Robust state preparation in a degenerate four-state system

A robust method utilizing a combination of optical pumping and a series of coherent excitation processes is developed for preparing any pure and a wide class of mixed quantum states in the decoherence-free ground-state subspace of a degenerate four-state system. For a given pulse sequence the same final state is obtained regardless of the initial state of the system. An example is presented where a pure state is prepared by a series of excitation processes in the two-dimensional dark subspace of the atom. (author)