The state of a multi-qubit system can either be a product state or an entangled state. In this video, David explains the very puzzling feature of quantum entanglement and how to quantify it. He also tells us about the Bell states and how they form a basis for all two-qubit states.

• A multi-qubit product state is such that it can be expressed as the tensorial product of the state of each of the qubits in the system.
• An entangled state |ψ⟩  cannot be expressed as a product state: |ψ⟩≠|a⟩⊗|b⟩
• The four Bell states are a set of entangled and orthogonal  states that form a basis for any two-qubit state.