6,511 research outputs found
Single-electron tunneling in InP nanowires
We report on the fabrication and electrical characterization of field-effect
devices based on wire-shaped InP crystals grown from Au catalyst particles by a
vapor-liquid-solid process. Our InP wires are n-type doped with diameters in
the 40-55 nm range and lengths of several microns. After being deposited on an
oxidized Si substrate, wires are contacted individually via e-beam fabricated
Ti/Al electrodes. We obtain contact resistances as low as ~10 kOhm, with minor
temperature dependence. The distance between the electrodes varies between 0.2
and 2 micron. The electron density in the wires is changed with a back gate.
Low-temperature transport measurements show Coulomb-blockade behavior with
single-electron charging energies of ~1 meV. We also demonstrate energy
quantization resulting from the confinement in the wire.Comment: 4 pages, 3 figure
Exponential complexity of an adiabatic algorithm for an NP-complete problem
We prove an analytical expression for the size of the gap between the ground
and the first excited state of quantum adiabatic algorithm for the
3-satisfiability, where the initial Hamiltonian is a projector on the subspace
complementary to the ground state. For large problem sizes the gap decreases
exponentially and as a consequence the required running time is also
exponential.Comment: 5 pages, 2 figures; v3. published versio
Neutrix Calculus and Finite Quantum Field Theory
In general, quantum field theories (QFT) require regularizations and infinite
renormalizations due to ultraviolet divergences in their loop calculations.
Furthermore, perturbation series in theories like QED are not convergent
series, but are asymptotic series. We apply neutrix calculus, developed in
connection with asymptotic series and divergent integrals, to QFT,obtaining
finite renormalizations. While none of the physically measurable results in
renormalizable QFT is changed, quantum gravity is rendered more manageable in
the neutrix framework.Comment: 10 pages; LaTeX; version to appear in J. Phys. A: Math. Gen. as a
Letter to the Edito
Spatial search by quantum walk
Grover's quantum search algorithm provides a way to speed up combinatorial
search, but is not directly applicable to searching a physical database.
Nevertheless, Aaronson and Ambainis showed that a database of N items laid out
in d spatial dimensions can be searched in time of order sqrt(N) for d>2, and
in time of order sqrt(N) poly(log N) for d=2. We consider an alternative search
algorithm based on a continuous time quantum walk on a graph. The case of the
complete graph gives the continuous time search algorithm of Farhi and Gutmann,
and other previously known results can be used to show that sqrt(N) speedup can
also be achieved on the hypercube. We show that full sqrt(N) speedup can be
achieved on a d-dimensional periodic lattice for d>4. In d=4, the quantum walk
search algorithm takes time of order sqrt(N) poly(log N), and in d<4, the
algorithm does not provide substantial speedup.Comment: v2: 12 pages, 4 figures; published version, with improved arguments
for the cases where the algorithm fail
Optimal parametrizations of adiabatic paths
The parametrization of adiabatic paths is optimal when tunneling is
minimized. Hamiltonian evolutions do not have unique optimizers. However,
dephasing Lindblad evolutions do. The optimizers are simply characterized by an
Euler-Lagrange equation and have a constant tunneling rate along the path
irrespective of the gap. Application to quantum search algorithms recovers the
Grover result for appropriate scaling of the dephasing. Dephasing rates that
beat Grover imply hidden resources in Lindblad operators.Comment: 4 pages, 2 figures; To prevent from misunderstanding, we clarified
the discussion of an apparent speedup in the Grover algorithm; figures
improved + minor change
Exponential speed-up with a single bit of quantum information: Testing the quantum butterfly effect
We present an efficient quantum algorithm to measure the average fidelity
decay of a quantum map under perturbation using a single bit of quantum
information. Our algorithm scales only as the complexity of the map under
investigation, so for those maps admitting an efficient gate decomposition, it
provides an exponential speed up over known classical procedures. Fidelity
decay is important in the study of complex dynamical systems, where it is
conjectured to be a signature of quantum chaos. Our result also illustrates the
role of chaos in the process of decoherence.Comment: 4 pages, 2 eps figure
Tératogénicité de l’acide 13-cis rétinoïque sur les ébauches mandibulo-optiques de l’embryon de souris: approche histologique et histochimique
Oral administration of 400 mg/kg of 13-cis retinoic acid to 9 day pregnant mice gives rise to important maxillo-facial malformations. The first manifestation of teratogenic effect is an increase of density of cell death arising in the dorsal part of the first two branchial arches at day 9.5. These two arches become hypoplastic at days 10 and 11, and the preskelettic anlagen appear too late in comparison to control embryos. Meckel’s cartilage is too curvilinear and medially situated. Pre-ossicular and pre-mandibular blastemata develop with spatial distorsions which are well analyzable at days 16 and 17. They give some arguments to discuss several features of normal early development of this area.L’administration orale de 400 mg/kg d’acide 13-cis rétinoïque à des souris gestantes à 9 jours de gestation est responsable d’importantes malformations de la sphère oto-maxillo-faciale. Le premier signe consiste en une augmentation de la densité des nécroses observées dans la partie dorsale des deux premiers arcs à 9.5 jours. Ceux-ci se caractérisent ultérieurement par une hypoplasie relative par rapport aux témoins et les ébauches présquelettiques y apparaissent avec retard. Le cartilage de Meckel apparaît d’emblée trop arciforme et médialement situé. Les blastèmes pré-ossiculaires et prémandibulaires subissent une importante distorsion spatiale et leurs déformations, bien observables à 16 et 17 jours, permettent de discuter certains aspects de la morphogenèse précoce normale de ces éléments
Critical collapse of a massive vector field
We perform numerical simulations of the critical gravitational collapse of a
massive vector field. The result is that there are two critical solutions. One
is equivalent to the Choptuik critical solution for a massless scalar field.
The other is periodic.Comment: 7 pages, 4 figure
Optimal phase estimation in quantum networks
We address the problem of estimating the phase phi given N copies of the
phase rotation u(phi) within an array of quantum operations in finite
dimensions. We first consider the special case where the array consists of an
arbitrary input state followed by any arrangement of the N phase rotations, and
ending with a POVM. We optimise the POVM for a given input state and fixed
arrangement. Then we also optimise the input state for some specific cost
functions. In all cases, the optimal POVM is equivalent to a quantum Fourier
transform in an appropriate basis. Examples and applications are given.Comment: 9 pages, 2 figures; this is an extended version of
arXiv:quant-ph/0609160. v2: minor corrections in reference
Enhanced Peculiar Velocities in Brane-Induced Gravity
The mounting evidence for anomalously large peculiar velocities in our
Universe presents a challenge for the LCDM paradigm. The recent estimates of
the large scale bulk flow by Watkins et al. are inconsistent at the nearly 3
sigma level with LCDM predictions. Meanwhile, Lee and Komatsu have recently
estimated that the occurrence of high-velocity merging systems such as the
Bullet Cluster (1E0657-57) is unlikely at a 6.5-5.8 sigma level, with an
estimated probability between 3.3x10^{-11} and 3.6x10^{-9} in LCDM cosmology.
We show that these anomalies are alleviated in a broad class of
infrared-modifed gravity theories, called brane-induced gravity, in which
gravity becomes higher-dimensional at ultra large distances. These theories
include additional scalar forces that enhance gravitational attraction and
therefore speed up structure formation at late times and on sufficiently large
scales. The peculiar velocities are enhanced by 24-34% compared to standard
gravity, with the maximal enhancement nearly consistent at the 2 sigma level
with bulk flow observations. The occurrence of the Bullet Cluster in these
theories is 10^4 times more probable than in LCDM cosmology.Comment: 15 pages, 6 figures. v2: added reference
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