Information Causality is the Unique Optimality Principle: Quantum Mechanics as the IC-Optimal Theory

We prove that quantum mechanics is the unique optimal physical theory under Information Causality (IC): it is the only rotationally covariant bipartite correlation theory that both respects IC and maximises non-local coordination. Concretely, among all IC-admissible positive semi-definite (PSD) kernels E(d) = −∑ₗ≥₁ aₗPₗ(cos d) with aₗ ≥ 0, the quantum correlation E(d) = −cos d is the unique maximiser of the CHSH value. The proof combines the Eigenvector Protocol (IC implies aₗ ≥ 0) with the Tsirelson uniqueness result into a single optimality statement. No second independent axiom is required: Tsirelson saturation is not an additional assumption but a consequence of optimality under IC. This provides the tightest known single-principle characterisation of quantum correlations, establishing that quantum mechanics is not merely consistent with IC — it is the optimal realisation of IC.