Quantum correlations with no causal order (1105.4464v3)

Abstract: 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? We address these questions from the standpoint of quantum mechanics in a new framework for multipartite correlations which 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.

• The paper introduces a framework demonstrating that quantum processes can occur without a predetermined causal order.
• It reveals that quantum correlations can violate causal inequalities, analogous to violations of Bell inequalities.
• The research implies that classical causality may emerge from underlying quantum indeterminacy, inspiring unified theories.

Quantum Correlations with No Causal Order

The paper "Quantum correlations with no causal order" by Ognyan Oreshkov, Fabio Costa, and Časlav Brukner presents an innovative framework within the domain of quantum mechanics, challenging the conventional understanding of causal relationships among events. It argues that the typically assumed global causal order does not necessarily underpin all quantum processes, positing a scenario in which events may not adhere to a definitive sequence.

Framework and Findings

The authors introduce a novel framework that accounts for multipartite correlations without presupposing a global causal structure, maintaining only the local application of quantum mechanics within specialized laboratories. This framework encapsulates both space-like and time-like correlations, which are traditionally underpinned by causal orders. A remarkable finding of this work is the identification of correlations that violate what the authors refer to as a "causal inequality." This violation is analogous to the more familiar violation of Bell inequalities, in which quantum mechanics contradicts the assumptions of local realism. While Bell's inequality pertains to non-signalling correlations, the causal inequality pertains to signalling correlations between parties.

Within the proposed framework, the researchers demonstrate that more general forms of correlations are possible—ones that exceed the framework of traditional quantum mechanics assuming pre-defined causal structures. Employing this framework, they illustrate a scenario in which correlations materialize without adhering to a predetermined causal sequence. This is showcased through a "communication task" that shares structure with the CHSH-Bell inequality, suggesting fundamental insights and challenges to current causal paradigms.

Implications and Speculations

The results of this paper prompt profound theoretical ramifications concerning the nature of space-time and causality in quantum theory. Classically, causal structures are neatly embedded within the relativistic fabric of space-time, where the causal order is definitive and invariable. However, Oreshkov et al.’s demonstration of possibly indefinite causal orders could inspire reconsiderations of quantum mechanics' compatibility with general relativity, especially in regimes where both are expected to converge, such as quantum gravity.

The authors also discuss potential implications concerning the classical limit. They demonstrate that when classical mechanics is applied within their framework, causal orders naturally emerge. This conjures the notion that the apparent determinate causality in classical physics might emerge from quantum reality, which lacks intrinsic causal order.

Future Developments

This work opens avenues for further inquiry into quantum processes' causal underpinnings and their implications for theories aiming to unify quantum mechanics and general relativity. Hypothetic exploration of 'non-causal' structures could unveil new principles underpinning quantum mechanics’ actual limits and, more broadly, a deeper understanding of reality's fabric.

The extension of this framework could bolster novel computational models that exploit non-classical causal strategies. As such, this research awakens possibilities of leveraging these indeterminate causal orders in quantum computing and communication technologies, potentially demonstrating new degrees of freedom within quantum information theories.

In conclusion, "Quantum correlations with no causal order" presents a conceptual leap in understanding the foundational elements of quantum mechanics by demonstrating the plausibility of quantum processes uncoupled from classical causal orders. This work not only sparks re-evaluation of established paradigms but also encourages the pursuit of fundamentally novel quantum theories and applications.