2,172 research outputs found
An Operational Approach to Quantum State Reduction
An operational approach to quantum state reduction, the state change of the
measured system caused by a measurement of an observable conditional upon the
outcome of measurement, is founded without assuming the projection postulate in
any stages of the measuring process. Whereas the conventional formula assumes
that the probe measurement satisfies the projection postulate, a new formula
for determining the state reduction shows that the state reduction does not
depend on how the probe observable is measured, or in particular does not
depend on whether the probe measurement satisfies the projection postulate or
not, contrary to the longstanding attempts in showing how the macroscopic
nature of probe detection provokes state reduction.Comment: 17 pages, LaTeX, no figures, to appear in Ann. Phys. (N.Y.
Correcting 100 years of misunderstanding: electric fields in superconductors, hole superconductivity, and the Meissner effect
From the outset of superconductivity research it was assumed that no
electrostatic fields could exist inside superconductors, and this assumption
was incorporated into conventional London electrodynamics. Yet the London
brothers themselves initially (in 1935) had proposed an electrodynamic theory
of superconductors that allowed for static electric fields in their interior,
which they unfortunately discarded a year later. I argue that the Meissner
effect in superconductors necessitates the existence of an electrostatic field
in their interior, originating in the expulsion of negative charge from the
interior to the surface when a metal becomes superconducting. The theory of
hole superconductivity predicts this physics, and associated with it a
macroscopic spin current in the ground state of superconductors ("Spin Meissner
effect"), qualitatively different from what is predicted by conventional
BCS-London theory. A new London-like electrodynamic description of
superconductors is proposed to describe this physics. Within this theory
superconductivity is driven by lowering of quantum kinetic energy, the fact
that the Coulomb repulsion strongly depends on the character of the charge
carriers, namely whether electron- or hole-like, and the spin-orbit
interaction. The electron-phonon interaction does not play a significant role,
yet the existence of an isotope effect in many superconductors is easily
understood. In the strong coupling regime the theory appears to favor local
charge inhomogeneity. The theory is proposed to apply to all superconducting
materials, from the elements to the high cuprates and pnictides, is
highly falsifiable, and explains a wide variety of experimental observations.Comment: Proceedings of the conference "Quantum phenomena in complex matter
2011 - Stripes 2011", Rome, 10 July -16 July 2011, to be published in J.
Supercond. Nov. Mag
Quantum Mechanics and Perspectivalism
Experimental evidence of the last decades has made the status of ``collapses of the wave function'' even more shaky than it already was on conceptual grounds: interference effects turn out to be detectable even when collapses are typically expected to occur. Non-collapse interpretations should consequently be taken seriously. In this paper we argue that such interpretations suggest a perspectivalism according to which quantum objects are not characterized by monadic properties, but by relations to other systems. Accordingly, physical systems may possess different properties with respect to different ``reference systems''. We discuss some of the relevant arguments, and argue that perspectivalism both evades recent arguments that single-world interpretations are inconsistent and eliminates the need for a privileged rest frame in the relativistic case
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In crystallo-screening for discovery of human norovirus 3C-like protease inhibitors.
Outbreaks of human epidemic nonbacterial gastroenteritis are mainly caused by noroviruses. Viral replication requires a 3C-like cysteine protease (3CLpro) which processes the 200 kDa viral polyprotein into six functional proteins. The 3CLpro has attracted much interest due to its potential as a target for antiviral drugs. A system for growing high-quality crystals of native Southampton norovirus 3CLpro (SV3CP) has been established, allowing the ligand-free crystal structure to be determined to 1.3 Å in a tetrameric state. This also allowed crystal-based fragment screening to be performed with various compound libraries, ultimately to guide drug discovery for SV3CP. A total of 19 fragments were found to bind to the protease out of the 844 which were screened. Two of the hits were located at the active site of SV3CP and showed good inhibitory activity in kinetic assays. Another 5 were found at the enzyme's putative RNA-binding site and a further 11 were located in the symmetric central cavity of the tetramer
(Never) Mind your p's and q's: Von Neumann versus Jordan on the Foundations of Quantum Theory
In two papers entitled "On a new foundation [Neue Begr\"undung] of quantum
mechanics," Pascual Jordan (1927b,g) presented his version of what came to be
known as the Dirac-Jordan statistical transformation theory. As an alternative
that avoids the mathematical difficulties facing the approach of Jordan and
Paul A. M. Dirac (1927), John von Neumann (1927a) developed the modern Hilbert
space formalism of quantum mechanics. In this paper, we focus on Jordan and von
Neumann. Central to the formalisms of both are expressions for conditional
probabilities of finding some value for one quantity given the value of
another. Beyond that Jordan and von Neumann had very different views about the
appropriate formulation of problems in quantum mechanics. For Jordan, unable to
let go of the analogy to classical mechanics, the solution of such problems
required the identication of sets of canonically conjugate variables, i.e., p's
and q's. For von Neumann, not constrained by the analogy to classical
mechanics, it required only the identication of a maximal set of commuting
operators with simultaneous eigenstates. He had no need for p's and q's. Jordan
and von Neumann also stated the characteristic new rules for probabilities in
quantum mechanics somewhat differently. Jordan (1927b) was the first to state
those rules in full generality. Von Neumann (1927a) rephrased them and, in a
subsequent paper (von Neumann, 1927b), sought to derive them from more basic
considerations. In this paper we reconstruct the central arguments of these
1927 papers by Jordan and von Neumann and of a paper on Jordan's approach by
Hilbert, von Neumann, and Nordheim (1928). We highlight those elements in these
papers that bring out the gradual loosening of the ties between the new quantum
formalism and classical mechanics.Comment: New version. The main difference with the old version is that the
introduction has been rewritten. Sec. 1 (pp. 2-12) in the old version has
been replaced by Secs. 1.1-1.4 (pp. 2-31) in the new version. The paper has
been accepted for publication in European Physical Journal
Quantum Trajectories and Quantum Measurement Theory
Beyond their use as numerical tools, quantum trajectories can be ascribed a
degree of reality in terms of quantum measurement theory. In fact, they arise
naturally from considering continuous observation of a damped quantum system. A
particularly useful form of quantum trajectories is as linear (but non-unitary)
stochastic Schrodinger equations. In the limit where a strong local oscillator
is used in the detection, and where the system is not driven, these quantum
trajectories can be solved. This gives an alternate derivation of the
probability distributions for completed homodyne and heterodyne detection
schemes. It also allows the previously intractable problem of real-time
adaptive measurements to be treated. The results for an analytically soluble
example of adaptive phase measurements are presented, and future developments
discussed.Comment: 17 pages. A review article publihsed in 1996 which has been picking
up some citations, so I thought I would post it her
Omega-3 Index and Sudden Cardiac Death
Sudden cardiac death (SCD) is an unresolved health issue, and responsible for 15% of all deaths in Western countries. Epidemiologic evidence, as well as evidence from clinical trials, indicates that increasing intake and high levels of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) protect from SCD and other major adverse cardiac events. Levels of EPA+DHA are best assessed by the Omega-3 Index, representing the red cell fatty acid content of EPA+DHA. Work is in progress that will further define the value of the Omega-3 Index as a risk factor for SCD, other cardiac events, and as target for treatment with EPA+DHA
A consistent quantum model for continuous photodetection processes
We are modifying some aspects of the continuous photodetection theory,
proposed by Srinivas and Davies [Optica Acta 28, 981 (1981)], which describes
the non-unitary evolution of a quantum field state subjected to a continuous
photocount measurement. In order to remedy inconsistencies that appear in their
approach, we redefine the `annihilation' and `creation' operators that enter in
the photocount superoperators. We show that this new approach not only still
satisfies all the requirements for a consistent photocount theory according to
Srinivas and Davies precepts, but also avoids some weird result appearing when
previous definitions are used.Comment: 12 pages, 4 figure
The Pondicherry interpretation of quantum mechanics: An overview
An overview of the Pondicherry interpretation of quantum mechanics is
presented. This interpretation proceeds from the recognition that the
fundamental theoretical framework of physics is a probability algorithm, which
serves to describe an objective fuzziness (the literal meaning of Heisenberg's
term "Unschaerfe," usually mistranslated as "uncertainty") by assigning
objective probabilities to the possible outcomes of unperformed measurements.
Although it rejects attempts to construe quantum states as evolving ontological
states, it arrives at an objective description of the quantum world that owes
nothing to observers or the goings-on in physics laboratories. In fact, unless
such attempts are rejected, quantum theory's true ontological implications
cannot be seen. Among these are the radically relational nature of space, the
numerical identity of the corresponding relata, the incomplete spatiotemporal
differentiation of the physical world, and the consequent top-down structure of
reality, which defies attempts to model it from the bottom up, whether on the
basis of an intrinsically differentiated spacetime manifold or out of a
multitude of individual building blocks.Comment: 18 pages, 1 eps figure, v3: with corrections made in proo
Design considerations in a clinical trial of a cognitive behavioural intervention for the management of low back pain in primary care : Back Skills Training Trial
Background
Low back pain (LBP) is a major public health problem. Risk factors for the development and persistence of LBP include physical and psychological factors. However, most research activity has focused on physical solutions including manipulation, exercise training and activity promotion.
Methods/Design
This randomised controlled trial will establish the clinical and cost-effectiveness of a group programme, based on cognitive behavioural principles, for the management of sub-acute and chronic LBP in primary care. Our primary outcomes are disease specific measures of pain and function. Secondary outcomes include back beliefs, generic health related quality of life and resource use. All outcomes are measured over 12 months. Participants randomised to the intervention arm are invited to attend up to six weekly sessions each of 90 minutes; each group has 6–8 participants. A parallel qualitative study will aid the evaluation of the intervention.
Discussion
In this paper we describe the rationale and design of a randomised evaluation of a group based cognitive behavioural intervention for low back pain
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