  An essential quantity in quantum information theory is the von Neumann
entropy which depends entirely on the quantum density operator. Once known, the
density operator reveals the statistics of observables in a quantum process,
and the corresponding von Neumann Entropy yields the full information content.
However, the state, or density operator, of a given system may be unknown.
Quantum state observers have been proposed to infer the unknown state of a
quantum system. In this note, we show (i) that the von Neumann entropy of the
state estimate produced by our quantum state observer is exponentially
convergent to that of the system's true state, and (ii) the relative entropy
between the system and observer's state converges exponentially to zero as long
as the system starts in a full-rank state.
