Mutually unbiased bases as extremal probes of isotropic random Hamiltonians

Abstract

Mutually unbiased bases (MUBs) are central finite structures in quantum information. We ask whether complete MUB systems have an extremal probing property beyond their projective \(2\)-design identities. For an isotropic Gaussian traceless Hamiltonian, we prove that, among labeled unions of \(d+1\) orthonormal bases, a complete MUB union has the stochastically largest sampled maximum. Each basis induces the same regular-simplex Gaussian block; mutual unbiasedness eliminates cross-block covariance, joint Gaussianity yields independence, and a centered-convex Gaussian correlation inequality makes this independent coupling extremal. We also derive a radial-mixture extension and prove exact MUB-family collapse for fully matched diagonal-cost constructions.