Quantum correlations with no causal order

By Ognyan Oreshkov, Fabio Costa, Č. Brukner. Nature Communications, 2011

803 citations recorded by the source corpus.

The idea that events obey a definite causal order is deeply rooted in our understanding of the world and at the basis of the very notion of time. But where does causal order come from, and is it a necessary property of nature? Here, we address these questions from the standpoint of quantum mechanics in a new framework for multipartite correlations that does not assume a pre-defined global causal structure but only the validity of quantum mechanics locally. All known situations that respect causal order, including space-like and time-like separated experiments, are captured by this framework in a unified way. Surprisingly, we find correlations that cannot be understood in terms of definite causal order. These correlations violate a 'causal inequality' that is satisfied by all space-like and time-like correlations. We further show that in a classical limit causal order always arises, which suggests that space-time may emerge from a more fundamental structure in a quantum-to-classical transition. Causal order is a concept that is engrained in the standard understanding of time, both in classical and quantum mechanics. Oreshkovet al.generalize the standard formalism of quantum theory to a framework with no pre-existing causal order, and find a new class of correlations that have no analogue in the classical world.

Subfields: Artificial Intelligence, Atomic and Molecular Physics, and Optics, Statistical and Nonlinear Physics

View Paper: https://www.semanticscholar.org/paper/cf88116ab3d7c04473a91038194467660854d439

How researchers see this paper

The researchers whose trusted networks rank it highest, strongest advocate first. Every researcher gets a different order.

In Artificial Intelligence

In Atomic and Molecular Physics, and Optics

In Statistical and Nonlinear Physics

Citations in the corpus

Quantum machine learning (2016, neutral): “The starting point is correlations—this is already problematic if we study quantum correlations which may not have a deﬁnite causal order [96], as it has been experimentally probed [97].”
Quantum common causes and quantum causal models (2016, background): “Finally, it would be interesting to extend the formalism to explore the possibility that certain quantum scenarios are best understood as involving a quantum coherent combination of different causal structures [46,48,83,84].”
A Postquantum Theory of Classical Gravity? (2018, neutral): “While this difficulty may be merely technical, background-independent approaches such as loop quantum gravity, string field theory [5], and others [6,7] have yet to demonstrate that this is possible.”
Experimental superposition of orders of quantum gates (2014, background): “From a foundational point of view, our experiment can be seen as the first realization of a “superposition of causal orders”, which represents an instance of an indefinite causal structure [25].”
Causal and causally separable processes (2015, methodology): “[4] only used certain consequences of the requirement that the causal configuration of the parties must be independent of the parties’ settings, but not the full requirement.”