The distribution of Compton scattered annihilation photons, and the Einstein-Podolsky-Rosen argument

The relative polarization of the two photons emitted when a positron annihilates at rest has been re-investigated with high precision and a different method of data analysis. An experiment using a pair of ideal polarization analyzers to measure this relative polarization would be a special case of the general class of thought experiments discussed by Einstein, Podolsky, and Rosen (EPR). EPR argued from these thought experiments that a physical system can exist in a state with definite values for two non-commuting variables. Since quantum mechanics can not describe such a state, EPR called quantum mechanics "incomplete". But EPR believed a complete theory -sometimes called a hidden variable theory- is possible. (This argument of EPR is sometimes called the Einstein-Podolsky-Rosen "paradox".) Our experimental results, together with a theorem due to Bell, provide strong evidence that a local "hidden variable" theory is not possible. The results also rule out a hypothetical modification of quantum mechanics, suggested by Bohm and Aharonov, which was motivated by the EPR thought experiments. Compton scattering was used to analyze the linear polarization. But the theorem of Bell, mentioned above, applies to relatively "ideal" polarization measurements. Therefore, it was necessary to prove the existence, and find the explicit form of the function f relating Compton and ideal linear polarization measurements. The existence of f is shown here to follow from general principles of quantum mechanics, plus parity and angular momentum conservation; the explicit form of f is deduced from the Klein-Nishina equation. Experimental evidence is cited against the argument that f may be different in a local "hidden variable" theory

Minimum detection efficiency for a loophole-free atom-photon Bell experiment

In Bell experiments, one problem is to achieve high enough photodetection to ensure that there is no possibility of describing the results via a local hidden-variable model. Using the Clauser-Horne inequality and a two-photon non-maximally entangled state, a photodetection efficiency higher than 0.67 is necessary. Here we discuss atom-photon Bell experiments. We show that, assuming perfect detection efficiency of the atom, it is possible to perform a loophole-free atom-photon Bell experiment whenever the photodetection efficiency exceeds 0.50.Comment: REVTeX4, 4 pages, 1 figur

Designing for designers: Towards the development of accessible ICT products and services using the VERITAS framework

Among key design practices which contribute to the development of inclusive ICT products and services is user testing with people with disabilities. Traditionally, this involves partial or minimal user testing through the usage of standard heuristics, employing external assisting devices, and the direct feedback of impaired users. However, efficiency could be improved if designers could readily analyse the needs of their target audience. The VERITAS framework simulates and systematically analyses how users with various impairments interact with the use of ICT products and services. Findings show that the VERITAS framework is useful to designers, offering an intuitive approach to inclusive design.The work presented in this article forms part of VERITAS, which is funded by the European Commission's 7th Framework Programme (FP7) (grant agreement # 247765 FP7-ICT-2009.7.2)

Philosophy Enters the Optics Laboratory: Bell's Theorem and its First Experimental Tests (1965-1982)

This paper deals with the ways that the issue of completing quantum mechanics was brought into laboratories and became a topic in mainstream quantum optics. It focuses on the period between 1965, when Bell published what now we call Bell's theorem, and 1982, when Aspect published the results of his experiments. I argue that what was considered good physics after Aspect's experiments was once considered by many a philosophical matter instead of a scientific one, and that the path from philosophy to physics required a change in the physics community's attitude about the status of the foundations of quantum mechanics.Comment: 57 pages, accepted by Studies in History and Philosophy of Modern Physic

Quantum Mechanics and Leggett's Inequalities

We show that when the proper description of the behaviour of individual photons or spin 1/2 particles in a spherically symmetric entangled pair is done through the use of the density matrix, the Leggett's inequality is not violated by quantum mechanics.Comment: 7 pages, no figures. A missing global sign in the r.h.s. of eq. (4.10) in section 4 of version 1 (v1) invalidates the conclusion of that particular section, which is then suppressed in the present version (v2

Does Clauser-Horne-Shimony-Holt Correlation or Freedman-Clauser Correlation lead to the largest violation of Bell's Inequality?

An inequality is deduced from Einstein's locality and a supplementary assumption. This inequality defines an experiment which can actually be performed with present technology to test local realism. Quantum mechanics violate this inequality a factor of 1.5. In contrast, quantum mechanics violates previous inequalities (for example, Clauser-Horne-Shimony-Holt inequality of 1969, Freedman-Clauser inequality of 1972, Clauser-Horne inequality of 1974) by a factor of $\sqrt 2$. Thus the magnitude of violation of the inequality derived in this paper is approximately $20.7$ larger than the magnitude of violation of previous inequalities. This result can be particularly important for the experimental test of locality.Comment: 15 pages, LaTeX file, no figure