10,819 research outputs found
Discovering Quantum Causal Models (final)
Costa and Shrapnel [2016] have recently proposed an interventionist theory of quantum causation. The formalism generalises the classical methods of Pearl [2000] and allows for the discovery of quantum causal structure via localised interventions. Classical causal structure is presented as a special case of this more general framework. I introduce the account and consider whether this formalism provides a causal explanation for the Bell correlations
Discovering quantum causal models
Costa and Shrapnel ([2016]) have recently proposed an interventionist theory of quantum causation. The formalism generalizes the classical methods of Pearl ([2000]) and allows for the discovery of quantum causal structure via localized interventions. Classical causal structure is presented as a special case of this more general framework. I introduce the account and consider whether this formalism provides a causal explanation for the Bell correlations. 1 Introduction 2 Classical Causal Models 3 What's the (Quantum) Problem? 4 Quantum Causal Models: Why Bother? 5 Markov Quantum Causal Models 6 The Bell Experiment 7 Bell's Objections
Quantifying Quantum Causal Influences
Causal influences are at the core of any empirical science, the reason why
its quantification is of paramount relevance for the mathematical theory of
causality and applications. Quantum correlations, however, challenge our notion
of cause and effect, implying that tools and concepts developed over the years
having in mind a classical world, have to be reevaluated in the presence of
quantum effects. Here, we propose the quantum version of the most common
causality quantifier, the average causal effect (ACE), measuring how much a
target quantum system is changed by interventions on its presumed cause. Not
only it offers an innate manner to quantify causation in two-qubit gates but
also in alternative quantum computation models such as the measurement-based
version, suggesting that causality can be used as a proxy for optimizing
quantum algorithms. Considering quantum teleportation, we show that any pure
entangled state offers an advantage in terms of causal effects as compared to
separable states. This broadness of different uses showcases that, just as in
the classical case, the quantification of causal influence has foundational and
applied consequences and can lead to a yet totally unexplored tool for quantum
information science.Comment: 12 pages, 3 figures. Comments welcome
Quantum causal models, faithfulness and retrocausality
Wood and Spekkens (2015) argue that any causal model explaining the EPRB
correlations and satisfying no-signalling must also violate the assumption that
the model faithfully reproduces the statistical dependences and
independences---a so-called "fine-tuning" of the causal parameters; this
includes, in particular, retrocausal explanations of the EPRB correlations. I
consider this analysis with a view to enumerating the possible responses an
advocate of retrocausal explanations might propose. I focus on the response of
N\"{a}ger (2015), who argues that the central ideas of causal explanations can
be saved if one accepts the possibility of a stable fine-tuning of the causal
parameters. I argue that, in light of this view, a violation of faithfulness
does not necessarily rule out retrocausal explanations of the EPRB
correlations, although it certainly constrains such explanations. I conclude by
considering some possible consequences of this type of response for retrocausal
explanations
Quantum Information Processing and Relativistic Quantum Fields
It is shown that an ideal measurement of a one-particle wave packet state of
a relativistic quantum field in Minkowski spacetime enables superluminal
signalling. The result holds for a measurement that takes place over an
intervention region in spacetime whose extent in time in some frame is longer
than the light-crossing time of the packet in that frame. Moreover, these
results are shown to apply not only to ideal measurements but also to unitary
transformations that rotate two orthogonal one-particle states into each other.
In light of these observations, possible restrictions on the allowed types of
intervention are considered. A more physical approach to such questions is to
construct explicit models of the interventions as interactions between the
field and other quantum systems such as detectors. The prototypical
Unruh-DeWitt detector couples to the field operator itself and so most likely
respects relativistic causality. On the other hand, detector models which
couple to a finite set of frequencies of field modes are shown to lead to
superluminal signalling. Such detectors do, however, provide successful
phenomenological models of atom-qubits interacting with quantum fields in a
cavity but are valid only on time scales many orders of magnitude larger than
the light-crossing time of the cavity.Comment: 16 pages, 2 figures. Improved abstract and discussion of 'ideal'
measurements. References to previous work adde
Which causal structures might support a quantum-classical gap?
A causal scenario is a graph that describes the cause and effect
relationships between all relevant variables in an experiment. A scenario is
deemed `not interesting' if there is no device-independent way to distinguish
the predictions of classical physics from any generalised probabilistic theory
(including quantum mechanics). Conversely, an interesting scenario is one in
which there exists a gap between the predictions of different operational
probabilistic theories, as occurs for example in Bell-type experiments. Henson,
Lal and Pusey (HLP) recently proposed a sufficient condition for a causal
scenario to not be interesting. In this paper we supplement their analysis with
some new techniques and results. We first show that existing graphical
techniques due to Evans can be used to confirm by inspection that many graphs
are interesting without having to explicitly search for inequality violations.
For three exceptional cases -- the graphs numbered 15,16,20 in HLP -- we show
that there exist non-Shannon type entropic inequalities that imply these graphs
are interesting. In doing so, we find that existing methods of entropic
inequalities can be greatly enhanced by conditioning on the specific values of
certain variables.Comment: 13 pages, 9 figures, 1 bicycle. Added an appendix showing that
e-separation is strictly more general than the skeleton method. Added journal
referenc
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