Much of the theory of quantum computing assumes the capacity to apply a chosen sequence of unitary transformations
to the state of a quantum register (sometimes called a memory). It is widely recognized that this
"application of a unitary transformation" requires an external influence. Here we relate the physics of external
influences to the well established framework of quantum-mechanical scattering problems, in order to show how
scattering is conceptually necessary to quantum computers, even in the idealization of zero temperature and no
For a single-qubit quantum register, infinitely slow limiting cases are shown in which scattering indeed results
in a unitary transformation of the register. Implications for "transformation-induced decoherence" are developed
and related to questions of errors and error correction.