necessary amount of classical communication in case of isotropic
transformation. Finally we study how the amount of sufficient classical
communication increases with weakening of entanglement used in the
teleportation process.Comment: Accepted in Quantum Info. Proces
Epistemic and Ontic Quantum Realities
Quantum theory has provoked intense discussions about its interpretation since its pioneer days. One of the few scientists who have been continuously engaged in this development from both physical and philosophical perspectives is Carl Friedrich von Weizsaecker. The questions he posed were and are inspiring for many, including the authors of this contribution. Weizsaecker developed Bohr's view of quantum theory as a theory of knowledge. We show that such an epistemic perspective can be consistently complemented by Einstein's ontically oriented position
On the notion of composite system
The notion of composite system made up of distinguishable parties is
investigated in the context of arbitrary convex spaces.Comment: 9 pages. Comments are welcom
Information-theoretic postulates for quantum theory
Why are the laws of physics formulated in terms of complex Hilbert spaces?
Are there natural and consistent modifications of quantum theory that could be
tested experimentally? This book chapter gives a self-contained and accessible
summary of our paper [New J. Phys. 13, 063001, 2011] addressing these
questions, presenting the main ideas, but dropping many technical details. We
show that the formalism of quantum theory can be reconstructed from four
natural postulates, which do not refer to the mathematical formalism, but only
to the information-theoretic content of the physical theory. Our starting point
is to assume that there exist physical events (such as measurement outcomes)
that happen probabilistically, yielding the mathematical framework of "convex
state spaces". Then, quantum theory can be reconstructed by assuming that (i)
global states are determined by correlations between local measurements, (ii)
systems that carry the same amount of information have equivalent state spaces,
(iii) reversible time evolution can map every pure state to every other, and
(iv) positivity of probabilities is the only restriction on the possible
measurements.Comment: 17 pages, 3 figures. v3: some typos corrected and references updated.
Summarizes the argumentation and results of arXiv:1004.1483. Contribution to
the book "Quantum Theory: Informational Foundations and Foils", Springer
Verlag (http://www.springer.com/us/book/9789401773027), 201
Gaussian bosonic synergy: quantum communication via realistic channels of zero quantum capacity
As with classical information, error-correcting codes enable reliable
transmission of quantum information through noisy or lossy channels. In
contrast to the classical theory, imperfect quantum channels exhibit a strong
kind of synergy: there exist pairs of discrete memoryless quantum channels,
each of zero quantum capacity, which acquire positive quantum capacity when
used together. Here we show that this "superactivation" phenomenon also occurs
in the more realistic setting of optical channels with attenuation and Gaussian
noise. This paves the way for its experimental realization and application in
real-world communications systems.Comment: 5 pages, 4 figures, one appendi
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