Quantum Bayesianism

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.

[5][6] 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.

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.

QBism begins by asserting that all probabilities, even those appearing in quantum theory, are most properly viewed as members of the latter category.

Specifically, QBism adopts a personalist Bayesian interpretation along the lines of Italian mathematician Bruno de Finetti[16] and English philosopher Frank Ramsey.

[23] Christopher Fuchs introduced the term "QBism" and outlined the interpretation in more or less its present form in 2010,[24] carrying further and demanding consistency of ideas broached earlier, notably in publications from 2002.

[22] Prior to the 2010 article, the term "quantum Bayesianism" was used to describe the developments which have since led to QBism in its present form.

[29] However, QBism itself was not influenced or motivated by Cubism and has no lineage to a potential connection between Cubist art and Bohr's views on quantum theory.

[13] Quantum theory, QBism claims, is fundamentally a guide for decision making which has been shaped by some aspects of physical reality.

Chief among the tenets of QBism are the following:[31] Reactions to the QBist interpretation have ranged from enthusiastic[13][28] to strongly negative.

[38] Further, while also raising concerns about the treatment of probability-one assignments, Timpson suggests that QBism may result in a reduction of explanatory power as compared to other interpretations.

[45] Similarly, Schlosshauer and Claringbold state that QBism is a consistent interpretation of quantum mechanics, but do not offer a verdict on whether it should be preferred.

Popularized or semi-popularized media coverage of QBism has appeared in New Scientist,[49] Scientific American,[50] Nature,[51] Science News,[52] the FQXi Community,[53] the Frankfurter Allgemeine Zeitung,[29] Quanta Magazine,[16] Aeon,[54] Discover,[55] Nautilus Quarterly,[56] and Big Think.

[57] In 2018, two popular-science books about the interpretation of quantum mechanics, Ball's Beyond Weird and Ananthaswamy's Through Two Doors at Once, devoted sections to QBism.

[58][59] Furthermore, Harvard University Press published a popularized treatment of the subject, QBism: The Future of Quantum Physics, in 2016.

This contrasts with older Copenhagen-type views, which hold that probabilities are given by quantum states that are in turn fixed by objective facts about preparation procedures.

Specifically, QBism posits that quantum theory is a normative tool which an agent may use to better navigate reality, rather than a set of mechanics governing it.

[6] These approaches differ from each other in what they consider quantum states to be information or expectations "about", as well as in the technical features of the mathematics they employ.

Furthermore, not all authors who advocate views of this type propose an answer to the question of what the information represented in quantum states concerns.

[68] However, they remain deliberately agnostic about what physical properties or entities quantum states are information (or beliefs) about, as opposed to QBism, which offers an answer to that question.

[68] Another approach, advocated by Bub and Pitowsky, argues that quantum states are information about propositions within event spaces that form non-Boolean lattices.

[72] Despite this difference, in Cabello's classification, the proposals of Zeilinger and Brukner are also designated as "participatory realism", as QBism and the Copenhagen-type interpretations are.

One important distinction between the two interpretations is their philosophy of probability: RQM does not adopt the Ramsey–de Finetti school of personalist Bayesianism.

[19][73] For example, some in the field of computer science have introduced a kind of quantum Bayesian network, which they argue could have applications in "medical diagnosis, monitoring of processes, and genetics".

One topic prominent in the reconstruction effort is the set of mathematical structures known as symmetric, informationally-complete, positive operator-valued measures (SIC-POVMs).

Fuchs, Schack, and others have taken to calling this restatement of the Born rule the urgleichung, from the German for "primal equation" (see Ur- prefix), because of the central role it plays in their reconstruction of quantum theory.

[19][97][98] The following discussion presumes some familiarity with the mathematics of quantum information theory, and in particular, the modeling of measurement procedures by POVMs.

The urgleichung, in contrast, is a relation between different contexts which finds its justification in the predictive success of quantum physics.

Those QBists who find this approach promising are pursuing a complete reconstruction of quantum theory featuring the urgleichung as the key postulate.

[100]) Comparisons between this approach and others not associated with QBism (or indeed with any particular interpretation) can be found in a book chapter by Fuchs and Stacey[101] and an article by Appleby et al.[97] As of 2017, alternative QBist reconstruction efforts are in the beginning stages.