Universal control of decoupled quantum systems

We show that if one can perform a restricted set of fast manipulations on a quantum system, one can implement a large class of dynamical evolutions by effectively removing or introducing selected Hamiltonians. The procedure c an be used to achieve universal noise-tolerant control based on purely unit ary open-loop transformations of the dynamics. As a result, it is in princi ple possible to perform noise-protected universal quantum computation using no extra space resources.