1,929 research outputs found
Bifurcation in Quantum Measurement
We present a generic model of (non-destructive) quantum measurement. Being
formulated within reversible quantum mechanics, the model illustrates a
mechanism of a measurement process --- a transition of the measured system to
an eigenstate of the measured observable. The model consists of a two-level
system interacting with a larger system , consisting of smaller
subsystems. The interaction is modelled as a scattering process. Restricting
the states of to product states leads to a bifurcation process: In the
limit of a large system , the initial states of that are efficient in
leading to a final state are divided into two separated subsets. For each of
these subsets, ends up in one of the eigenstates of the measured
observable. The probabilities obtained in this branching confirm the Born rule.Comment: A revised version that includes a more general presentation of the
model (in Sect. 4) and a larger revision of the Introductio
The hiphive package for the extraction of high-order force constants by machine learning
The efficient extraction of force constants (FCs) is crucial for the analysis
of many thermodynamic materials properties. Approaches based on the systematic
enumeration of finite differences scale poorly with system size and can rarely
extend beyond third order when input data is obtained from first-principles
calculations. Methods based on parameter fitting in the spirit of interatomic
potentials, on the other hand, can extract FC parameters from semi-random
configurations of high information density and advanced regularized regression
methods can recover physical solutions from a limited amount of data. Here, we
present the hiPhive Python package, that enables the construction of force
constant models up to arbitrary order. hiPhive exploits crystal symmetries to
reduce the number of free parameters and then employs advanced machine learning
algorithms to extract the force constants. Depending on the problem at hand
both over and underdetermined systems are handled efficiently. The FCs can be
subsequently analyzed directly and or be used to carry out e.g., molecular
dynamics simulations. The utility of this approach is demonstrated via several
examples including ideal and defective monolayers of MoS as well as bulk
nickel
Efficient construction of linear models in materials modeling and applications to force constant expansions
Linear models, such as force constant (FC) and cluster expansions, play a key
role in physics and materials science. While they can in principle be
parametrized using regression and feature selection approaches, the convergence
behavior of these techniques, in particular with respect to thermodynamic
properties is not well understood. Here, we therefore analyze the efficacy and
efficiency of several state-of-the-art regression and feature selection
methods, in particular in the context of FC extraction and the prediction of
different thermodynamic properties. Generic feature selection algorithms such
as recursive feature elimination with ordinary least-squares (OLS), automatic
relevance determination regression, and the adaptive least absolute shrinkage
and selection operator can yield physically sound models for systems with a
modest number of degrees of freedom. For large unit cells with low symmetry
and/or high-order expansions they come, however, with a non-negligible
computational cost that can be more than two orders of magnitude higher than
that of OLS. In such cases, OLS with cutoff selection provides a viable route
as demonstrated here for both second-order FCs in large low-symmetry unit cells
and high-order FCs in low-symmetry systems. While regression techniques are
thus very powerful, they require well-tuned protocols. Here, the present work
establishes guidelines for the design of protocols that are readily usable,
e.g., in high-throughput and materials discovery schemes. Since the underlying
algorithms are not specific to FC construction, the general conclusions drawn
here also have a bearing on the construction of other linear models in physics
and materials science.Comment: 15 pages, 12 figure
Modelling procurement effects on cooperation
Cooperative arrangements, such as partnering, have received increased interest in recent years. Several studies show however that cooperative relationships are not easily achieved in construction. Implementation of cooperative relationships requires changes in several elements of the traditional procurement procedures. The purpose of this paper is therefore to propose and test a sequential model regarding clients’ cooperative procurement procedures. We especially ask: what elements in clients’ procurement procedures facilitate the establishment of cooperation and trust in their relationships with contractors? The model was tested through structural equation modelling. The empirical data required for the test were collected through a survey responded to by 87 Swedish professional construction clients. The empirical results show that cooperative procurement procedures are triggered by clients’ wish to involve contractors early in specification, which has a simultaneous effect on procedures regarding bid invitation and compensation. Furthermore, these simultaneous effects breed a certain kind of partner selection based on task-related attributes, which also has a direct positive effect on trust and above all on cooperation in client–contractor relationships. Besides these implications from the model, the improvement of measurements for future modelling is discussed.Construction; cooperation; procurement;
Waveforms for the Massive MIMO Downlink: Amplifier Efficiency, Distortion and Performance
In massive MIMO, most precoders result in downlink signals that suffer from
high PAR, independently of modulation order and whether single-carrier or OFDM
transmission is used. The high PAR lowers the power efficiency of the base
station amplifiers. To increase power efficiency, low-PAR precoders have been
proposed. In this article, we compare different transmission schemes for
massive MIMO in terms of the power consumed by the amplifiers. It is found that
(i) OFDM and single-carrier transmission have the same performance over a
hardened massive MIMO channel and (ii) when the higher amplifier power
efficiency of low-PAR precoding is taken into account, conventional and low-PAR
precoders lead to approximately the same power consumption. Since downlink
signals with low PAR allow for simpler and cheaper hardware, than signals with
high PAR, therefore, the results suggest that low-PAR precoding with either
single-carrier or OFDM transmission should be used in a massive MIMO base
station
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