Fragile Entanglement: Fidelity in the [[4,2,2]] Code

A cinematic quantum-laboratory explainer follows four qubits initialized in |0000⟩ through the right-to-left sequence H₂ → CNOT₂→₁ → H₀ → CNOT₀→₃, producing two Bell pairs: |Φ⁺⟩₀₃ ⊗ |Φ⁺⟩₁₂. Independent two-qubit depolarizing noise follows each CNOT; XX, YY, and ZZ preserve Bell-state overlap while the other 12 Pauli errors produce orthogonal states. The result is the raw, unconditioned physical-state fidelity Fphysical(p) = (1−4p/5)²—without measurement, syndrome detection, ancillas, or post-selection.