13,040 research outputs found
Deriving Bell's nonlocality from nonlocality at detection
It is argued that Bell's nonlocality is a particular case of nonlocality at
detection, which appears already in single-particle interference experiments.
The unity of nonlocality and local causality is crucial to provide a consistent
description of the world.Comment: 9 pages, 2 figure
Delayed Choice, Complementarity, Entanglement and Measurement
It is well known that Wheeler proposed several delayed choice experiments in
order to show the impossibility to speak of the way a quantum system behaves
before being detected. In a double-slit experiment, when do photons decide to
travel by one way or by two ways? Delayed choice experiments seem to indicate
that, strangely, it is possible to change the decision of the photons until the
very last moment before they are detected. This led Wheeler to his famous
sentence: No elementary quantum phenomenon is a phenomenon until it is a
registered phenomenon, brought to a close by an irreversible act of
amplification. Nevertheless some authors wrote that backward in time effects
were needed to explain these results. I will show that in delayed choice
experiments involving only one particle, a simple explanation is possible
without invoking any backward in time effect. Delayed choice experiments
involving entangled particles such as the so called quantum eraser can also be
explained without invoking any backward in time effect but I will argue that
these experiments cannot be accounted for so simply because they rise the whole
problem of knowing what a measurement and a collapse are. A previously
presented interpretation, Convivial Solipsism, is a natural framework for
giving a simple explanation of these delayed choice experiments with entangled
particles. In this paper, I show how Convivial Solipsism helps clarifying the
puzzling questions raised by the collapse of the wave function of entangled
systems.Comment: 3 figure
Bell Correlations and the Common Future
Reichenbach's principle states that in a causal structure, correlations of
classical information can stem from a common cause in the common past or a
direct influence from one of the events in correlation to the other. The
difficulty of explaining Bell correlations through a mechanism in that spirit
can be read as questioning either the principle or even its basis: causality.
In the former case, the principle can be replaced by its quantum version,
accepting as a common cause an entangled state, leaving the phenomenon as
mysterious as ever on the classical level (on which, after all, it occurs). If,
more radically, the causal structure is questioned in principle, closed
space-time curves may become possible that, as is argued in the present note,
can give rise to non-local correlations if to-be-correlated pieces of classical
information meet in the common future --- which they need to if the correlation
is to be detected in the first place. The result is a view resembling Brassard
and Raymond-Robichaud's parallel-lives variant of Hermann's and Everett's
relative-state formalism, avoiding "multiple realities."Comment: 8 pages, 5 figure
Information Causality, the Tsirelson Bound, and the 'Being-Thus' of Things
The principle of `information causality' can be used to derive an upper
bound---known as the `Tsirelson bound'---on the strength of quantum mechanical
correlations, and has been conjectured to be a foundational principle of
nature. To date, however, it has not been sufficiently motivated to play such a
foundational role. The motivations that have so far been given are, as I argue,
either unsatisfactorily vague or appeal to little if anything more than
intuition. Thus in this paper I consider whether some way might be found to
successfully motivate the principle. And I propose that a compelling way of so
doing is to understand it as a generalisation of Einstein's principle of the
mutually independent existence---the `being-thus'---of spatially distant
things. In particular I first describe an argument, due to Demopoulos, to the
effect that the so-called `no-signalling' condition can be viewed as a
generalisation of Einstein's principle that is appropriate for an irreducibly
statistical theory such as quantum mechanics. I then argue that a compelling
way to motivate information causality is to in turn consider it as a further
generalisation of the Einsteinian principle that is appropriate for a theory of
communication. I describe, however, some important conceptual obstacles that
must yet be overcome if the project of establishing information causality as a
foundational principle of nature is to succeed.Comment: '*' footnote added to page 1; 24 pages, 1 figure; Forthcoming in
Studies in History and Philosophy of Modern Physic
Generalizations of Boxworld
Boxworld is a toy theory that can generate extremal nonlocal correlations
known as PR boxes. These have been well established as an important tool to
examine general nonlocal correlations, even beyond the correlations that are
possible in quantum theory. We modify boxworld to include new features. The
first modification affects the construction of joint systems such that the new
theory allows entangled measurements as well as entangled states in contrast to
the standard version of boxworld. The extension to multipartite systems and the
consequences for entanglement swapping are analysed. Another modification
provides continuous transitions between classical probability theory and
boxworld, including the algebraic expression for the maximal CHSH violation as
a function of the transition parameters.Comment: In Proceedings QPL 2011, arXiv:1210.029
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