Abstract
Causality is a seminal concept in science: Any research discipline, from sociology and medicine to physics and chemistry, aims at understanding the causes that could explain the correlations observed among some measured variables. While several methods exist to characterize classical causal models, no general construction is known for the quantum case. In this work, we present quantum inflation, a systematic technique to falsify if a given quantum causal model is compatible with some observed correlations. We demonstrate the power of the technique by reproducing known results and solving open problems for some paradigmatic examples of causal networks. Our results may find applications in many fields: from the characterization of correlations in quantum networks to the study of quantum effects in thermodynamic and biological processes.
- Received 29 May 2020
- Revised 1 December 2020
- Accepted 18 February 2021
DOI:https://doi.org/10.1103/PhysRevX.11.021043
Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.
Published by the American Physical Society
Physics Subject Headings (PhySH)
Popular Summary
Identifying the causes behind the correlations observed among some measured variables is one of the main challenges in any scientific discipline. Understanding this is crucial in many situations, such as the development of medical treatments, the design of new materials, and the theoretical modeling of experiments. While much effort has been put toward understanding cause-and-effect relations between classical systems, it is also essential to do so for quantum systems, since our current description of nature is ultimately quantum. Here, we provide a novel method to discern whether observations are incompatible with causal explanations that involve interactions between quantum systems.
We call our framework “quantum inflation,” a general method for deriving conditions that observations must satisfy to be compatible with a quantum causal explanation. Crucially, these conditions can be formulated as well-known and efficiently solvable optimization problems. In addition to its general formulation, we demonstrate the applicability of quantum inflation in a variety of situations, including the certification of causal structures, quantum cryptography, and the quantification of causal strengths.
Quantum inflation has already found applications in generalizations of entanglement theory and quantum information protocols for networks with many parties. It also provides a computationally tractable method for analyzing causal relations between classical systems. Moreover, because of the central role that causality has in science, we expect that quantum inflation will become a fundamental tool for analyzing causality in any situation where a quantum behavior is presumed.
