  A method is proposed for preparing any pure and a wide class of mixed quantum
states in the decoherence-free ground-state subspace of a degenerate multilevel
lambda system. The scheme is a combination of optical pumping and a series of
coherent excitation processes, and for a given pulse sequence the same final
state is obtained regardless of the initial state of the system. The method is
robust with respect to the fluctuation of the pulse areas, like in adiabatic
methods, however, the field amplitude can be adjusted in a larger range.
