In his recent paper, C. Fuchs formulates QBism in the form of eight postulates. We criticise QBism as an anti-realist position and propose an alternative – contextual quantum realism (QCR). 1. A quantum state is not “an agent’s personal judgement” (QBism), nor is it subjective (QBism), but objective (QCR). It describes not the current experience (QBism), but a state of a physical system in context (QCR). 2. A quantum measurement is a (literally) measurement of quantum (...) reality (QCR), rather than an agent’s action upon its external world (QBism). It can only be regarded as an action in the sense in which a cognitive Wittgensteinian language game is an action (QCR). 3. The result of a quantum meas- urement is objective, though context-sensitive (QCR), rather than subjective (QBism), personal to the agent performing the action (QBism). 4. The quantum formalism is normative (QCR and QBism) and at the same time descriptive (QCR). The wave function tells us what to expect and how a quantum experiment should be conducted (i.e. plays the role of a norm), and also describes the state of a quantum system in context (QCR). 5. Unitary evolution is objective (QCR), not subjective (QBism). It does not express an agent’s degrees of belief (QBism). 6. Probability 1 is ontic (QCR), not an agent’s maximum degree of subjective certainty without ontic content (QBism). 7. In general, measurement outcomes are not predetermined (QCR and QBism), i.e. “unperformed measurements have no outcomes” (for QCR, this is an analytical judgement; for QBism this is a thesis), but they are predeter- mined in the case of probabilities 1 and 0 (QCR). In the case of probabilities 0 and 1, one can speak of performed measurements (QCR). 8. Quantum theory is a universal Wittgen- steinian rule (norm), i.e. a rule (norm) rooted in experience, reality (QCR). It can be used by any competent subject (QCR and QBism). We illustrate the difference between QCR and QBism on the example of how they treat “Wigner’s friend” thought experiment and consider their attitude to phenomenology. (shrink)

The purpose of this book is to explain Quantum Bayesianism (‘QBism’) to “people without easy access to mathematical formulas and equations” (4-5). Qbism is an interpretation of quantum mechanics that “doesn’t meddle with the technical aspects of the theory [but instead] reinterprets the fundamental terms of the theory and gives them new meaning” (3). The most important motivation for QBism, enthusiastically stated on the book’s cover, is that QBism provides “a way past quantum theory’s paradoxes and (...) puzzles” such that much of the weirdness associated with quantum theory “dissolves under the lens of QBism”. (shrink)

According to QBism, quantum states, unitary evolutions, and measurement operators are all understood as personal judgments of the agent using the formalism. Meanwhile, quantum measurement outcomes are understood as the personal experiences of the same agent. Wigner’s conundrum of the friend, in which two agents ostensibly have different accounts of whether or not there is a measurement outcome, thus poses no paradox for QBism. Indeed the resolution of Wigner’s original thought experiment was central to the development of QBist (...) thinking. The focus of this paper concerns two very instructive modifications to Wigner’s puzzle: One, a recent no-go theorem by Frauchiger and Renner, and the other a thought experiment by Baumann and Brukner. We show that the paradoxical features emphasized in these works disappear once both friend and Wigner are understood as agents on an equal footing with regard to their individual uses of quantum theory. Wigner’s action on his friend then becomes, from the friend’s perspective, an action the friend takes on Wigner. Our analysis rests on a kind of quantum Copernican principle: When two agents take actions on each other, each agent has a dual role as a physical system for the other agent. No user of quantum theory is more privileged than any other. In contrast to the sentiment of Wigner’s original paper, neither agent should be considered as in “suspended animation.” In this light, QBism brings an entirely new perspective to understanding Wigner’s friend thought experiments. (shrink)

[issue 20211210] Qbism (quantum bayesism) is a philosophical interpretation of quantum mechanics (QM) that places the agent and its expectations at the heart of theory. The QBists advocate a "subjectivist" interpretation of probabilities that allows to understand the quantum laws of Born and to eliminate certain enigmas of interpretation of the QM going "beyond" the interpretation of Copenhagen. The Ontology of Knowledge (OK) is in agreement with the main ideas of the Qbism. For the OdC indeed: -The agent (...) is the focal point of the representation -Representation is specific to the agent -There are no "states of the world" but only "states of knowledge" -There is no probability of evolution of the "state of the world" but only probabilities, for the subject, of evolution of his Knowledge. -These probabilities lead to the future actions and experiences of the agent. The aim of this article is to propose, according to the OdC, ways for an extension of the explanatory power of the QBism. (shrink)

The following article will attempt to highlight four questions which, in my opinion, are left unanswered (or overlooked) by QBism and to show the answers that the Ontology of Knowledge (OK) can provide. ● How does the subject come to exist for itself, individuated and persistent? ● From what common reality do world, mind, and meaning emerge? ● How does meaning emerge from the mathematical fact of probabilistic expectation? ● Is meaning animated by its own nature?

In the Bayesian approach to quantum mechanics, probabilities—and thus quantum states—represent an agent’s degrees of belief, rather than corresponding to objective properties of physical systems. In this paper we investigate the concept of certainty in quantum mechanics. Particularly, we show how the probability-1 predictions derived from pure quantum states highlight a fundamental difference between our Bayesian approach, on the one hand, and Copenhagen and similar interpretations on the other. We first review the main arguments for the general claim that probabilities (...) always represent degrees of belief. We then argue that a quantum state prepared by some physical device always depends on an agent’s prior beliefs, implying that the probability-1 predictions derived from that state also depend on the agent’s prior beliefs. Quantum certainty is therefore always some agent’s certainty. Conversely, if facts about an experimental setup could imply agent-independent certainty for a measurement outcome, as in many Copenhagen-like interpretations, that outcome would effectively correspond to a preexisting system property. The idea that measurement outcomes occurring with certainty correspond to preexisting system properties is, however, in conflict with locality. We emphasize this by giving a version of an argument of Stairs [(1983). Quantum logic, realism, and value-definiteness. Philosophy of Science, 50, 578], which applies the Kochen–Specker theorem to an entangled bipartite system. (shrink)

The appearance of negative terms in quasiprobability representations of quantum theory is known to be inevitable, and, due to its equivalence with the onset of contextuality, of central interest in quantum computation and information. Until recently, however, nothing has been known about how much negativity is necessary in a quasiprobability representation. Zhu :120404, 2016) proved that the upper and lower bounds with respect to one type of negativity measure are saturated by quasiprobability representations which are in one-to-one correspondence with the (...) elusive symmetric informationally complete quantum measurements. We define a family of negativity measures which includes Zhu’s as a special case and consider another member of the family which we call “sum negativity.” We prove a sufficient condition for local maxima in sum negativity and find exact global maxima in dimensions 3 and 4. Notably, we find that Zhu’s result on the SICs does not generally extend to sum negativity, although the analogous result does hold in dimension 4. Finally, the Hoggar lines in dimension 8 make an appearance in a conjecture on sum negativity. (shrink)

In the quantum-Bayesian approach to quantum foundations, a quantum state is viewed as an expression of an agent’s personalist Bayesian degrees of belief, or probabilities, concerning the results of measurements. These probabilities obey the usual probability rules as required by Dutch-book coherence, but quantum mechanics imposes additional constraints upon them. In this paper, we explore the question of deriving the structure of quantum-state space from a set of assumptions in the spirit of quantum Bayesianism. The starting point is the representation (...) of quantum states induced by a symmetric informationally complete measurement or SIC. In this representation, the Born rule takes the form of a particularly simple modification of the law of total probability. We show how to derive key features of quantum-state space from (i) the requirement that the Born rule arises as a simple modification of the law of total probability and (ii) a limited number of additional assumptions of a strong Bayesian flavor. (shrink)

It is proposed a critique of existing interpretations of quantum mechanics, both anti-realistic and realistic, and, in particular, the Copenhagen interpretation, the interpretations with hidden variables, the metaphysical interpretation of H. Everett’s interpretation, the many-worlds interpretation by D. Wallace, QBism by C. Fuchs, D. Mermin and R. Schack, the relational interpretation by C. Rovelli, neo-Kantian and phenomenological interpretations by M. Bitbol, the informational interpretation by A. Zeilinger, the Nobel Prize Winner in Physics 2022, and others. As is known compared (...) to classical physics, quantum mechanics is a new physical paradigm. The author argues that from a philosophical point of view quantum mechanics is an anomaly within the paradigm of (post)modern philosophy. A new approach to understanding quantum mechanics and resolving its paradoxes, rejecting the assumptions and prejudices of (post)modern philosophy, is proposed – contextual quantum realism (CQR). It is a moderately deflationary approach that combines elements of (neo-)Copenhagen, informational and realist interpretations and recognizes the possibility of knowing the things themselves. From the point of view of CQR, the measurement problem – the basic problem of philosophy of quantum mechanics – is a pseudo-problem . (shrink)

The probabilities a Bayesian agent assigns to a set of events typically change with time, for instance when the agent updates them in the light of new data. In this paper we address the question of how an agent's probabilities at different times are constrained by Dutch-book coherence. We review and attempt to clarify the argument that, although an agent is not forced by coherence to use the usual Bayesian conditioning rule to update his probabilities, coherence does require the agent's (...) probabilities to satisfy van Fraassen's [1984] reflection principle (which entails a related constraint pointed out by Goldstein [1983]). We then exhibit the specialized assumption needed to recover Bayesian conditioning from an analogous reflection-style consideration. Bringing the argument to the context of quantum measurement theory, we show that "quantum decoherence" can be understood in purely personalist terms---quantum decoherence (as supposed in a von Neumann chain) is not a physical process at all, but an application of the reflection principle. From this point of view, the decoherence theory of Zeh, Zurek, and others as a story of quantum measurement has the plot turned exactly backward. (shrink)

A book review of _Phenomenological Approaches to Physics_ (2020) edited by H. A. Wiltsche and P. Berghofer.