Local indistinguishability of Bell-state phase
= Local indistinguishability of Bell-state phase
The Bell states $|\Phi^+\rangle=(|00\rangle+|11\rangle)/\sqrt2$ and $|\Phi^-\rangle=(|00\rangle-|11\rangle)/\sqrt2$ both have reduced state $I/2$ on either qubit. No measurement on one qubit alone can distinguish their relative sign.