Causal Quantum Theory and the Collapse Locality Loophole

Causal quantum theory is an umbrella term for ordinary quantum theory modified by two hypotheses: state vector reduction is a well-defined process, and strict local causality applies. The first of these holds in some versions of Copenhagen quantum theory and need not necessarily imply practically testable deviations from ordinary quantum theory. The second implies that measurement events which are spacelike separated have no non-local correlations. To test this prediction, which sharply differs from standard quantum theory, requires a precise theory of state vector reduction. Formally speaking, any precise version of causal quantum theory defines a local hidden variable theory. However, causal quantum theory is most naturally seen as a variant of standard quantum theory. For that reason it seems a more serious rival to standard quantum theory than local hidden variable models relying on the locality or detector efficiency loopholes. Some plausible versions of causal quantum theory are not refuted by any Bell experiments to date, nor is it obvious that they are inconsistent with other experiments. They evade refutation via a neglected loophole in Bell experiments -- the {\it collapse locality loophole} -- which exists because of the possible time lag between a particle entering a measuring device and a collapse taking place. Fairly definitive tests of causal versus standard quantum theory could be made by observing entangled particles separated by $\approx 0.1$ light seconds.Comment: Discussion expanded; typos corrected; references adde

Project 7708 understanding heritage crime in Kent and Medway – a data analytical approach

Our research was concerned with the geographical areas of Kent and Medway and involved the spatial and temporal analysis of ‘heritage-specific offences’, ‘targeted heritage crime’ and ‘crime within, at or close to heritage sites’. The crime data consisted of offence type and location details for the 1,122,180 crimes recorded by Kent Police during the period under study. The geographical data we utilised included locations of Conservation Areas, Listed Buildings, Scheduled Monuments, Registered Parks and Gardens, Registered Battlefields, World Heritage Sites, Protected Wreck Sites and ‘Heritage at Risk’ sites in Kent and Medway. Our best estimates suggest that currently approximately one in five Listed Buildings and one in four Places of Worship in Kent and Medway experience some form of crime each year. About one in ten Scheduled Monuments suffer crime, or it occurs nearby. Just over one half of Registered Parks or Gardens have one or more crimes a year within them. For Conservation Areas the proportion is (not unexpectedly) much larger, at closer to four in five. We utilised local Moran’s I to identify spatial clusters at a regional level using LSOA-level crime data. This revealed several LSOAs on the fringes of areas of high levels of crime that could be particularly vulnerable to the spread of crime and therefore heritage-specific locations within these areas could be managed to halt the ‘spread’ of crime towards the periphery of the town. In Kent and Medway Places of Worship (mostly Christian churches) are experiencing increasing numbers of crimes, and this has been particularly the case since around summer 2016. The rate of increase appears higher than that of all other crimes in the same period, both in general and at other heritage locations. There is clear statistical evidence that metal thefts from churches have also been increasing markedly since around summer 2016. The rate of increase appears higher than that of most other crimes. There is statistically significant correlation between metal thefts from churches in Kent and Medway with both the price of lead and mixed brass. Finally, we discovered that machine learning as a method of heritage crime prevention shows promise

Mantle heterogeneity during the formation of the North Atlantic Igneous Province: Constraints from trace element and Sr-Nd-Os-O isotope systematics of Baffin Island picrites

Sr-Nd-Os-O isotope and major and trace element data from ~62 Ma picrites from Baffin Island constrain the composition of mantle sources sampled at the inception of North Atlantic Igneous Province (NAIP) magmatism. We recognize two compositional types. Depleted (N-type) lavas have low 87Sr/86Sri (0.702990–0.703060) and 187Os/188Osi (0.1220–0.1247) and high 143Nd/144Ndi (0.512989–0.512999) and are depleted in incompatible elements relative to primitive mantle. Enriched (E-type) lavas have higher 87Sr/86Sri (0.703306–0.703851) and 187Os/188Osi (0.1261–0.1303), lower 143Nd/144Ndi (0.512825–0.512906), and incompatible element concentrations similar to, or more enriched than, primitive mantle. There is also a subtle difference in oxygen isotope composition; E-type lavas are marginally lower in δ18Oolivine value (5.16–4.84‰) than N-type lavas (5.15–5.22‰). Chemical and isotopic variations between E- and N-type lavas are inconsistent with assimilation of crust and/or subcontinental lithospheric mantle and appear to instead reflect mixing between melts derived from two distinct mantle sources. Strontium-Nd-O isotope compositions and incompatible trace element abundances of N-type lavas suggest these are largely derived from the depleted upper mantle. The 187Os/188Osi ratios of N-type lavas can also be explained by such a model but require that the depleted upper mantle had γOs of approximately −5 to −7 at 62 Ma. This range overlaps the lowest γOs values measured in abyssal peridotites. Baffin Island lava compositions are also permissive of a model involving recharging of depleted upper mantle with 3He-rich material from the lower mantle (Stuart et al., Nature, 424, 57–59, 2003), with the proviso that recharge had no recognizable effect on the lithophile trace element and Sr-Nd-Os-O isotope composition. The origin of the enriched mantle component sampled by Baffin Island lavas is less clear but may be metasomatized and high-temperature-altered recycled oceanic lithosphere transported within the proto Iceland plume. Differences between Baffin Island lavas and modern Icelandic basalts suggest that a range of enriched and depleted mantle sources have been tapped since the inception of magmatism in the province. Similarities between Baffin Island lavas erupted and those of similar age from East and West Greenland also suggest that the enriched component in Baffin Island lavas may have been sampled by lavas erupted over a wide geographic range

Current Induced Excitations in Cu/Co/Cu Single Ferromagnetic Layer Nanopillars

Current-induced magnetic excitations in Cu/Co/Cu single layer nanopillars (~50 nm in diameter) have been studied experimentally as a function of Co layer thickness at low temperatures for large applied fields perpendicular to the layers. For asymmetric junctions current induced excitations are observed at high current densities for only one polarity of the current and are absent at the same current densities in symmetric junctions. These observations confirm recent predictions of spin-transfer torque induced spin wave excitations in single layer junctions with a strong asymmetry in the spin accumulation in the leads.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let

Monte Carlo energy and variance minimization techniques for optimizing many-body wave functions

We investigate Monte Carlo energy and variance minimization techniques for optimizing many-body wave functions. Several variants of the basic techniques are studied, including limiting the variations in the weighting factors which arise in correlated sampling estimations of the energy and its variance. We investigate the numerical stability of the techniques and identify two reasons why variance minimization exhibits superior numerical stability to energy minimization. The characteristics of each method are studied using a non-interacting 64-electron model of crystalline silicon. While our main interest is in solid state systems, the issues investigated are relevant to Monte Carlo studies of atoms, molecules and solids. We identify a robust and efficient variance minimization scheme for optimizing wave functions for large systems.Comment: 14 pages, including 7 figures. To appear in Phys. Rev. B. For related publications see http://www.tcm.phy.cam.ac.uk/Publications/many_body.htm

Beable-Guided Quantum Theories: Generalising Quantum Probability Laws

We introduce the idea of a {\it beable-guided quantum theory}. Beable-guided quantum theories (BGQT) are generalisations of quantum theory, inspired by Bell's concept of beables. They modify the quantum probabilities for some specified set of fundamental events, histories, or other elements of quasiclassical reality by probability laws that depend on the realised configuration of beables. For example, they may define an additional probability weight factor for a beable configuration, independent of the quantum dynamics. BGQT can be fitted to observational data to provide foils against which to compare explanations based on standard quantum theory. For example, a BGQT could, in principle, characterise the effects attributed to dark energy or dark matter, or any other deviation from the predictions of standard quantum dynamics, without introducing extra fields or a cosmological constant. The complexity of the beable-guided theory would then parametrise how far we are from a standard quantum explanation. Less conservatively, we give reasons for taking suitably simple beable-guided quantum theories as serious phenomenological theories in their own right. Among these are that cosmological models defined by BGQT might in fact fit the empirical data better than any standard quantum explanation, and that BGQT suggest potentially interesting non-standard ways of coupling quantum matter to gravity.Comment: Minor corrections and edits; closer but not identical to published versio

Zinc-blende and wurtzite AlxGa1-xN bulk crystals grown by molecular beam epitaxy

There is a significant difference in the lattice parameters of GaN and AlN and for many device applications AlxGa1-xN substrates would be preferable to either GaN or AlN. We have studied the growth of free-standing zinc-blende and wurtzite AlxGa1-xN bulk crystals by plasma-assisted molecular beam epitaxy (PA-MBE). Thick (similar to 10 mu m) zinc-blende and wurtzite AlxGa1-xN films were grown by PA-MBE on 2-in. GaAs (0 0 1) and GaAs (1 1 1)B substrates respectively and were removed from the GaAs substrate after the growth. We demonstrate that free-standing zinc-blende and wurtzite AlxGa1-xN wafers can be achieved by PA-MBE for a wide range of Al compositions. (C) 2011 Elsevier B.V. All rights reserved

X-Ray Detection of Transient Magnetic Moments Induced by a Spin Current in Cu

We have used a MHz lock-in x-ray spectro-microscopy technique to directly detect changes of magnetic moments in Cu due to spin injection from an adjacent Co layer. The elemental and chemical specificity of x-rays allows us to distinguish two spin current induced effects. We detect the creation of transient magnetic moments of $3\times 10^{-5}$ $\mu_\mathrm{B}$ on Cu atoms within the bulk of the 28 nm thick Cu film due to spin-accumulation. The moment value is compared to predictions by Mott's two current model. We also observe that the hybridization induced existing magnetic moments on Cu interface atoms are transiently increased by about 10% or $4\times 10^{-3}$ $\mu_\mathrm{B}$. This reveals the dominance of spin-torque alignment over Joule heat induced disorder of the interfacial Cu moments during current flow

Current-Induced Effective Magnetic Fields in Co/Cu/Co Nanopillars

We present a method to measure the effective field contribution to spin-transfer-induced interactions between the magnetic layers in a trilayer nanostructure, which enables spin-current effects to be distinguished from the usual charge-current-induced magnetic fields. This technique is demonstrated on submicron Co/Cu/Co nanopillars. The hysteresis loop of one of the magnetic layers in the trilayer is measured as a function of current while the direction of magnetization of the other layer is kept fixed, first in one direction and then in the opposite direction. These measurements show a current-dependent shift of the hysteresis loop which, based on the symmetry of the magnetic response, we associate with spin-transfer. The observed loop-shift with applied current at room temperature is reduced in measurements at 4.2 K. We interprete these results both in terms of a spin-current dependent effective activation barrier for magnetization reversal and a spin-current dependent effective magnetic field. From data at 4.2 K we estimate the magnitude of the spin-transfer induced effective field to be $\sim 1.5 \times 10^{-7}$ Oe cm$^2$/A, about a factor of 5 less than the spin-transfer torque.Comment: 6 pages, 4 figure

Speakable in Quantum Mechanics

At the 1927 Como conference Bohr spoke the now famous words "It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature." However, if the Copenhagen interpretation really holds on to this motto, why then is there this feeling of conflict when comparing it with realist interpretations? Surely what one can say about nature should in a certain sense be interpretation independent. In this paper I take Bohr's motto seriously and develop a quantum logic that avoids assuming any form of realism as much as possible. To illustrate the non-triviality of this motto a similar result is first derived for classical mechanics. It turns out that the logic for classical mechanics is a special case of the derived quantum logic. Finally, some hints are provided in how these logics are to be used in practical situations and I discuss how some realist interpretations relate to these logics