It is difficult to manipulate an entire large quantum system directly. When we try to do so, huge noise will be injected into the system. However, if we can indirectly control the system via a restricted part of it, we will be able to suppress the injected noise. In this research, we investigated the effect of the restriction mathematically, and succeeded in completely categorizing the set of operations in the case of indirect control. This result indicates that, if the degree of the freedom of the controllable part is more than two, we can universally control the whole quantum system in effect even when the degree of freedom of the uncontrollable part is very large. This knowledge provides a new strategy for constructing a noise-less quantum computer or any other noise-less device for quantum information processing. If we can construct such a device, we can realize quantum information processing, e.g. factorization of huge numbers with a quantum computer.
/ Computing Theory Research Group, Media Information Laboratory