1,707 research outputs found
Robustness of the BB84 quantum key distribution protocol against general coherent attacks
It is demonstrated that for the entanglement-based version of the
Bennett-Brassard (BB84) quantum key distribution protocol, Alice and Bob share
provable entanglement if and only if the estimated qubit error rate is below
25% or above 75%. In view of the intimate relation between entanglement and
security, this result sheds also new light on the unconditional security of the
BB84 protocol in its original prepare-and-measure form. In particular, it
indicates that for small qubit error rates 25% is the ultimate upper security
bound for any prepare-and-measure BB84-type QKD protocol. On the contrary, for
qubit error rates between 25% and 75% we demonstrate that the correlations
shared between Alice and Bob can always be explained by separable states and
thus, no secret key can be distilled in this regime.Comment: New improved version. A minor mistake has been eliminate
Wave Solutions of Evolution Equations and Hamiltonian Flows on Nonlinear Subvarieties of Generalized Jacobians
The algebraic-geometric approach is extended to study solutions of
N-component systems associated with the energy dependent Schrodinger operators
having potentials with poles in the spectral parameter, in connection with
Hamiltonian flows on nonlinear subvariaties of Jacobi varieties. The systems
under study include the shallow water equation and Dym type equation. The
classes of solutions are described in terms of theta-functions and their
singular limits by using new parameterizations. A qualitative description of
real valued solutions is provided
Completely positive covariant two-qubit quantum processes and optimal quantum NOT operations for entangled qubit pairs
The structure of all completely positive quantum operations is investigated
which transform pure two-qubit input states of a given degree of entanglement
in a covariant way. Special cases thereof are quantum NOT operations which
transform entangled pure two-qubit input states of a given degree of
entanglement into orthogonal states in an optimal way. Based on our general
analysis all covariant optimal two-qubit quantum NOT operations are determined.
In particular, it is demonstrated that only in the case of maximally entangled
input states these quantum NOT operations can be performed perfectly.Comment: 14 pages, 2 figure
On Soliton-type Solutions of Equations Associated with N-component Systems
The algebraic geometric approach to -component systems of nonlinear
integrable PDE's is used to obtain and analyze explicit solutions of the
coupled KdV and Dym equations. Detailed analysis of soliton fission, kink to
anti-kink transitions and multi-peaked soliton solutions is carried out.
Transformations are used to connect these solutions to several other equations
that model physical phenomena in fluid dynamics and nonlinear optics.Comment: 43 pages, 16 figure
Antisymmetric multi-partite quantum states and their applications
Entanglement is a powerful resource for processing quantum information. In
this context pure, maximally entangled states have received considerable
attention. In the case of bipartite qubit-systems the four orthonormal
Bell-states are of this type. One of these Bell states, the singlet Bell-state,
has the additional property of being antisymmetric with respect to particle
exchange. In this contribution we discuss possible generalizations of this
antisymmetric Bell-state to cases with more than two particles and with
single-particle Hilbert spaces involving more than two dimensions. We review
basic properties of these totally antisymmetric states. Among possible
applications of this class of states we analyze a new quantum key sharing
protocol and methods for comparing quantum states
On the visualization of universal degeneracy in the IMRT problem
BACKGROUND: In general, the IMRT optimisation problem possesses many equivalent solutions. This makes it difficult to decide whether a result produced by an IMRT planning algorithm can be further improved, e.g. by adding more beams, or whether it is close to the globally best solution. RESULTS: It is conjectured that the curvature properties of the objective function around any globally optimum dose distribution are universal. This allows an assessment of optimality of dose distributions that are generated by different beam arrangements in a complementary manner to the objective function value alone. A tool to visualize the curvature structure of the objective function is devised. CONCLUSION: In an example case, it is demonstrated how the assessment of the curvature space can indicate the equivalence of rival beam configurations and their proximity to the global optimum
Continuous macroscopic limit of a discrete stochastic model for interaction of living cells
In the development of multiscale biological models it is crucial to establish
a connection between discrete microscopic or mesoscopic stochastic models and
macroscopic continuous descriptions based on cellular density. In this paper a
continuous limit of a two-dimensional Cellular Potts Model (CPM) with excluded
volume is derived, describing cells moving in a medium and reacting to each
other through both direct contact and long range chemotaxis. The continuous
macroscopic model is obtained as a Fokker-Planck equation describing evolution
of the cell probability density function. All coefficients of the general
macroscopic model are derived from parameters of the CPM and a very good
agreement is demonstrated between CPM Monte Carlo simulations and numerical
solution of the macroscopic model. It is also shown that in the absence of
contact cell-cell interactions, the obtained model reduces to the classical
macroscopic Keller-Segel model. General multiscale approach is demonstrated by
simulating spongy bone formation from loosely packed mesenchyme via the
intramembranous route suggesting that self-organizing physical mechanisms can
account for this developmental process.Comment: 4 pages, 3 figure
Preparation of entangled states of two photons in several spatial modes
We describe a protocol capable of preparing an arbitrary state of two photons
in several spatial modes using pairs of photons generated by spontaneous
parametric down-conversion, linear optical elements and single-photon detectors
or post-selection. The protocol involves unitary and non-unitary
transformations realizable by beam splitters and phase shifters. Non-unitary
transformations are implemented by attenuation filters. The protocol contains
several optimization capabilities with the goal of improving overall
probability of its success. We also show how entangled two-photon states
required for quantum computing with linear optics can be prepared using a very
simple and feasible scheme.Comment: 9 pages, 9 figures, REVTeX
Thermal entanglement witness for materials with variable local spin lengths
We show that the thermal entanglement in a spin system using only magnetic
susceptibility measurements is restricted to the insulator materials. We
develop a generalization of the thermal entanglement witness that allows us to
get information about the system entanglement with variable local spin lengths
that can be used experimentally in conductor or insulator materials. As an
application, we study thermal entanglement for the half-filled Hubbard model
for linear, square and cubic clusters. We note that it is the itinerancy of
electrons that favors the entanglement. Our results suggest a weak dependence
between entanglement and external spin freedom degrees.Comment: 4 pages, 3 figure
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