8,643 research outputs found
Directed quantum communication
We raise the question whether there is a way to characterize the quantum
information transport properties of a medium or material. For this analysis the
special features of quantum information have to be taken into account. We find
that quantum communication over an isotropic medium, as opposed to classical
information transfer, requires the transmitter to direct the signal towards the
receiver. Furthermore, for large classes of media there is a threshold, in the
sense that `sufficiently much' of the signal has to be collected. Therefore,
the medium's capacity for quantum communication can be characterized in terms
of how the size of the transmitter and receiver has to scale with the
transmission distance to maintain quantum information transmission. To
demonstrate the applicability of this concept, an n-dimensional spin lattice is
considered, yielding a sufficient scaling of d^(n/3) with the distance d
Unconstrained distillation capacities of a pure-loss bosonic broadcast channel
Bosonic channels are important in practice as they form a simple model for
free-space or fiber-optic communication. Here we consider a single-sender
two-receiver pure-loss bosonic broadcast channel and determine the
unconstrained capacity region for the distillation of bipartite entanglement
and secret key between the sender and each receiver, whenever they are allowed
arbitrary public classical communication. We show how the state merging
protocol leads to achievable rates in this setting, giving an inner bound on
the capacity region. We also evaluate an outer bound on the region by using the
relative entropy of entanglement and a `reduction by teleportation' technique.
The outer bounds match the inner bounds in the infinite-energy limit, thereby
establishing the unconstrained capacity region for such channels. Our result
could provide a useful benchmark for implementing a broadcasting of
entanglement and secret key through such channels. An important open question
relevant to practice is to determine the capacity region in both this setting
and the single-sender single-receiver case when there is an energy constraint
on the transmitter.Comment: v2: 6 pages, 3 figures, introduction revised, appendix added where
the result is extended to the 1-to-m pure-loss bosonic broadcast channel. v3:
minor revision, typo error correcte
Classical capacity of the lossy bosonic channel: the exact solution
The classical capacity of the lossy bosonic channel is calculated exactly. It
is shown that its Holevo information is not superadditive, and that a
coherent-state encoding achieves capacity. The capacity of far-field,
free-space optical communications is given as an example.Comment: 4 pages, 2 figures (revised version
Strong Converse for a Degraded Wiretap Channel via Active Hypothesis Testing
We establish an upper bound on the rate of codes for a wiretap channel with
public feedback for a fixed probability of error and secrecy parameter. As a
corollary, we obtain a strong converse for the capacity of a degraded wiretap
channel with public feedback. Our converse proof is based on a reduction of
active hypothesis testing for discriminating between two channels to coding for
wiretap channel with feedback.Comment: This paper was presented at Allerton 201
Quantum Limitations on the Storage and Transmission of Information
Information must take up space, must weigh, and its flux must be limited.
Quantum limits on communication and information storage leading to these
conclusions are here described. Quantum channel capacity theory is reviewed for
both steady state and burst communication. An analytic approximation is given
for the maximum signal information possible with occupation number signal
states as a function of mean signal energy. A theorem guaranteeing that these
states are optimal for communication is proved. A heuristic "proof" of the
linear bound on communication is given, followed by rigorous proofs for signals
with specified mean energy, and for signals with given energy budget. And
systems of many parallel quantum channels are shown to obey the linear bound
for a natural channel architecture. The time--energy uncertainty principle is
reformulated in information language by means of the linear bound. The quantum
bound on information storage capacity of quantum mechanical and quantum field
devices is reviewed. A simplified version of the analytic proof for the bound
is given for the latter case. Solitons as information caches are discussed, as
is information storage in one dimensional systems. The influence of signal
self--gravitation on communication is considerd. Finally, it is shown that
acceleration of a receiver acts to block information transfer.Comment: Published relatively inaccessible review on a perennially interesting
subject. Plain TeX, 47 pages, 5 jpg figures (not embedded
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