When will two agents agree on a quantum measurement outcome? Intersubjective agreement in QBism

TL;DR

This paper defends QBism's view that quantum measurement outcomes are personal and not necessarily mutually consistent, clarifying misconceptions about intersubjectivity and proposing conditions for agreement between agents.

Contribution

It refutes Khrennikov's claim by clarifying that QBism does not require agents to agree on measurement outcomes and discusses conditions for intersubjective agreement.

Findings

Khrennikov's claim is based on an additional assumption, not Ozawa's theorem.

Agents can create conditions for measurement outcome agreement in QBism.

Ozawa's assumptions can serve as a basis for intersubjective agreement.

Abstract

In the QBist approach to quantum mechanics, a measurement is an action an agent takes on the world external to herself. A measurement device is an extension of the agent and both measurement outcomes and their probabilities are personal to the agent. According to QBism, nothing in the quantum formalism implies either that the quantum state assignments of two agents or their respective measurement outcomes need to be mutually consistent. Recently, Khrennikov has claimed that QBism's personalist theory of quantum measurement is invalidated by Ozawa's so-called intersubjectivity theorem. Here, following Stacey, we refute Khrennikov's claim by showing that it is not Ozawa's mathematical theorem but an additional assumption made by Khrennikov that QBism is incompatible with. We then address the question of intersubjective agreement in QBism more generally. Even though there is never a necessity for two agents to agree on their respective measurement outcomes, a QBist agent can strive to create conditions under which she would expect another agent's reported measurement outcome to agree with hers. It turns out that the assumptions of Ozawa's theorem provide an example for just such a condition.