QBism

QBism, short for Quantum Bayesianism, is an interpretation of quantum mechanics. It proposes that quantum states do not represent objective properties of the physical world, but rather subjective degrees of belief held by an agent. The probabilities in quantum mechanics are therefore seen as expressions of an individual's uncertainty about the outcomes of future measurements. This perspective contrasts with many other interpretations that ascribe to quantum states a more direct description of reality independent of an observer.

Key to QBism is the concept of an agent. The agent is the one who experiences and makes predictions about the world. When an agent makes a measurement, they update their beliefs based on the outcome. This update process is governed by the rules of quantum mechanics, but the focus is on the agent's personal experience and information. Therefore, quantum probabilities are personal and context-dependent, rather than being inherent properties of systems themselves.

QBism also emphasizes the role of action. Agents use their beliefs to guide their actions in the world, aiming to achieve desired outcomes. Quantum theory, from this viewpoint, provides a tool for agents to navigate the world more effectively by helping them make better predictions and thus take better actions. The 'quantum' aspect refers to the specific mathematical formalism used to describe these beliefs and predictions, which is quantum probability theory.