3,297 research outputs found
Reconstructing the world trade multiplex: the role of intensive and extensive biases
In economic and financial networks, the strength of each node has always an
important economic meaning, such as the size of supply and demand, import and
export, or financial exposure. Constructing null models of networks matching
the observed strengths of all nodes is crucial in order to either detect
interesting deviations of an empirical network from economically meaningful
benchmarks or reconstruct the most likely structure of an economic network when
the latter is unknown. However, several studies have proved that real economic
networks and multiplexes are topologically very different from configurations
inferred only from node strengths. Here we provide a detailed analysis of the
World Trade Multiplex by comparing it to an enhanced null model that
simultaneously reproduces the strength and the degree of each node. We study
several temporal snapshots and almost one hundred layers (commodity classes) of
the multiplex and find that the observed properties are systematically well
reproduced by our model. Our formalism allows us to introduce the (static)
concept of extensive and intensive bias, defined as a measurable tendency of
the network to prefer either the formation of extra links or the reinforcement
of link weights, with respect to a reference case where only strengths are
enforced. Our findings complement the existing economic literature on (dynamic)
intensive and extensive trade margins. More in general, they show that
real-world multiplexes can be strongly shaped by layer-specific local
constraints
Universal fault-tolerant gates on concatenated stabilizer codes
It is an oft-cited fact that no quantum code can support a set of
fault-tolerant logical gates that is both universal and transversal. This no-go
theorem is generally responsible for the interest in alternative universality
constructions including magic state distillation. Widely overlooked, however,
is the possibility of non-transversal, yet still fault-tolerant, gates that
work directly on small quantum codes. Here we demonstrate precisely the
existence of such gates. In particular, we show how the limits of
non-transversality can be overcome by performing rounds of intermediate
error-correction to create logical gates on stabilizer codes that use no
ancillas other than those required for syndrome measurement. Moreover, the
logical gates we construct, the most prominent examples being Toffoli and
controlled-controlled-Z, often complete universal gate sets on their codes. We
detail such universal constructions for the smallest quantum codes, the 5-qubit
and 7-qubit codes, and then proceed to generalize the approach. One remarkable
result of this generalization is that any nondegenerate stabilizer code with a
complete set of fault-tolerant single-qubit Clifford gates has a universal set
of fault-tolerant gates. Another is the interaction of logical qubits across
different stabilizer codes, which, for instance, implies a broadly applicable
method of code switching.Comment: 18 pages + 5 pages appendix, 12 figure
Models of free quantum field theories on curved backgrounds
Free quantum field theories on curved backgrounds are discussed via three
explicit examples: the real scalar field, the Dirac field and the Proca field.
The first step consists of outlining the main properties of globally hyperbolic
spacetimes, that is the class of manifolds on which the classical dynamics of
all physically relevant free fields can be written in terms of a Cauchy
problem. The set of all smooth solutions of the latter encompasses the
dynamically allowed configurations which are used to identify via a suitable
pairing a collection of classical observables. As a last step we use such
collection to construct a -algebra which encodes the information on the
dynamics and on the canonical commutation or anti-commutation relations
depending whether the underlying field is a Fermion or a Boson.Comment: 41 page
An overview on different strategies for the stemness maintenance of MSCs
Recent evidence suggests that mesenchymal stem cells (MSCs) have promising therapeutic potential for a broad range of diseases. Because the percentage of MSCs obtained from tissues is very low for cell therapy applications, ex vivo expansion of MSCs is necessary, but aging, loss of stemness and undesired differentiation of them during in vitro cultivation reduces their effectiveness. For achieving ideal therapeutic potential of MSCs in tissue regenerative purposes, it is necessary to retain their stemness properties in vitro. This review emphasis on the last updates in preserving the self-renewal capability of stem cells through in vitro expansion with different parameters
Irreducible network backbones: unbiased graph filtering via maximum entropy
Networks provide an informative, yet non-redundant description of complex systems only if links represent truly dyadic relationships that cannot be directly traced back to node-specific properties such as size, importance, or coordinates in some embedding space. In any real-world network, some links may be reducible, and others irreducible, to such local properties. This dichotomy persists despite the steady increase in data availability and resolution, which actually determines an even stronger need for filtering techniques aimed at discerning essential links from non-essential ones. Here we introduce a rigorous method that, for any desired level of statistical significance, outputs the network backbone that is irreducible to the local properties of nodes, i.e. their degrees and strengths. Unlike previous approaches, our method employs an exact maximum-entropy formulation guaranteeing that the filtered network encodes only the links that cannot be inferred from local information. Extensive empirical analysis confirms that this approach uncovers essential backbones that are otherwise hidden amidst many redundant relationships and inaccessible to other methods. For instance, we retrieve the hub-and-spoke skeleton of the US airport network and many specialised patterns of international trade. Being irreducible to local transportation and economic constraints of supply and demand, these backbones single out genuinely higher-order wiring principles
Modeling Complex Material Systems Using Stochastic Reconstruction and Lattice Particle Simulation
abstract: In this dissertation, three complex material systems including a novel class of hyperuniform composite materials, cellularized collagen gel and low melting point alloy (LMPA) composite are investigated, using statistical pattern characterization, stochastic microstructure reconstruction and micromechanical analysis. In Chapter 1, an introduction of this report is provided, in which a brief review is made about these three material systems. In Chapter 2, detailed discussion of the statistical morphological descriptors and a stochastic optimization approach for microstructure reconstruction is presented. In Chapter 3, the lattice particle method for micromechanical analysis of complex heterogeneous materials is introduced. In Chapter 4, a new class of hyperuniform heterogeneous material with superior mechanical properties is investigated. In Chapter 5, a bio-material system, i.e., cellularized collagen gel is modeled using correlation functions and stochastic reconstruction to study the collective dynamic behavior of the embed tumor cells. In chapter 6, LMPA soft robotic system is generated by generalizing the correlation functions and the rigidity tunability of this smart composite is discussed. In Chapter 7, a future work plan is presented.Dissertation/ThesisDoctoral Dissertation Materials Science and Engineering 201
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