QBism


In physics and the philosophy of physics, QBism 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, does not depend on any particular reconstruction.

History and development

, a promoter of the use of Bayesian probability in statistical physics, once suggested that quantum theory is " 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. 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.

Background

There are many interpretations of probability theory. Broadly speaking, these interpretations fall into one of three categories: those which assert that a probability is an objective property of reality, those who assert that probability is an objective property of the measuring process, and those which assert that a probability is a cognitive construct which an agent may use to quantify their ignorance or degree of belief in a proposition. 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 and English philosopher Frank Ramsey.
According to QBists, the advantages of adopting this view of probability are twofold. First, for QBists the role of quantum states, such as the wavefunctions of particles, is to efficiently encode probabilities; so quantum states are ultimately degrees of belief themselves. Regarding quantum states as degrees of belief implies that the event of a quantum state changing when a measurement occurs—the "collapse of the wave function"—is simply the agent updating her beliefs in response to a new experience. Second, it suggests that quantum mechanics can be thought of as a local theory, because the Einstein–Podolsky–Rosen criterion of reality can be rejected. The EPR criterion states: "If, without in any way disturbing a system, we can predict with certainty the value of a physical quantity, then there exists an element of reality corresponding to that quantity." Arguments that quantum mechanics should be considered a nonlocal theory depend upon this principle, but to a QBist, it is invalid, because a personalist Bayesian considers all probabilities, even those equal to unity, to be degrees of belief. Therefore, while many interpretations of quantum theory conclude that quantum mechanics is a nonlocal theory, QBists do not.

Core positions

According to QBism, quantum theory is a tool which an agent may use to help manage their expectations, more like probability theory than a conventional physical theory. 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:
  1. All probabilities, including those equal to zero or one, are valuations that an agent ascribes to their degrees of belief in possible outcomes. As they define and update probabilities, quantum states, channels, and measurements are also the personal judgements of an agent.
  2. The Born rule is normative, not descriptive. It is a relation to which an agent should strive to adhere in their probability and quantum-state assignments.
  3. Quantum measurement outcomes are personal experiences for the agent gambling on them. Different agents may confer and agree upon the consequences of a measurement, but the outcome is the experience each of them individually has.
  4. A measurement apparatus is conceptually an extension of the agent. It should be considered analogous to a sense organ or prosthetic limb—simultaneously a tool and a part of the individual.

    Reception and criticism

Reactions to the QBist interpretation have ranged from enthusiastic to strongly negative. On the supportive side, Theodor Hänsch has characterized QBism as sharpening older Copenhagen-type views and making them more consistent. Some who have criticized QBism claim that it fails to meet the goal of resolving paradoxes in quantum theory. Bacciagaluppi argues that QBism's treatment of measurement outcomes does not ultimately resolve the issue of nonlocality, and Jaeger finds QBism's supposition that the interpretation of probability is key for the resolution to be unnatural and unconvincing. Norsen has accused QBism of solipsism, and Wallace identifies QBism as an instance of instrumentalism; QBists have argued insistently that these characterizations are misunderstandings, and that QBism is neither solipsist nor instrumentalist. A critical article by Nauenberg in the American Journal of Physics prompted a reply by Fuchs, Mermin, and Schack.
Some assert that there may be inconsistencies; for example, Stairs argues that when a probability assignment equals one, it cannot be a degree of belief as QBists say. Further, while also raising concerns about the treatment of probability-one assignments, Timpson suggests that quantum-Bayesian approaches may result in a reduction of explanatory power as compared to other interpretations. Fuchs and Schack replied to these concerns in a later article. Mermin advocated QBism in a 2012 Physics Today article, which prompted considerable discussion. Several further critiques of QBism which arose in response to Mermin's article, and Mermin's replies to these comments, may be found in the Physics Today readers' forum. Section 2 of the Stanford Encyclopedia of Philosophy entry on QBism also contains a summary of objections to the interpretation, and some replies. Ballentine argues that "the initial assumption of QBism is not valid" because the inferential probability of Bayesian theory used by QBism is not applicable to quantum mechanics. Others are opposed to QBism on more general philosophical grounds; for example, Mohrhoff criticizes QBism from the standpoint of Kantian philosophy.
Certain authors find QBism internally self-consistent, but do not subscribe to the interpretation. For example, Marchildon finds QBism well-defined in a way that, to him, many-worlds interpretations are not, but he ultimately prefers a Bohmian interpretation. 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. In addition, some agree with most, but perhaps not all, of the core tenets of QBism; Barnum's position, as well as Appleby's, are examples.
Popularized or semi-popularized media coverage of QBism has appeared in New Scientist, ''Scientific American, Nature, Science News, the FQXi Community, the Frankfurter Allgemeine Zeitung, Quanta Magazine, Aeon, Discover, Nautilus Quarterly, and Big Think. 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. Furthermore, Harvard University Press published von Baeyer's popularized treatment of the subject, QBism: The Future of Quantum Physics'', in 2016.
The philosophy literature has also discussed QBism from the viewpoints of structural realism and of phenomenology.