  The existence of universal quantum computers has been theoretically well
established. However, building up a real quantum computer system not only
relies on the theory of universality, but also needs methods to satisfy
requirements on other features, such as programmability, modularity,
scalability, etc. To this end, we study the recently proposed model of quantum
von Neumann architecture, by putting it in a practical and broader setting,
namely, the hierarchical design of a computer system. We analyze the structures
of quantum CPU and quantum control unit, and draw their connections with
computational advantages. We also point out that a recent demonstration of our
model would require less than 20 qubits.
