Quantum birthday problems: geometrical aspects of quantum random coding

This article explores the asymptotics of randomly generated vectors on extended Hilbert spaces. In particular, we are interested to know how "orthogonal" these vectors are. We investigate two types of asymptotic orthogonality, the weak orthogonality and the strong orthogonality, that are regarded as quantum analogs of the classical birthday problem and its variant. As regards the weak orthogonality, a new characterization of the von Neumann entropy is derived, and a mechanism behind the noiseless quantum channel coding theorem is clarified. As regards the strong orthogonality, on the other hand, a characterization of the quantum Renyl (1965) entropy of degree 2 is derived.