146 research outputs found
Graph editing to a fixed target
For a fixed graph H, the H-Minor Edit problem takes as input a graph G and an integer k and asks whether G can be modified into H by a total of at most k edge contractions, edge deletions and vertex deletions. Replacing edge contractions by vertex dissolutions yields the H-Topological Minor Edit problem. For each problem we show polynomial-time solvable and NP-complete cases depending on the choice of H. Moreover, when G is AT-free, chordal or planar, we show that H-Minor Edit is polynomial-time solvable for all graphs H
L-Drawings of Directed Graphs
We introduce L-drawings, a novel paradigm for representing directed graphs
aiming at combining the readability features of orthogonal drawings with the
expressive power of matrix representations. In an L-drawing, vertices have
exclusive - and -coordinates and edges consist of two segments, one
exiting the source vertically and one entering the destination horizontally.
We study the problem of computing L-drawings using minimum ink. We prove its
NP-completeness and provide a heuristics based on a polynomial-time algorithm
that adds a vertex to a drawing using the minimum additional ink. We performed
an experimental analysis of the heuristics which confirms its effectiveness.Comment: 11 pages, 7 figure
All-Electrical Quantum Computation with Mobile Spin Qubits
We describe and discuss a solid state proposal for quantum computation with
mobile spin qubits in one-dimensional systems, based on recent advances in
spintronics. Static electric fields are used to implement a universal set of
quantum gates, via the spin-orbit and exchange couplings. Initialization and
measurement can be performed either by spin injection from/to ferromagnets, or
by using spin filters and mesoscopic spin polarizing beam-splitters. The
vulnerability of this proposal to various sources of error is estimated by
numerical simulations. We also assess the suitability of various materials
currently used in nanotechnology for an actual implementation of our model.Comment: 10 pages, 6 figs, RevTeX
Electron Spins in Artificial Atoms and Molecules for Quantum Computing
Achieving control over the electron spin in quantum dots (artificial atoms)
or real atoms promises access to new technologies in conventional and in
quantum information processing. Here we review our proposal for quantum
computing with spins of electrons confined to quantum dots. We discuss the
basic requirements for implementing spin-qubits, and describe a complete set of
quantum gates for single- and two-qubit operations. We show how a quantum dot
attached to leads can be used for spin filtering and spin read-out, and as a
spin-memory device. Finally, we focus on the experimental characterization of
the quantum dot systems, and discuss transport properties of a double-dot and
show how Kondo correlations can be used to measure the Heisenberg exchange
interaction between the spins of two dots.Comment: 13 pages, 8 figures, Invited Review (Semiconductor Spintronics,
Special Issue of SST
The readout of the fullerene-based quantum computing by a scanning tunneling microscope
We consider to detect the electron spin of a doped atom, i.e., a nitrogen or
a phosphorus, caged in a fullerene by currently available technique of the
scanning tunneling microscope (STM), which actually corresponds to the readout
of a qubit in the fullerene-based quantum computing. Under the conditions of
polarized STM current and Coulomb blockade, we investigate the tunneling matrix
elements involving the exchange coupling between the tunneling polarized
electrons and the encapsulated polarized electron, and calculate the variation
of the tunneling current with respect to different orientations of the
encapsulated electron spin. The experimental feasibility of our scheme is
discussed under the consideration of some imperfect factors.Comment: RevTex file, 3 figures. To appear in New Journal of Physic
Bezpieczeństwo i ryzyko podróżowania tanimi liniami lotniczymi
Golovach, E. P.; Rubahov, A. I. Safety and risks of traveling with low-cost airlinesDecydując się na podróż samolotem, jednym z najważniejszych czynników jakie potencjalny turysta bieżę pod uwagę jest kwestia bezpieczeństwa. Szczególnego znaczenia ten czynnik nabiera zwłaszcza w dobie nagłaśnianych i szeroko omawianych katastrof lotniczych w różnych częściach świata oraz bankructwie tanich linii lotniczych w ostatnich latach. W referacie zostali omówione czynniki mające wpływ na bezpieczeństwo i ryzyko podróżowania tanimi liniami lotniczymi. Przedstawione obszary z największą liczbą ofiar wypadków cyvilnych statków powietrznych
Line-distortion, Bandwidth and Path-length of a graph
We investigate the minimum line-distortion and the minimum bandwidth problems
on unweighted graphs and their relations with the minimum length of a
Robertson-Seymour's path-decomposition. The length of a path-decomposition of a
graph is the largest diameter of a bag in the decomposition. The path-length of
a graph is the minimum length over all its path-decompositions. In particular,
we show:
- if a graph can be embedded into the line with distortion , then
admits a Robertson-Seymour's path-decomposition with bags of diameter at most
in ;
- for every class of graphs with path-length bounded by a constant, there
exist an efficient constant-factor approximation algorithm for the minimum
line-distortion problem and an efficient constant-factor approximation
algorithm for the minimum bandwidth problem;
- there is an efficient 2-approximation algorithm for computing the
path-length of an arbitrary graph;
- AT-free graphs and some intersection families of graphs have path-length at
most 2;
- for AT-free graphs, there exist a linear time 8-approximation algorithm for
the minimum line-distortion problem and a linear time 4-approximation algorithm
for the minimum bandwidth problem
Spin relaxation at the singlet-triplet crossing in a quantum dot
We study spin relaxation in a two-electron quantum dot in the vicinity of the
singlet-triplet crossing. The spin relaxation occurs due to a combined effect
of the spin-orbit, Zeeman, and electron-phonon interactions. The
singlet-triplet relaxation rates exhibit strong variations as a function of the
singlet-triplet splitting. We show that the Coulomb interaction between the
electrons has two competing effects on the singlet-triplet spin relaxation. One
effect is to enhance the relative strength of spin-orbit coupling in the
quantum dot, resulting in larger spin-orbit splittings and thus in a stronger
coupling of spin to charge. The other effect is to make the charge density
profiles of the singlet and triplet look similar to each other, thus
diminishing the ability of charge environments to discriminate between singlet
and triplet states. We thus find essentially different channels of
singlet-triplet relaxation for the case of strong and weak Coulomb interaction.
Finally, for the linear in momentum Dresselhaus and Rashba spin-orbit
interactions, we calculate the singlet-triplet relaxation rates to leading
order in the spin-orbit interaction, and find that they are proportional to the
second power of the Zeeman energy, in agreement with recent experiments on
triplet-to-singlet relaxation in quantum dots.Comment: 29 pages, 14 figures, 1 tabl
On retracts, absolute retracts, and folds in cographs
Let G and H be two cographs. We show that the problem to determine whether H
is a retract of G is NP-complete. We show that this problem is fixed-parameter
tractable when parameterized by the size of H. When restricted to the class of
threshold graphs or to the class of trivially perfect graphs, the problem
becomes tractable in polynomial time. The problem is also soluble when one
cograph is given as an induced subgraph of the other. We characterize absolute
retracts of cographs.Comment: 15 page
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