5,695 research outputs found
Entanglement and the linearity of quantum mechanics
Optimal universal entanglement processes are discussed which entangle two
quantum systems in an optimal way for all possible initial states. It is
demonstrated that the linear character of quantum theory which enforces the
peaceful coexistence of quantum mechanics and relativity imposes severe
restrictions on the structure of the resulting optimally entangled states.
Depending on the dimension of the one-particle Hilbert space such a universal
process generates either a pure Bell state or mixed entangled states. In the
limit of very large dimensions of the one-particle Hilbert space the
von-Neumann entropy of the optimally entangled state differs from the one of
the maximally mixed two-particle state by one bit only.Comment: Proceedings of the X International Symposium on Theoretical
Electrical Engineering, ISTET 99, Magdebur
Disentanglement and Inseparability correlation : in two-qubit system
Started from local universal isotropic disentanglement, a threshold
inequality on reduction factors is proposed, which is necessary and sufficient
for this type of disentanglement processes. Furthermore, we give the conditions
realizing ideal disentanglement processes provided that some information on
quantum states is known. In addition, based on fully entangled fraction, a
concept called inseparability correlation is presented. Some properties on
inseparability correlation coefficient are studied.Comment: 10 Pages, 2 Figures, REVTeX; to appear in PR
Asymmetric quantum telecloning of d-level systems and broadcasting of entanglement to different locations using the "many-to-many" communication protocol
We propose a generalization of quantum teleportation: the so-called
many-to-many quantum communication of the information of a d-level system from
N spatially separated senders to M>N receivers situated at different locations.
We extend the concept of asymmetric telecloning from qubits to d-dimensional
systems. We investigate the broadcasting of entanglement by using local 1->2
optimal universal asymmetric Pauli machines and show that the maximal
fidelities of the two final entangled states are obtained when symmetric
machines are applied. Cloning of entanglement is studied using a nonlocal
optimal universal asymmetric cloning machine and we show that the symmetric
machine optimally copies the entanglement. The "many-to-many" teleportation
scheme is applied in order to distribute entanglement shared between two
observers to two pairs of spatially separated observers.Comment: 17 pages, 1 figur
Optimal manipulations with qubits: Universal quantum entanglers
We analyze various scenarios for entangling two initially unentangled qubits.
In particular, we propose an optimal universal entangler which entangles a
qubit in unknown state with a qubit in a reference (known) state
. That is, our entangler generates the output state which is as close as
possible to the pure (symmetrized) state . The most
attractive feature of this entangling machine, is that the fidelity of its
performance (i.e. the distance between the output and the ideally entangled --
symmetrized state) does not depend on the input and takes the constant value
. We also analyze how to optimally generate
from a single qubit initially prepared in an unknown state |\Psi\r a two
qubit entangled system which is as close as possible to a Bell state
, where \l\Psi|\Psi^\perp\r =0.Comment: 11 pages, 3 eps figures, accepted for publication in Phys. Rev.
Broadcasting of three qubit entanglement via local copying and entanglement swapping
In this work,We investigate the problem of secretly broadcasting of
three-qubit entangled state between two distant partners. The interesting
feature of this problem is that starting from two particle entangled state
shared between two distant partners we find that the action of local cloner on
the qubits and the measurement on the machine state vector generates
three-qubit entanglement between them. The broadcasting of entanglement is made
secret by sending the measurement result secretly using cryptographic scheme
based on orthogonal states. Further we show that this idea can be extended to
generate three particle entangled state between three distant partners.Comment: 18 pages, 4 figures, Accepted in Physical Review
Universal Dynamical Control of Local Decoherence for Multipartite and Multilevel Systems
A unified theory is given of dynamically modified decay and decoherence of
field-driven multilevel multipartite entangled states that are weakly coupled
to zero-temperature baths or undergo random phase fluctuations. The theory
allows for arbitrary local differences in their coupling to the environment.
Due to such differences, the optimal driving-field modulation to ensure maximal
fidelity is found to substantially differ from conventional ``Bang-Bang'' or
-phase flips of the single-qubit evolution.Comment: 22 pages, 6 figure
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