Optimal discrimination of single-qubit mixed states

We consider the problem of minimum-error quantum state discrimination for single-qubit mixed states. We present a method which uses the Helstrom conditions constructively and analytically; this algebraic approach is complementary to existing geometric methods, and solves the problem for any number of arbitrary signal states with arbitrary prior probabilities. It has long been known that the minimum-error probability is given by the trace of the Lagrange operator Γ. The remarkable feature of our approach is the central role played not by Γ, but by its inverse.