Artículos con la etiqueta ‘física cuántica’

Entropy is in Flux

Por • 30 mar, 2014 • Category: Crítica

The science of thermodynamics was put together in the Nineteenth Century to describe large systems in equilibrium. One part of thermodynamics defines entropy for equilibrium systems and demands an ever-increasing entropy for non-equilibrium ones. However, starting with the work of Ludwig Boltzmann in 1872, and continuing to the present day, various models of non-equilibrium behavior have been put together with the specific aim of generalizing the concept of entropy to non-equilibrium situations. This kind of entropy has been termed {\em kinetic entropy} to distinguish it from the thermodynamic variety. Knowledge of kinetic entropy started from Boltzmann’s insight about his equation for the time dependence of gaseous systems. In this paper, his result is stated as a definition of kinetic entropy in terms of a local equation for the entropy density. This definition is then applied to Landau’s theory of the Fermi liquid thereby giving the kinetic entropy within that theory.

Information Content of Elementary Systems as a Physical Principle

Por • 30 mar, 2014 • Category: Ambiente

Quantum physics has remarkable characteristics such as quantum correlations, uncertainty relations, no cloning, which make for an interpretative and conceptual gap between the classical and the quantum world. To provide more unified framework the generalized probabilistic theories were formulated. Recently, it turned out that such theories include so called «postquantum» ones which share many of the typical quantum characteristics but predict supraquantum effects such as correlations stronger than quantum ones. As a result we reveal even more dramatic gap between classical/quantum and post-quantum world. Therefore it is imperative to search for information principles characterizing physical theories. In recent years, different principles has been proposed, however all of the principles considered so far has been correlation ones. Here, we introduce an elementary system information content principle (ICP) whose basic ingredient is the phenomenon of Heisenberg uncertainty.

An intuition behind quantum measurement

Por • 29 mar, 2014 • Category: Opinion

An attempt is made to give a heuristic explanation of the distinguished role of measurement in the quantum theory. We question the notion of «naive» reductionism by stressing the difference between an isolated quantum and classical object. It is argued that the transition from the micro- to the macroscopic description should be made along some parameters not characterized by the quantum theory.

Explanation and Randomness

Por • 29 mar, 2014 • Category: Educacion

The aim of this paper is to elaborate a notion of explanation which is applicable to stochastic processes such as quantum processes. The model-theoretic approach was adopted in order to delimit appropriate classes, by defining set-theoretical predicates, of different kinds of physical transformations that quantum systems suffer, either of transitions or of transmutations, by interaction or in a spontaneous manner. To explain a singular quantum process consists in showing that it is feasible to model it as an indeterministic process of certain specified kind.

Quantum Humeanism,or: physicalism without properties

Por • 27 mar, 2014 • Category: Ciencia y tecnología

In recent literature, it has become clear that quantum physics does not refute Humeanism. This point has so far been made with respect to Bohms quantum theory. Against this background, this paper seeks to achieve the following four results: to generalize the option of quantum Humeanism from Bohmian mechanics to primitive ontology theories in general, to show that this option applies also to classical mechanics, to establish that it requires a commitment to matter as primitive stuff, but no commitment to natural properties (physicalism without properties, to point out that by removing the commitment to properties, the stock metaphysical objections against Humeanism from quidditism and humility no longer apply. In that way, quantum physics strengthens Humeanism instead of refuting it.

Quantum Theory and Human Perception of the Macro-World

Por • 21 mar, 2014 • Category: Filosofía

We investigate the question of ‘why customary macroscopic entities appear to us humans as they do, i.e. as bounded entities occupying space and persisting through time’, starting from our knowledge of quantum theory, how it affects the behavior of such customary macroscopic entities, and how it influences our perception of them. For this purpose, we approach the question from three perspectives. The specific and very classical perception of human seeing — light as a geometric theory — and human touching — only ruled by Pauli’s exclusion principle — plays a role in our perception of macroscopic entities as ontologically stable entities in space. To ascertain quantum behavior in such macroscopic entities, we will need measuring apparatuses capable of its detection. Future experimental research will have to show if sharp quantum effects — as they occur in smaller entities — appear to be ontological aspects of customary macroscopic entities.

Solving the Hard Problem of Bertrand’s Paradox

Por • 19 mar, 2014 • Category: Leyes

Bertrand’s paradox is a famous problem of probability theory, pointing to a possible inconsistency in Laplace’s principle of insufficient reason. In this article we show that Bertrand’s paradox contains two different problems: an «easy» problem and a «hard» problem. The easy problem can be solved by formulating Bertrand’s question in sufficiently precise terms, so allowing for a non ambiguous modelization of the entity subjected to the randomization. We then show that once the easy problem is settled, also the hard problem becomes solvable, provided Laplace’s principle of insufficient reason is applied not to the outcomes of the experiment, but to the different possible «ways of selecting» an interaction between the entity under investigation and that producing the randomization.

Logic gates at the surface code threshold: Superconducting qubits poised for fault-tolerant quantum computing

Por • 6 mar, 2014 • Category: Leyes

A quantum computer can solve hard problems – such as prime factoring, database searching, and quantum simulation – at the cost of needing to protect fragile quantum states from error. Quantum error correction provides this protection, by distributing a logical state among many physical qubits via quantum entanglement. Superconductivity is an appealing platform, as it allows for constructing large quantum circuits, and is compatible with microfabrication. For superconducting qubits the surface code is a natural choice for error correction, as it uses only nearest-neighbour coupling and rapidly-cycled entangling gates. The gate fidelity requirements are modest: The per-step fidelity threshold is only about 99%. Here, we demonstrate a universal set of logic gates in a superconducting multi-qubit processor, achieving an average single-qubit gate fidelity of 99.92% and a two-qubit gate fidelity up to 99.4%.

Schwinger’s Quantum Action Principle. I. From Dirac’s Formulation through Feynman’s Path Integrals to the Schwinger-Keldysh Method

Por • 23 feb, 2014 • Category: Leyes

Starting from the earlier notions of stationary action principles, we show how Schwinger’s Quantum Action Principle descended from Dirac’s formulation, which independently led Feynman to his path-integral formulation of quantum mechanics. The connection between the two is brought out, and applications are discussed. The Keldysh-Schwinger time-cycle method of extracting matrix elements is described. Part II will discuss the variational formulation of quantum electrodynamics and the development of source theory.

Particles, waves and trajectories: 210 years after Young’s experiment

Por • 20 feb, 2014 • Category: Leyes

Mermin’s «shut up and calculate!» somehow summarizes the most widely accepted view on quantum mechanics. This conception has led to a rather constraining way to think and understand the quantum world. Nonetheless, a closer look at the principles and formal body of this theory shows that, beyond longstanding prejudices, there is still room enough for alternative tools. This is the case, for example, of Bohmian mechanics. As it is discussed here, there is nothing contradictory or wrong with this hydrodynamical representation, which enhances the dynamical role of the quantum phase to the detriment (to some extent) of the probability density. The possibility to describe the evolution of quantum systems in terms of trajectories or streamlines is just a direct consequence of the fact that Bohmian mechanics (quantum hydrodynamics) is just a way to recast quantum mechanics in the more general language of the theory of characteristics.