Global phase¶

Global phase multiplies a quantum state by e^(i*phi) without changing measurement probabilities.

Why it is unobservable in single-state measurement¶

Expectation values cancel global phase factors:

<e^(i*phi)psi| A |e^(i*phi)psi> = <psi|A|psi>

Global phase can become relative phase in composite systems, where it can affect measurable outcomes.