Bell State

Bell states are the four maximally entangled states of two qubits, named after physicist John Bell. They are: |Φ+⟩ = (|00⟩+|11⟩)/√2, |Φ-⟩ = (|00⟩−|11⟩)/√2, |Ψ+⟩ = (|01⟩+|10⟩)/√2, |Ψ-⟩ = (|01⟩−|10⟩)/√2. The Bell state |Φ+⟩ is created by applying a Hadamard gate to qubit 0 then a CNOT gate with qubit 0 as control and qubit 1 as target. When measured, both qubits always return the same value (both 0 or both 1 with equal probability) — this correlation is the signature of entanglement. Bell states are used in quantum teleportation, quantum key distribution (BB84, E91), superdense coding, and entanglement verification (Bell inequality tests). "Hello World" quantum programs typically create and measure a Bell state.