557 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
Optimal universal quantum cloning and state estimation
We derive a tight upper bound for the fidelity of a universal N to M qubit
cloner, valid for any M \geq N, where the output of the cloner is required to
be supported on the symmetric subspace. Our proof is based on the concatenation
of two cloners and the connection between quantum cloning and quantum state
estimation. We generalise the operation of a quantum cloner to mixed and/or
entangled input qubits described by a density matrix supported on the symmetric
subspace of the constituent qubits. We also extend the validity of optimal
state estimation methods to inputs of this kind.Comment: 4 pages (RevTeX
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
Bounds for state-dependent quantum cloning
Due to the no-cloning theorem, the unknown quantum state can only be cloned
approximately or exactly with some probability. There are two types of cloners:
universal and state-dependent cloner. The optimal universal cloner has been
found and could be viewed as a special state-dependent quantum cloner which has
no information about the states. In this paper, we investigate the
state-dependent cloning when the state-set contains more than two states. We
get some bounds of the global fidelity for these processes. This method is not
dependent on the number of the states contained in the state-set. It is also
independent of the numbers of copying.Comment: 13 pages, 1 figure, to appear in Phys. Rev.
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
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
Universal Quantum Cloning in Cavity QED
We propose an implementation of an universal quantum cloning machine [UQCM,
Hillery and Buzek, Phys. Rev. A {\bf 56}, 3446 (1997)] in a Cavity Quantum
Electrodynamics (CQED) experiment. This UQCM acts on the electronic states of
atoms that interact with the electromagnetic field of a high cavity. We
discuss here the specific case of the cloning process using either a
one- or a two-cavity configuration
The rencontre problem
Let be independent sequences of
Bernoulli random variables with success-parameters
respectively, where is a positive integer, and for all
Let \begin{equation*} S^{j}(n) = \sum_{i=1}^{n} X^{j}_{i} =
X^{j}_{1} + X^{j}_{2} + \cdots + X^{j}_{n}, \quad n =1,2 , \cdots.
\end{equation*} We declare a "rencontre" at time , or, equivalently, say
that is a "rencontre-time," if \begin{equation*} S^{1}(n) = S^{2}(n) =
\cdots = S^{d}(n). \end{equation*} We motivate and study the distribution of
the first (provided it is finite) rencontre time.Comment: 40 pages, 1 tabl
Reversibility of continuous-variable quantum cloning
We analyze a reversibility of optimal Gaussian quantum cloning of a
coherent state using only local operations on the clones and classical
communication between them and propose a feasible experimental test of this
feature. Performing Bell-type homodyne measurement on one clone and anti-clone,
an arbitrary unknown input state (not only a coherent state) can be restored in
the other clone by applying appropriate local unitary displacement operation.
We generalize this concept to a partial LOCC reversal of the cloning and we
show that this procedure converts the symmetric cloner to an asymmetric cloner.
Further, we discuss a distributed LOCC reversal in optimal Gaussian
cloning of coherent states which transforms it to optimal cloning for
. Assuming the quantum cloning as a possible eavesdropping attack on
quantum communication link, the reversibility can be utilized to improve the
security of the link even after the attack.Comment: 7 pages, 5 figure
Optimal N-to-M Cloning of Quantum Coherent States
The cloning of continuous quantum variables is analyzed based on the concept
of Gaussian cloning machines, i.e., transformations that yield copies that are
Gaussian mixtures centered on the state to be copied. The optimality of
Gaussian cloning machines that transform N identical input states into M output
states is investigated, and bounds on the fidelity of the process are derived
via a connection with quantum estimation theory. In particular, the optimal
N-to-M cloning fidelity for coherent states is found to be equal to
MN/(MN+M-N).Comment: 3 pages, RevTe
- …