Bell states are the four maximally entangled two-qubit states. They form a complete orthonormal basis for two-qubit Hilbert space and are fundamental to quantum communication, quantum cryptography, and quantum teleportation.

1. Bell 00 (|Φ⁺⟩): $|\Phi^+\rangle = \frac{1}{\sqrt{2}}(|00\rangle + |11\rangle)$
   • Correlated outcomes (00 or 11)
   • Eigenstate of $Z_1 Z_2$ with eigenvalue +1
2. Bell 11 (|Φ⁻⟩): $|\Phi^-\rangle = \frac{1}{\sqrt{2}}(|00\rangle - |11\rangle)$
   • Correlated outcomes (00 or 11)
   • Eigenstate of $Z_1 Z_2$ with eigenvalue +1
3. Bell 01 (|Ψ⁺⟩): $|\Psi^+\rangle = \frac{1}{\sqrt{2}}(|01\rangle + |10\rangle)$
   • Anti-correlated outcomes (01 or 10)
   • Eigenstate of $Z_1 Z_2$ with eigenvalue -1
4. Bell 10 (|Ψ⁻⟩): $|\Psi^-\rangle = \frac{1}{\sqrt{2}}(|01\rangle - |10\rangle)$
   • Anti-correlated outcomes (01 or 10)
   • Eigenstate of $Z_1 Z_2$ with eigenvalue -1

All four Bell states are:

• Maximally entangled: non-factorable, represent maximum correlation
• Eigenstates of certain Pauli products: distinguishable via measurement operators
• Equal superposition: two basis states with equal amplitude
• Pure states: no mixing, zero purity loss

A Bell measurement distinguishes all four states by measuring two-qubit observables. This requires entangling the qubits via gates before measurement or using post-selected measurement schemes in photonic systems.

• Quantum teleportation: Bell pair enables teleportation of unknown qubit state
• Superdense coding: Bell pair allows sender to encode two classical bits by manipulating one qubit
• Quantum cryptography: Bell states used in E91 protocol for quantum key distribution
• Entanglement swapping: combining Bell states extends entanglement across quantum networks
• Bell inequality tests: Bell states violate classical inequalities demonstrating non-locality

Create Φ⁺ (most common):

1. Apply Hadamard to first qubit of $|00\rangle$: $\frac{1}{\sqrt{2}}(|0\rangle + |1\rangle) \otimes |0\rangle$
2. Apply CNOT with first as control: yields $|\Phi^+\rangle$

Other Bell states created by applying single-qubit rotations (Z or X gates) before or after CNOT.

Bell states are building blocks for larger entangled states. Two Bell pairs can be swapped to extend entanglement; Bell states generalize to GHZ, W, and cluster states for multi-qubit systems.