Entanglement

A correlation structure where a joint quantum state cannot be factored into states of its subsystems.

A two-qubit state is separable if it can be written as $|\psi\rangle = |a\rangle\otimes|b\rangle$ . If it cannot, it’s entangled.

Canonical example

The Bell state:

|\Phi^+\rangle = (|00\rangle + |11\rangle)/\sqrt{2}.