469 research outputs found
Unsharp Values, Domains and Topoi
The so-called topos approach provides a radical reformulation of quantum
theory. Structurally, quantum theory in the topos formulation is very similar
to classical physics. There is a state object, analogous to the state space of
a classical system, and a quantity-value object, generalising the real numbers.
Physical quantities are maps from the state object to the quantity-value object
-- hence the `values' of physical quantities are not just real numbers in this
formalism. Rather, they are families of real intervals, interpreted as `unsharp
values'. We will motivate and explain these aspects of the topos approach and
show that the structure of the quantity-value object can be analysed using
tools from domain theory, a branch of order theory that originated in
theoretical computer science. Moreover, the base category of the topos
associated with a quantum system turns out to be a domain if the underlying von
Neumann algebra is a matrix algebra. For general algebras, the base category
still is a highly structured poset. This gives a connection between the topos
approach, noncommutative operator algebras and domain theory. In an outlook, we
present some early ideas on how domains may become useful in the search for new
models of (quantum) space and space-time.Comment: 32 pages, no figures; to appear in Proceedings of Quantum Field
Theory and Gravity, Regensburg (2010
Partial and Total Ideals of Von Neumann Algebras
A notion of partial ideal for an operator algebra is a weakening the notion
of ideal where the defining algebraic conditions are enforced only in the
commutative subalgebras. We show that, in a von Neumann algebra, the
ultraweakly closed two-sided ideals, which we call total ideals, correspond to
the unitarily invariant partial ideals. The result also admits an equivalent
formulation in terms of central projections. We place this result in the
context of an investigation into notions of spectrum of noncommutative
-algebras.Comment: 14 page
On monogamy of non-locality and macroscopic averages: examples and preliminary results
We explore a connection between monogamy of non-locality and a weak
macroscopic locality condition: the locality of the average behaviour. These
are revealed by our analysis as being two sides of the same coin.
Moreover, we exhibit a structural reason for both in the case of Bell-type
multipartite scenarios, shedding light on but also generalising the results in
the literature [Ramanathan et al., Phys. Rev. Lett. 107, 060405 (2001);
Pawlowski & Brukner, Phys. Rev. Lett. 102, 030403 (2009)]. More specifically,
we show that, provided the number of particles in each site is large enough
compared to the number of allowed measurement settings, and whatever the
microscopic state of the system, the macroscopic average behaviour is local
realistic, or equivalently, general multipartite monogamy relations hold.
This result relies on a classical mathematical theorem by Vorob'ev [Theory
Probab. Appl. 7(2), 147-163 (1962)] about extending compatible families of
probability distributions defined on the faces of a simplicial complex -- in
the language of the sheaf-theoretic framework of Abramsky & Brandenburger [New
J. Phys. 13, 113036 (2011)], such families correspond to no-signalling
empirical models, and the existence of an extension corresponds to locality or
non-contextuality. Since Vorob'ev's theorem depends solely on the structure of
the simplicial complex, which encodes the compatibility of the measurements,
and not on the specific probability distributions (i.e. the empirical models),
our result about monogamy relations and locality of macroscopic averages holds
not just for quantum theory, but for any empirical model satisfying the
no-signalling condition.
In this extended abstract, we illustrate our approach by working out a couple
of examples, which convey the intuition behind our analysis while keeping the
discussion at an elementary level.Comment: In Proceedings QPL 2014, arXiv:1412.810
A comonadic view of simulation and quantum resources
We study simulation and quantum resources in the setting of the
sheaf-theoretic approach to contextuality and non-locality. Resources are
viewed behaviourally, as empirical models. In earlier work, a notion of
morphism for these empirical models was proposed and studied. We generalize and
simplify the earlier approach, by starting with a very simple notion of
morphism, and then extending it to a more useful one by passing to a co-Kleisli
category with respect to a comonad of measurement protocols. We show that these
morphisms capture notions of simulation between empirical models obtained via
`free' operations in a resource theory of contextuality, including the type of
classical control used in measurement-based quantum computation schemes.Comment: To appear in Proceedings of LiCS 201
A complete characterisation of All-versus-Nothing arguments for stabiliser states
An important class of contextuality arguments in quantum foundations are the
All-versus-Nothing (AvN) proofs, generalising a construction originally due to
Mermin. We present a general formulation of All-versus-Nothing arguments, and a
complete characterisation of all such arguments which arise from stabiliser
states. We show that every AvN argument for an n-qubit stabiliser state can be
reduced to an AvN proof for a three-qubit state which is local
Clifford-equivalent to the tripartite GHZ state. This is achieved through a
combinatorial characterisation of AvN arguments, the AvN triple Theorem, whose
proof makes use of the theory of graph states. This result enables the
development of a computational method to generate all the AvN arguments in
on n-qubit stabiliser states. We also present new insights into
the stabiliser formalism and its connections with logic.Comment: 18 pages, 6 figure
Sheaf representation of monoidal categories
Every small monoidal category with universal (finite) joins of central
idempotents is monoidally equivalent to the category of global sections of a
sheaf of (sub)local monoidal categories on a topological space. Every small
stiff monoidal category monoidally embeds into such a category of global
sections. These representation results are functorial and subsume the
Lambek-Moerdijk-Awodey sheaf representation for toposes, the Stone
representation of Boolean algebras, and the Takahashi representation of Hilbert
modules as continuous fields of Hilbert spaces. Many properties of a monoidal
category carry over to the stalks of its sheaf, including having a trace,
having exponential objects, having dual objects, having limits of some shape,
and the central idempotents forming a Boolean algebra.Comment: 39 page
The Logic of Contextuality
Contextuality is a key signature of quantum non-classicality, which has been
shown to play a central role in enabling quantum advantage for a wide range of
information-processing and computational tasks. We study the logic of
contextuality from a structural point of view, in the setting of partial
Boolean algebras introduced by Kochen and Specker in their seminal work. These
contrast with traditional quantum logic \`a la Birkhoff and von Neumann in that
operations such as conjunction and disjunction are partial, only being defined
in the domain where they are physically meaningful.
We study how this setting relates to current work on contextuality such as
the sheaf-theoretic and graph-theoretic approaches. We introduce a general free
construction extending the commeasurability relation on a partial Boolean
algebra, i.e. the domain of definition of the binary logical operations. This
construction has a surprisingly broad range of uses. We apply it in the study
of a number of issues, including:
- establishing the connection between the abstract measurement scenarios
studied in the contextuality literature and the setting of partial Boolean
algebras;
- formulating various contextuality properties in this setting, including
probabilistic contextuality as well as the strong, state-independent notion of
contextuality given by Kochen-Specker paradoxes, which are logically
contradictory statements validated by partial Boolean algebras, specifically
those arising from quantum mechanics;
- investigating a Logical Exclusivity Principle, and its relation to the
Probabilistic Exclusivity Principle widely studied in recent work on
contextuality as a step towards closing in on the set of quantum-realisable
correlations;
- developing some work towards a logical presentation of the Hilbert space
tensor product, using logical exclusivity to capture some of its salient
quantum features.Comment: 18 pages, to appear in Proceedings of 29th EACSL Annual Conference on
Computer Science Logic (CSL 2021
The Cohomology of Non-Locality and Contextuality
In a previous paper with Adam Brandenburger, we used sheaf theory to analyze
the structure of non-locality and contextuality. Moreover, on the basis of this
formulation, we showed that the phenomena of non-locality and contextuality can
be characterized precisely in terms of obstructions to the existence of global
sections.
Our aim in the present work is to build on these results, and to use the
powerful tools of sheaf cohomology to study the structure of non-locality and
contextuality. We use the Cech cohomology on an abelian presheaf derived from
the support of a probabilistic model, viewed as a compatible family of
distributions, in order to define a cohomological obstruction for the family as
a certain cohomology class. This class vanishes if the family has a global
section. Thus the non-vanishing of the obstruction provides a sufficient (but
not necessary) condition for the model to be contextual.
We show that for a number of salient examples, including PR boxes, GHZ
states, the Peres-Mermin magic square, and the 18-vector configuration due to
Cabello et al. giving a proof of the Kochen-Specker theorem in four dimensions,
the obstruction does not vanish, thus yielding cohomological witnesses for
contextuality.Comment: In Proceedings QPL 2011, arXiv:1210.029
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