397 research outputs found
Optimal quantum repeaters for qubits and qudits
A class of optimal quantum repeaters for qubits is suggested. The schemes are
minimal, i.e. involve a single additional probe qubit, and optimal, i.e.
provide the maximum information adding the minimum amount of noise. Information
gain and state disturbance are quantified by fidelities which, for our schemes,
saturate the ultimate bound imposed by quantum mechanics for randomly
distributed signals. Special classes of signals are also investigated, in order
to improve the information-disturbance trade-off. Extension to higher
dimensional signals (qudits) is straightforward.Comment: Revised version. To appear in PR
Approximate quantum cloning and the impossibility of superluminal information transfer
We show that nonlocality of quantum mechanics cannot lead to superluminal
transmission of information, even if most general local operations are allowed,
as long as they are linear and trace preserving. In particular, any quantum
mechanical approximate cloning transformation does not allow signalling. On the
other hand, the no-signalling constraint on its own is not sufficient to
prevent a transformation from surpassing the known cloning bounds. We
illustrate these concepts on the basis of some examples.Comment: 4 pages, 1eps figur
Quantum entanglement and classical communication through a depolarising channel
We analyse the role of entanglement for transmission of classical information
through a memoryless depolarising channel. Using the isotropic character of
this channel we prove analytically that the mutual information cannot be
increased by encoding classical bits into entangled states of two qubits.Comment: 6 pages, 2 figures; contribution to special issue of JMO on the
physics of quantum information; 2nd version: slight modifications and
improved presentatio
Depolarization channels with zero-bandwidth noises
A simple model describing depolarization channels with zero-bandwidth
environment is presented and exactly solved. The environment is modelled by
Lorentzian, telegraphic and Gaussian zero-bandwidth noises. Such channels can
go beyond the standard Markov dynamics and therefore can illustrate the
influence of memory effects of the noisy communication channel on the
transmitted information. To quantify the disturbance of quantum states the
entanglement fidelity between arbitrary input and output states is
investigated.Comment: 15 pages, 3 figure
Approximate quantum data storage and teleportation
In this paper we present an optimal protocol by which an unknown state on a
Hilbert space of dimension can be approximately stored in an
-dimensional quantum system or be approximately teleported via an
-dimensional quantum channel. The fidelity of our procedure is determined
for pure states as well as for mixed states and states which are entangled with
auxiliary quantum systems of varying Hilbert space dimension, and it is
compared with theoretical results for the maximally achievable fidelity.Comment: More detailed discussion of teleportation of entangled and mixed
states. Added reference to work by Banaszek. 8 pages, 1 figur
Optimal quantum teleportation with an arbitrary pure state
We derive the maximum fidelity attainable for teleportation using a shared
pair of d-level systems in an arbitrary pure state. This derivation provides a
complete set of necessary and sufficient conditions for optimal teleportation
protocols. We also discuss the information on the teleported particle which is
revealed in course of the protocol using a non-maximally entangled state.Comment: 10 pages, REVTe
Maximization of capacity and p-norms for some product channels
It is conjectured that the Holevo capacity of a product channel \Omega
\otimes \Phi is achieved when product states are used as input. Amosov, Holevo
and Werner have also conjectured that the maximal p-norm of a product channel
is achieved with product input states. In this paper we establish both of these
conjectures in the case that \Omega is arbitrary and \Phi is a CQ or QC channel
(as defined by Holevo). We also establish the Amosov, Holevo and Werner
conjecture when \Omega is arbitrary and either \Phi is a qubit channel and p=2,
or \Phi is a unital qubit channel and p is integer. Our proofs involve a new
conjecture for the norm of an output state of the half-noisy channel I \otimes
\Phi, when \Phi is a qubit channel. We show that this conjecture in some cases
also implies additivity of the Holevo capacity
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