L6-09 · Quantum counterfactual questions

Learn¶

A classical counterfactual asks what would have occurred under another action for the same relevant background. It requires a structural model that connects observed and alternative worlds. Observational probabilities alone do not generally determine that connection, even before quantum systems are introduced.

Quantum alternatives can involve incompatible measurements and different disturbances. There is no universal joint classical record of all unperformed outcomes. A proposed quantum counterfactual must specify its semantics, available operations, conditioning event, and physical model. Several research approaches make different commitments.

Do not assign simultaneous definite values to all incompatible measurements without an explicit additional model. For an operational question, often an intervention probability is sufficient and better defined. Clearly distinguish a repeatable experimental prediction from a claim about an unperformed event in a particular past trial.

Practice¶

Check your understanding¶

What must accompany a quantum counterfactual claim?

• A. Only an observed frequency

• B. A statement that all measurements commute

• C. Explicit semantics and a physical model

Sources¶

This lesson draws on Leifer & Spekkens, 2013Allen et al., 2017.

Next: L6-10 · Design a quantum causal experiment