Alice holds an unknown qubit state and one particle of a pre-shared entangled pair. She performs a joint Bell-basis measurement on both, which yields 2 random classical bits and destroys the original state at her end (no-cloning theorem).
- Entanglement distributes instantly — but a raw entangled particle carries no usable information by itself, only correlation.
- The 2 classical bits must physically travel to Bob through an ordinary channel, capped at light-speed. Only once they arrive can Bob apply the matching correction (I, X, Z or XZ) to recover the original state.
- Try "Force early correction": Bob guesses before the bits land — the result is wrong, proving this is not faster-than-light signaling.