572 research outputs found
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Generation of Micro Mechanical Devices Using Stereo Lithography
A high resolution machining setup for creating three-dimensional precision components from a
UV-curable photo-resin has been developed. By using frequency-converted diode-pumped solid
state lasers, functional micro-mechanical devices are directly fabricated in a successive layer-bylayer fashion. Within this paper, the direct generation of micro assemblies having
moving components without further assembly of parts will be presented. The micro system
design is based on user-defined 3D-CAD data and will completively be built up within the
fabrication cycle. By using specially developed μSL materials with suitable properties for micromechanical parts, the development from Rapid Prototyping towards Rapid Production of small
series is intended.Mechanical Engineerin
Stochastic Matrix Product States
The concept of stochastic matrix product states is introduced and a natural
form for the states is derived. This allows to define the analogue of Schmidt
coefficients for steady states of non-equilibrium stochastic processes. We
discuss a new measure for correlations which is analogous to the entanglement
entropy, the entropy cost , and show that this measure quantifies the bond
dimension needed to represent a steady state as a matrix product state. We
illustrate these concepts on the hand of the asymmetric exclusion process
Conical: an extended module for computing a numerically satisfactory pair of solutions of the differential equation for conical functions
Conical functions appear in a large number of applications in physics and
engineering. In this paper we describe an extension of our module CONICAL for
the computation of conical functions. Specifically, the module includes now a
routine for computing the function , a
real-valued numerically satisfactory companion of the function for . In this way, a natural basis for solving
Dirichlet problems bounded by conical domains is provided.Comment: To appear in Computer Physics Communication
Quantum Metropolis Sampling
The original motivation to build a quantum computer came from Feynman who
envisaged a machine capable of simulating generic quantum mechanical systems, a
task that is believed to be intractable for classical computers. Such a machine
would have a wide range of applications in the simulation of many-body quantum
physics, including condensed matter physics, chemistry, and high energy
physics. Part of Feynman's challenge was met by Lloyd who showed how to
approximately decompose the time-evolution operator of interacting quantum
particles into a short sequence of elementary gates, suitable for operation on
a quantum computer. However, this left open the problem of how to simulate the
equilibrium and static properties of quantum systems. This requires the
preparation of ground and Gibbs states on a quantum computer. For classical
systems, this problem is solved by the ubiquitous Metropolis algorithm, a
method that basically acquired a monopoly for the simulation of interacting
particles. Here, we demonstrate how to implement a quantum version of the
Metropolis algorithm on a quantum computer. This algorithm permits to sample
directly from the eigenstates of the Hamiltonian and thus evades the sign
problem present in classical simulations. A small scale implementation of this
algorithm can already be achieved with today's technologyComment: revised versio
Resonantly enhanced pair production in a simple diatomic model
A new mechanism for the production of electron-positron pairs from the
interaction of a laser field and a fully stripped diatomic molecule in the
tunneling regime is presented. When the laser field is turned off, the Dirac
operator has resonances in both the positive and the negative energy continua
while bound states are in the mass gap. When this system is immersed in a
strong laser field, the resonances move in the complex energy plane: the
negative energy resonances are pushed to higher energies while the bound states
are Stark shifted. It is argued here that there is a pair production
enhancement at the crossing of resonances by looking at a simple 1-D model: the
nuclei are modeled simply by Dirac delta potential wells while the laser field
is assumed to be static and of finite spatial extent. The average rate for the
number of electron-positron pairs produced is evaluated and the results are
compared to the single nucleus and to the free cases. It is shown that
positrons are produced by the Resonantly Enhanced Pair Production (REPP)
mechanism, which is analogous to the resonantly enhanced ionization of
molecular physics. This phenomenon could be used to increase the number of
pairs produced at low field strength, allowing the study of the Dirac vacuum.Comment: 11 pages, 4 figure
Fermions on one or fewer Kinks
We find the full spectrum of fermion bound states on a Z_2 kink. In addition
to the zero mode, there are int[2 m_f/m_s] bound states, where m_f is the
fermion and m_s the scalar mass. We also study fermion modes on the background
of a well-separated kink-antikink pair. Using a variational argument, we prove
that there is at least one bound state in this background, and that the energy
of this bound state goes to zero with increasing kink-antikink separation, 2L,
and faster than e^{-a2L} where a = min(m_s, 2 m_f). By numerical evaluation, we
find some of the low lying bound states explicitly.Comment: 7 pages, 4 figure
Computation of a numerically satisfactory pair of solutions of the differential equation for conical functions of non-negative integer orders
We consider the problem of computing satisfactory pairs of solutions of the
differential equation for Legendre functions of non-negative integer order
and degree , where is a non-negative real
parameter. Solutions of this equation are the conical functions
and ,
. An algorithm for computing a numerically satisfactory pair of solutions
is already available when (see \cite{gil:2009:con},
\cite{gil:2012:cpc}).In this paper, we present a stable computational scheme
for a real valued numerically satisfactory companion of the function
for , the function
. The
proposed algorithm allows the computation of the function on a large parameter
domain without requiring the use of extended precision arithmetic.Comment: To be published in Numerical Algoritm
Computing solutions of the modified Bessel differential equation for imaginary orders and positive arguments
Computation of the coefficients appearing in the uniform asymptotic expansions of integrals
The coefficients that appear in uniform asymptotic expansions for integrals
are typically very complicated. In the existing literature the majority of the
work only give the first two coefficients. In a limited number of papers where
more coefficients are given the evaluation of the coefficients near the
coalescence points is normally highly numerically unstable. In this paper, we
illustrate how well-known Cauchy type integral representations can be used to
compute the coefficients in a very stable and efficient manner. We discuss the
cases: (i) two coalescing saddles, (ii) two saddles coalesce with two branch
points, (iii) a saddle point near an endpoint of the interval of integration.
As a special case of (ii) we give a new uniform asymptotic expansion for Jacobi
polynomials in terms of Laguerre polynomials
as that holds uniformly for near .
Several numerical illustrations are included.Comment: 18 page
Crossover between ballistic and diffusive transport: The Quantum Exclusion Process
We study the evolution of a system of free fermions in one dimension under
the simultaneous effects of coherent tunneling and stochastic Markovian noise.
We identify a class of noise terms where a hierarchy of decoupled equations for
the correlation functions emerges. In the special case of incoherent,
nearest-neighbour hopping the equation for the two-point functions is solved
explicitly. The Green's function for the particle density is obtained
analytically and a timescale is identified where a crossover from ballistic to
diffusive behaviour takes place. The result can be interpreted as a competition
between the two types of conduction channels where diffusion dominates on large
timescales.Comment: 20 pages, 5 figure
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