Von Neumann Entropy and Quantum Algorithmic Randomness

Abstract

A state $\rho=(\rho_n)_{n=1}^{\infty}$ is a sequence such that $\rho_n$ is a density matrix on $n$ qubits. It formalizes the notion of an infinite sequence of qubits. The von Neumann entropy $H(d)$ of a density matrix $d$ is the Shannon entropy of its eigenvalue distribution. We show: (1) If $\rho$ is a computable quantum Schnorr random state then $\lim_n [H(\rho_n )/n] = 1$. (2) We define quantum s-tests for $s\in [0,1]$, show that $\liminf_n [H(\rho_n)/n]\geq \{ s: \rho$ is covered by a quantum s-test $\}$ for computable $\rho$ and construct states where this inequality is an equality. (3) If $\exists c \exists^\infty n H(\rho_n)> n-c$ then $\rho$ is strong quantum random. Strong quantum randomness is a randomness notion which implies quantum Schnorr randomness relativized to any oracle. (4) A computable state $(\rho_n)_{n=1}^{\infty}$ is quantum Schnorr random iff the family of distributions of the $\rho_n$'s is uniformly integrable. We show that the implications in (1) and (3) are strict.