7,413 research outputs found
An Operational Interpretation of Negative Probabilities and No-Signalling Models
Negative probabilities have long been discussed in connection with the
foundations of quantum mechanics. We have recently shown that, if signed
measures are allowed on the hidden variables, the class of probability models
which can be captured by local hidden-variable models are exactly the
no-signalling models. However, the question remains of how negative
probabilities are to be interpreted. In this paper, we present an operational
interpretation of negative probabilities as arising from standard probabilities
on signed events. This leads, by virtue of our previous result, to a systematic
scheme for simulating arbitrary no-signalling models.Comment: 13 pages, 2 figure
No-Signalling Is Equivalent To Free Choice of Measurements
No-Signalling is a fundamental constraint on the probabilistic predictions
made by physical theories. It is usually justified in terms of the constraints
imposed by special relativity. However, this justification is not as clear-cut
as is usually supposed. We shall give a different perspective on this condition
by showing an equivalence between No-Signalling and Lambda Independence, or
"free choice of measurements", a condition on hidden-variable theories which is
needed to make no-go theorems such as Bell's theorem non-trivial. More
precisely, we shall show that a probability table describing measurement
outcomes is No-Signalling if and only if it can be realized by a
Lambda-Independent hidden-variable theory of a particular canonical form, in
which the hidden variables correspond to non-contextual deterministic
predictions of measurement outcomes. The key proviso which avoids contradiction
with Bell's theorem is that we consider hidden-variable theories with signed
probability measures over the hidden variables - i.e. negative probabilities.
Negative probabilities have often been discussed in the literature on quantum
mechanics. We use a result proved previously in "The Sheaf-theoretic Structure
of Locality and Contextuality" by Abramsky and Brandenburger, which shows that
they give rise to, and indeed characterize, the entire class of No-Signalling
behaviours. In the present paper, we put this result in a broader context,
which reveals the surprising consequence that the No-Signalling condition is
equivalent to the apparently completely different notion of free choice of
measurements.Comment: In Proceedings QPL 2013, arXiv:1412.791
On the 'Reality' of Observable Properties
This note contains some initial work on attempting to bring recent
developments in the foundations of quantum mechanics concerning the nature of
the wavefunction within the scope of more logical and structural methods. A
first step involves generalising and reformulating a criterion for the reality
of the wavefunction proposed by Harrigan & Spekkens, which was central to the
PBR theorem. The resulting criterion has several advantages, including the
avoidance of certain technical difficulties relating to sets of measure zero.
By considering the 'reality' not of the wavefunction but of the observable
properties of any ontological physical theory a novel characterisation of
non-locality and contextuality is found.
Secondly, a careful analysis of preparation independence, one of the key
assumptions of the PBR theorem, leads to an analogy with Bell locality, and
thence to a proposal to weaken it to an assumption of
`no-preparation-signalling' in analogy with no-signalling. This amounts to
introducing non-local correlations in the joint ontic state, which is, at
least, consistent with the Bell and Kochen-Specker theorems. The question of
whether the PBR result can be strengthened to hold under this relaxed
assumption is therefore posed.Comment: 8 pages, re-written with new section
The Leggett-Garg Inequalities and No-Signalling in Time: A Quasi-Probability Approach
The Leggett-Garg (LG) inequalities were proposed in order to assess whether
sets of pairs of sequential measurements on a single quantum system can be
consistent with an underlying notion of macrorealism. Here, the LG inequalities
are explored using a simple quasi-probability linear in the projection
operators to describe the properties of the system at two times. We show that
this quasi-probability is measurable, has the same correlation function as the
usual two-time measurement probability (for the bivalent variables considered
here) and has the key property that the probabilities for the later time are
independent of whether an earlier measurement was made, a generalization of the
no-signalling in time condition of Kofler and Brukner. We argue that this
quasi-probability, appropriately measured, provides a non-invasive measure of
macrorealism per se at the two time level. This measure, when combined with the
LG inequalities, provides a characterization of macrorealism more detailed than
that provided by the LG inequalities alone. When the quasi-probability is
non-negative, the LG system has a natural parallel with the EPRB system and
Fine's theorem. A simple spin model illustrating key features of the approach
is exhibited.Comment: 23 pages. Significant revisions. Change of titl
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