QBism

In physics and the philosophy of physics, QBism (pronounced "cubism") is an interpretation of quantum mechanics that takes an agent's actions and experiences as the central concerns of the theory. It is the most prominent and extreme form of quantum Bayesianism, a collection of related approaches that all involve interpreting quantum probabilities as Bayesian in some manner. QBism deals with common questions in the interpretation of quantum theory about the nature of wavefunction superposition, quantum measurement, and entanglement. According to QBism, many, but not all, aspects of the quantum formalism are subjective in nature. For example, in this interpretation, a quantum state is not an element of reality—instead, it represents the degrees of belief an agent has about the possible outcomes of measurements. For this reason, some philosophers of science have deemed QBism a form of anti-realism. The originators of the interpretation disagree with this characterization, proposing instead that the theory more properly aligns with a kind of realism they call "participatory realism", wherein reality consists of more than can be captured by any putative third-person account of it. This interpretation is distinguished by its use of a subjective Bayesian account of probabilities to understand the quantum mechanical Born rule as a normative addition to good decision-making. Rooted in the prior work of Carlton Caves, Christopher Fuchs, and Rüdiger Schack during the early 2000s, QBism itself is primarily associated with Fuchs and Schack and has more recently been adopted by David Mermin. QBism draws from the fields of quantum information and Bayesian probability and aims to eliminate the interpretational conundrums that have beset quantum theory. The QBist interpretation is historically derivative of the views of the various physicists that are often grouped together as "the" Copenhagen interpretation, but is itself distinct from them. In addition to presenting an interpretation of the existing mathematical structure of quantum theory, some QBists have advocated a research program of reconstructing quantum theory from basic physical principles whose QBist character is manifest. The ultimate goal of this research is to identify what aspects of the ontology of the physical world make quantum theory a good tool for agents to use. However, the QBist interpretation itself, as described in § Core positions, does not depend on any particular reconstruction.

History and development

E. T. Jaynes, a promoter of the use of Bayesian probability in statistical physics, once suggested that quantum theory is "[a] peculiar mixture describing in part realities of Nature, in part incomplete human information about Nature—all scrambled up by Heisenberg and Bohr into an omelette that nobody has seen how to unscramble". This point of view inspired the development of quantum Bayesianism. Jaynes pointed out that a mixed quantum state can be written in multiple different ways as a statistical mixture of pure states. (This is part of what would later be known as the Schrödinger–HJW theorem.) So, if pure states are supposed to represent objective uncertainty while the weights in the mixture are subjective probabilities representing an observer's uncertainty as to which pure state is truly present, then the subjective and objective have become completely intermingled: many distinct combinations of subjective and objective entities yield exactly the same physical predictions. A 2002 paper by Carlton Caves, Christopher A. Fuchs and Ruediger Schack proposed interpreting quantum probability as a form of Bayesian probability. However, all three authors grew dissatisfied with that paper, and ultimately diverged in their views on how to resolve the problems; Fuchs and Schack developed their position into QBism. Christopher Fuchs introduced the term "QBism" and outlined the interpretation in more or less its present form in 2010, carrying further and demanding consistency of ideas broached earlier, notably in publications from 2002. Several subsequent works have expanded and elaborated upon these foundations, notably a Reviews of Modern Physics article by Fuchs and Schack; an American Journal of Physics article by Fuchs, Mermin, and Schack; and Enrico Fermi Summer School lecture notes by Fuchs and Stacey. Fuchs chose to call the interpretation "QBism", pronounced "cubism", preserving the Bayesian spirit via the CamelCase in the first two letters, but distancing it from Bayesianism more broadly. As this neologism is a homophone of Cubism the art movement, it has motivated conceptual comparisons between the two, and media coverage of QBism has been illustrated with art by Picasso and Gris.

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