  We show that the discrete-time evolution of an open quantum system generated
by a single quantum channel $T$ can be embedded in the discrete-time evolution
of an enlarged closed quantum system, i.e. we construct a unitary dilation of
the discrete-time quantum-dynamical semigroup $(T^n)_{n \in \mathbb N_0}$. In
the case of a cyclic channel $T$, the auxiliary space may be chosen (partially)
finite-dimensional. We further investigate discrete-time quantum control
systems generated by finitely many commuting quantum channels and prove a
similar unitary dilation result as in the case of a single channel.
