Separable Counterexamples to Complementary Quantum Correlations, and Why Random Search Missed Them

Abstract

The complementary quantum correlations (CQC) relation bounds the sum of two classical mutual informations, obtained from local mutually unbiased measurements, by the quantum mutual information of the premeasurement state. We refute it. Separable rank-two counterexamples exist in every local dimension pair \(m\times n\) with \(m,n\ge3\), with closed-form excess at least \(1/(8m^2n^2)\) nats, and in every qubit--qudit pair \(2\times n\) with \(n\ge3\) except \(n=3,5\); every covered pair also admits full-rank separable counterexamples. We then analyse the two residual qubit--qudit dimensions. A dimension-free entropy envelope replaces the natural quadratic majorant and lowers the requirement for closing the equal-prior orthogonal two-ray family from a triangular-discrimination bound \(S\le8/3\) to \(S\le3.8265583\ldots\); the associated gate matrix has trace exactly two, so its spectral test collapses to a single eigenvalue-free scalar; and the exact identity \(X=1-4\operatorname{Var}(c)\) turns the prime-Fourier full-spark barrier into a variance bound. A \(128\)-bit interval cover then closes that family at \(2\times3\) with gap at least \(0.012021\) nats, and at \(2\times5\) an exact saturator attaining \(S=(14+2\sqrt5)/5\) refutes three competing routes. We also give a state-dependent corrected inequality that is universal, and show it is incomparable with CQC already at \(2\times2\). Finally we quantify why the original searches had essentially no power to find these states: the violating set is a sliver against the low-rank boundary, the witness lies \(7.3\) standard deviations below the Hilbert--Schmidt mean, the bases must be aligned to about nine degrees (\(\sim10^{-21}\) of frames), and extrapolating the sample minimum demands \(10^{10}\) to \(10^{18}\) samples against the \(10^7\) ever run.