Zenith Grant Awardee

Giulio Chiribella

University of Hong Kong

Co-Investigators

Adán Cabello, University of Seville; Matthias Kleinmann, University of the Basque Country

Project Title

The Observer Observed: a Bayesian Route to the Reconstruction of Quantum Theory

Project Summary

Quantum theory portrays a world where the outcomes of individual measurements cannot be predicted with certainty. But despite this apparent weakness, the quantum predictions are strikingly successful in many applications. Such a success demands for an explanation: Is quantum theory successful because it captures a bundle of primitive facts about the world? Or is it successful because it is a general-purpose tool for guessing the outcomes of our experiments? A new quantum interpretation, called Quantum Bayesianism (QBism), vigorously argues for the second alternative, regarding quantum theory as a \"manual for betting agents\". Still, it is not clear why an agent should opt for quantum theory, instead of classical probability theory or more exotic alternatives. Also, it is not clear how the agent-centric approach of QBism can be reconciled with the requirement that the laws of physics should transcend the viewpoint of individual observers. We believe that the solution to both puzzles is hidden in the curious way in which quantum theory combines the perspectives of different observers. Following this clue, we are seeking a derivation of quantum theory from basic principles regulating how different agents can formulate their pictures of reality in a consistent and maximally predictive fashion.

Technical Abstract

The role of the observer in quantum theory is central and controversial at the same time. The standard Copenhagen interpretation assumes a cut between observer and observed, while other quantum interpretations advocate the need for an observer-free account of nature. Recently, quantum Bayesianism (QBism) has stirred the debate with a radical proposal, putting the observer right at the foundation of quantum theory. QBism asserts that quantum framework is a normative toolbox prescribing how rational agents should organize and update their beliefs about reality. But why should quantum theory be better suited to this task than classical theory or other exotic alternatives? The goals of our project are 1) to provide an axiomatic foundation for QBism, by reconstructing quantum theory directly from principles of Bayesian nature, and 2) to reconcile the agent-centric approach of QBism with the requirement that agents should be describable as physical systems. We will develop a framework for Bayesian theories of physics, embracing classical and quantum Bayesianism as special cases. Within this framework we will explore the consistency conditions that arise when the viewpoints of different observers are compared and we will explore the picture of reality that emerges from the accounts of multiple interacting agents.