925 research outputs found
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
Efficient bipartite quantum state purification in arbitrary dimensional Hilbert spaces
A new purification scheme is proposed which applies to arbitrary dimensional
bipartite quantum systems. It is based on the repeated application of a special
class of nonlinear quantum maps and a single, local unitary operation. This
special class of nonlinear quantum maps is generated in a natural way by a
hermitian generalized XOR-gate. The proposed purification scheme offers two
major advantages, namely it does not require local depolarization operations at
each step of the purification procedure and it purifies more efficiently than
other know purification schemes.Comment: This manuscript is based on results of our previous manuscript
'Generalized quantum XOR-gate for quantum teleportation and state
purification in arbitrary dimensional Hilbert spaces
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
Sequential Quantum Cloning
Not all unitary operations upon a set of qubits can be implemented by
sequential interactions between each qubit and an ancillary system. We analyze
the specific case of sequential quantum cloning 1->M and prove that the minimal
dimension D of the ancilla grows linearly with the number of clones M. In
particular, we obtain D = 2M for symmetric universal quantum cloning and D =
M+1 for symmetric phase-covariant cloning. Furthermore, we provide a recipe for
the required ancilla-qubit interactions in each step of the sequential
procedure for both cases.Comment: 4 pages, no figures. New version with changes. Accepted in Physical
Review Letter
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
Class of PPT bound entangled states associated to almost any set of pure entangled states
We analyze a class of entangled states for bipartite systems,
with non-prime. The entanglement of such states is revealed by the
construction of canonically associated entanglement witnesses. The structure of
the states is very simple and similar to the one of isotropic states: they are
a mixture of a separable and a pure entangled state whose supports are
orthogonal. Despite such simple structure, in an opportune interval of the
mixing parameter their entanglement is not revealed by partial transposition
nor by the realignment criterion, i.e. by any permutational criterion in the
bipartite setting. In the range in which the states are Positive under Partial
Transposition (PPT), they are not distillable; on the other hand, the states in
the considered class are provably distillable as soon as they are Nonpositive
under Partial Transposition (NPT). The states are associated to any set of more
than two pure states. The analysis is extended to the multipartite setting. By
an opportune selection of the set of multipartite pure states, it is possible
to construct mixed states which are PPT with respect to any choice of bipartite
cuts and nevertheless exhibit genuine multipartite entanglement. Finally, we
show that every -positive but not completely positive map is associated to a
family of nondecomposable maps.Comment: 12 pages, 3 figures. To appear in Phys. Rev.
Analysis of the flavonoid component of bioactive New Zealand mÄnuka (Leptospermum scoparium) honey and the isolation, characterisation and synthesis of an unusual pyrrole
The flavonoid components of New Zealand mÄnuka (Leptospermum scoparium) honey have been quantified in a series of 31 honeys of varying non-peroxide antibacterial activity to clarify discrepancies between previous studies reported in the literature. Total flavonoid content was 1.16 mg/100 g honey. The principal flavonoids present were pinobanksin, pinocembrin, luteolin and chrysin and together these represented 61% of the total flavonoid content. 1, 2-formyl-5-(2-methoxyphenyl)-pyrrole, which was weakly correlated with the non-peroxide antibacterial activity, was isolated from the flavonoid fraction and separately synthesised. 1 did not display inhibitory activity against Staphylococcus aureus in vitro and thus the origin of the correlation, which is still unknown, is not a direct contribution
Optimal copying of entangled two-qubit states
We investigate the problem of copying pure two-qubit states of a given degree
of entanglement in an optimal way. Completely positive covariant quantum
operations are constructed which maximize the fidelity of the output states
with respect to two separable copies. These optimal copying processes hint at
the intricate relationship between fundamental laws of quantum theory and
entanglement.Comment: 13 pages, 7 figure
Quantum error correction of coherent errors by randomization
A general error correction method is presented which is capable of correcting
coherent errors originating from static residual inter-qubit couplings in a
quantum computer. It is based on a randomization of static imperfections in a
many-qubit system by the repeated application of Pauli operators which change
the computational basis. This Pauli-Random-Error-Correction (PAREC)-method
eliminates coherent errors produced by static imperfections and increases
significantly the maximum time over which realistic quantum computations can be
performed reliably. Furthermore, it does not require redundancy so that all
physical qubits involved can be used for logical purposes.Comment: revtex 4 pages, 3 fig
Elastic Interfacial Waves in Discrete and Continuous Media
Phonon spectra of bicrystals with relaxed grain-boundary structure display a variety of localized modes including long-wavelength acoustic modes. Continuum solutions for localized waves that incorporate atomic-level elastic properties of the interface via discontinuity relations agree well with the latter modes. In contrast, classical solutions that depend only on bulk elastic properties do not. This demonstrates that the distinct atomic structure of the interface is a controlling factor, and it is shown how local, atomic-level properties can be incorporated into continuum analyses of interfacial phenomena
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