259 research outputs found
Grover's algorithm on a Feynman computer
We present an implementation of Grover's algorithm in the framework of
Feynman's cursor model of a quantum computer. The cursor degrees of freedom act
as a quantum clocking mechanism, and allow Grover's algorithm to be performed
using a single, time-independent Hamiltonian. We examine issues of locality and
resource usage in implementing such a Hamiltonian. In the familiar language of
Heisenberg spin-spin coupling, the clocking mechanism appears as an excitation
of a basically linear chain of spins, with occasional controlled jumps that
allow for motion on a planar graph: in this sense our model implements the idea
of "timing" a quantum algorithm using a continuous-time random walk. In this
context we examine some consequences of the entanglement between the states of
the input/output register and the states of the quantum clock
POROUS MEDIA MICROSTRUCTURE RECONSTRUCTION USING PIXEL-BASED AND OBJECT-BASED SIMULATED ANNEALING – COMPARISON WITH OTHER RECONSTRUCTION METHODS
In this contribution the issue of the stochastic reconstruction of particulatemedia from 2D photomicrographic images is addressed with particular reference to pore space connectivity. The reconstruction of porous bodies in 2D or 3D space was achieved by using simulated annealing techniques. Two methods were proposed to reconstruct a well connected pore space. The first, named PSA (Pixel-based Simulated Annealing), a pixel-movement based, three constraints were found to be necessary for the successful reconstruction of well connected pore space: the two-pointcorrelation function, the d3-4 distance transform distribution and the linealpath function for the pore phase. The second, named OSA (Object-based Simulated Annealing), only constrains the two-point correlation function. Following several researches which tried to reconstruct porous media using pixel-movement based simulated techniques, we propose a new parameter to add a microstructure descriptor, but we also propose a new technique, based in moving the microstructure grains (spheres) instead of the pixels. Both methods were applied to reconstruct reservoir rocks microstructures, and the 2D and 3D results were compared with microstructures reconstructed by truncated Gaussian methods. The PSA resulted in microstructures characterized by poor pore space connectivity, and by artificial patterns, while the OSA reconstructed microstructures with good pore space connectivity. These results indicate that the OSA method can reconstruct better microstructures than the present methods
The digital whomanities project. Best practices for digital pedagogy in the pandemic era
This paper aims to enter the ongoing debate about the critical issues of digital pedagogy through the presentation of Digital WHOmanities, a series of online conferences and workshops held at the University of Bologna. Distance learning has become one of the most discussed topics in educational institutions during the spread of Covid-19, revealing a discrepancy between the rapid development of technology and the ability of learning environments to adapt to this turn. In view of this ongoing debate, Digital WHOmanities tried to define the complex and multifaceted figure of the digital humanist and to provide a methodological framework that could foster further online academic initiatives. Specifically, the accurate organization of timing and contents and the adoption of synchronous and asynchronous approaches have highlighted the effectiveness of flexible digital didactics
The power of quantum systems on a line
We study the computational strength of quantum particles (each of finite
dimensionality) arranged on a line. First, we prove that it is possible to
perform universal adiabatic quantum computation using a one-dimensional quantum
system (with 9 states per particle). This might have practical implications for
experimentalists interested in constructing an adiabatic quantum computer.
Building on the same construction, but with some additional technical effort
and 12 states per particle, we show that the problem of approximating the
ground state energy of a system composed of a line of quantum particles is
QMA-complete; QMA is a quantum analogue of NP. This is in striking contrast to
the fact that the analogous classical problem, namely, one-dimensional
MAX-2-SAT with nearest neighbor constraints, is in P. The proof of the
QMA-completeness result requires an additional idea beyond the usual techniques
in the area: Not all illegal configurations can be ruled out by local checks,
so instead we rule out such illegal configurations because they would, in the
future, evolve into a state which can be seen locally to be illegal. Our
construction implies (assuming the quantum Church-Turing thesis and that
quantum computers cannot efficiently solve QMA-complete problems) that there
are one-dimensional systems which take an exponential time to relax to their
ground states at any temperature, making them candidates for being
one-dimensional spin glasses.Comment: 21 pages. v2 has numerous corrections and clarifications, and most
importantly a new author, merged from arXiv:0705.4067. v3 is the published
version, with additional clarifications, publisher's version available at
http://www.springerlink.co
Quantum Annealing and Analog Quantum Computation
We review here the recent success in quantum annealing, i.e., optimization of
the cost or energy functions of complex systems utilizing quantum fluctuations.
The concept is introduced in successive steps through the studies of mapping of
such computationally hard problems to the classical spin glass problems. The
quantum spin glass problems arise with the introduction of quantum
fluctuations, and the annealing behavior of the systems as these fluctuations
are reduced slowly to zero. This provides a general framework for realizing
analog quantum computation.Comment: 22 pages, 7 figs (color online); new References Added. Reviews of
Modern Physics (in press
Quantum Simulations of Classical Annealing Processes
We describe a quantum algorithm that solves combinatorial optimization
problems by quantum simulation of a classical simulated annealing process. Our
algorithm exploits quantum walks and the quantum Zeno effect induced by
evolution randomization. It requires order steps to find an
optimal solution with bounded error probability, where is the minimum
spectral gap of the stochastic matrices used in the classical annealing
process. This is a quadratic improvement over the order steps
required by the latter.Comment: 4 pages - 1 figure. This work differs from arXiv:0712.1008 in that
the quantum Zeno effect is implemented via randomization in the evolutio
On the path integral representation for quantum spin models and its application to the quantum cavity method and to Monte Carlo simulations
The cavity method is a well established technique for solving classical spin
models on sparse random graphs (mean-field models with finite connectivity).
Laumann et al. [arXiv:0706.4391] proposed recently an extension of this method
to quantum spin-1/2 models in a transverse field, using a discretized
Suzuki-Trotter imaginary time formalism. Here we show how to take analytically
the continuous imaginary time limit. Our main technical contribution is an
explicit procedure to generate the spin trajectories in a path integral
representation of the imaginary time dynamics. As a side result we also show
how this procedure can be used in simple heat-bath like Monte Carlo simulations
of generic quantum spin models. The replica symmetric continuous time quantum
cavity method is formulated for a wide class of models, and applied as a simple
example on the Bethe lattice ferromagnet in a transverse field. The results of
the methods are confronted with various approximation schemes in this
particular case. On this system we performed quantum Monte Carlo simulations
that confirm the exactness of the cavity method in the thermodynamic limit.Comment: 25 pages, 15 figures, typos correcte
Oleate but not stearate induces the regulatory phenotype of myeloid suppressor cells
Tumor infiltrating myeloid cells play contradictory roles in the tumor
development. Dendritic cells and classical activated macrophages support anti-
tumor immune activity via antigen presentation and induction of pro-
inflammatory immune responses. Myeloid suppressor cells (MSCs), for instance
myeloid derived suppressor cells (MDSCs) or tumor associated macrophages play
a critical role in tumor growth. Here, treatment with sodium oleate, an
unsaturated fatty acid, induced a regulatory phenotype in the myeloid
suppressor cell line MSC-2 and resulted in an increased suppression of
activated T cells, paralleled by increased intracellular lipid droplets
formation. Furthermore, sodium oleate potentiated nitric oxide (NO) production
in MSC-2, thereby increasing their suppressive capacity. In primary polarized
bone marrow cells, sodium oleate (C18:1) and linoleate (C18:2), but not
stearate (C18:0) were identified as potent FFA to induce a regulatory
phenotype. This effect was abrogated in MSC-2 as well as primary cells by
specific inhibition of droplets formation while the inhibition of de novo FFA
synthesis proved ineffective, suggesting a critical role for exogenous FFA in
the functional induction of MSCs. Taken together our data introduce a new
unsaturated fatty acid-dependent pathway shaping the functional phenotype of
MSCs, facilitating the tumor escape from the immune system
POROUS MEDIA MICROSTRUCTURE RECONSTRUCTION USING PIXEL-BASED AND OBJECT-BASED SIMULATED ANNEALING – COMPARISON WITH OTHER RECONSTRUCTION METHODS
In this contribution the issue of the stochastic reconstruction of particulatemedia from 2D photomicrographic images is addressed with particular reference to pore space connectivity. The reconstruction of porous bodies in 2D or 3D space was achieved by using simulated annealing techniques. Two methods were proposed to reconstruct a well connected pore space. The first, named PSA (Pixel-based Simulated Annealing), a pixel-movement based, three constraints were found to be necessary for the successful reconstruction of well connected pore space: the two-pointcorrelation function, the d3-4 distance transform distribution and the linealpath function for the pore phase. The second, named OSA (Object-based Simulated Annealing), only constrains the two-point correlation function. Following several researches which tried to reconstruct porous media using pixel-movement based simulated techniques, we propose a new parameter to add a microstructure descriptor, but we also propose a new technique, based in moving the microstructure grains (spheres) instead of the pixels. Both methods were applied to reconstruct reservoir rocks microstructures, and the 2D and 3D results were compared with microstructures reconstructed by truncated Gaussian methods. The PSA resulted in microstructures characterized by poor pore space connectivity, and by artificial patterns, while the OSA reconstructed microstructures with good pore space connectivity. These results indicate that the OSA method can reconstruct better microstructures than the present methods
The diminishing role of hubs in dynamical processes on complex networks
It is notoriously difficult to predict the behaviour of a complex
self-organizing system, where the interactions among dynamical units form a
heterogeneous topology. Even if the dynamics of each microscopic unit is known,
a real understanding of their contributions to the macroscopic system behaviour
is still lacking. Here we develop information-theoretical methods to
distinguish the contribution of each individual unit to the collective
out-of-equilibrium dynamics. We show that for a system of units connected by a
network of interaction potentials with an arbitrary degree distribution, highly
connected units have less impact on the system dynamics as compared to
intermediately connected units. In an equilibrium setting, the hubs are often
found to dictate the long-term behaviour. However, we find both analytically
and experimentally that the instantaneous states of these units have a
short-lasting effect on the state trajectory of the entire system. We present
qualitative evidence of this phenomenon from empirical findings about a social
network of product recommendations, a protein-protein interaction network, and
a neural network, suggesting that it might indeed be a widespread property in
nature.Comment: Published versio
- …