252 research outputs found
Time correlated quantum amplitude damping channel
We analyze the problem of sending classical information through qubit
channels where successive uses of the channel are correlated. This work extends
the analysis of C. Macchiavello and G. M. Palma to the case of a non-Pauli
channel - the amplitude damping channel. Using the channel description outlined
in S. Daffer, et al, we derive the correlated amplitude damping channel. We
obtain a similar result to C. Macchiavello and G. M. Palma, that is, that under
certain conditions on the degree of channel memory, the use of entangled input
signals may enhance the information transmission compared to the use of product
input signals.Comment: 9 pages, REVTex
Optimal dimensionality for quantum cryptography
We perform a comparison of two protocols for generating a cryptographic key
composed from d-valued symbols: one exploiting a string of independent qubits
and another one utilizing d-level systems prepared in states belonging to d+1
mutually unbiased bases. We show that the protocol based on qubits is optimal
for quantum cryptography, since it provides higher security and higher key
generation rate.Comment: Revtex, 4 pages, 1 eps figur
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
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
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
Symmetric qubits from cavity states
Two-mode cavities can be prepared in quantum states which represent symmetric
multi-qubit states. However, the qubits are impossible to address individually
and as such cannot be independently measured or otherwise manipulated. We
propose two related schemes to coherently transfer the qubits which the cavity
state represents onto individual atoms, so that the qubits can then be
processed individually. In particular, our scheme can be combined with the
quantum cloning scheme of Simon and coworkers [C. Simon et al, PRL 84, 2993
(2000)] to allow the optimal clones which their scheme produces to be spatially
separated and individually utilized.Comment: 8 pages, 4 figures, minor typographical errors correcte
Distributed phase-covariant cloning with atomic ensembles via quantum Zeno dynamics
We propose an interesting scheme for distributed orbital state quantum
cloning with atomic ensembles based on the quantum Zeno dynamics. These atomic
ensembles which consist of identical three-level atoms are trapped in distant
cavities connected by a single-mode integrated optical star coupler. These
qubits can be manipulated through appropriate modulation of the coupling
constants between atomic ensemble and classical field, and the cavity decay can
be largely suppressed as the number of atoms in the ensemble qubits increases.
The fidelity of each cloned qubit can be obtained with analytic result. The
present scheme provides a new way to construct the quantum communication
network.Comment: 5 pages, 4 figure
Broadcasting of Inseparability
Suppose we are given an entangled pair and then one can ask how well we can
produce two entangled pairs starting from a given entangled pair using only
local operations. To give response of the above asked question, we study
broadcasting of entanglement using state dependent quantum cloning machine as a
local copier. We show that the length of the interval for
probability-amplitude-squared for broadcasting of entanglement using state
dependent cloner can be made larger than the length of the interval for
probability-amplitude-squared for broadcasting entanglement using state
independent cloner. Further we show that there exists local state dependent
cloner which gives better quality copy (in terms of average fidelity) of an
entangled pair than the local universal cloner.Comment: 13 pages, 2 tables, Accepted in Journal of Physics
The quantum capacity is properly defined without encodings
We show that no source encoding is needed in the definition of the capacity
of a quantum channel for carrying quantum information. This allows us to use
the coherent information maximized over all sources and and block sizes, but
not encodings, to bound the quantum capacity. We perform an explicit
calculation of this maximum coherent information for the quantum erasure
channel and apply the bound in order find the erasure channel's capacity
without relying on an unproven assumption as in an earlier paper.Comment: 19 pages revtex with two eps figures. Submitted to Phys. Rev. A.
Replaced with revised and simplified version, and improved references, etc.
Why can't the last line of the comments field end with a period using this
web submission form
Non-linear operations in quantum information theory
Quantum information theory is used to analize various non-linear operations
on quantum states. The universal disentanglement machine is shown to be
impossible, and partial (negative) results are obtained in the state-dependent
case. The efficiency of the transformation of non-orthogonal states into
orthogonal ones is discussed.Comment: 11 pages, LaTeX, 3 figures on separate page
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