Paired Return of Phase and Excitation in a Four-Port Quantum Contact

We study what can be inferred from a coherent two-copy return on a qubit code. An antisymmetric input reduces the return operator to its determinant. A fixed measurement recovers the mean complex amplitude and the joint code-return probability while retaining loss. For diagonal masks whose single-pass ensemble is a complete dephasing, we derive a dimension-dependent lower bound on incorrect-pair return. A four-port tetrahedral construction attains the bound 1/3 on every incorrect pair and every input. We construct exact counterexamples to recovery of coherence from excitation counts and to calibration without a loss standard. An independent one-copy measurement, implemented either with a spectator qubit or four unentangled inputs, quantifies the deviation of the successful operation from a target isometry. The accompanying matrix calculations, circuit replays and prospective statistical tests specify how to distinguish the modeled alternatives. The results concern declared finite-dimensional models. No laboratory data are used.