  The impossibility of undoing a mixing process is analysed in the context of
quantum information theory. The optimal machine to undo the mixing process is
studied in the case of pure states, focusing on qubit systems. Exploiting the
symmetry of the problem we parametrise the optimal machine in such a way that
the number of parameters grows polynomially in the size of the problem. This
simplification makes the numerical methods feasible. For simple but non-trivial
cases we computed the analytical solution, comparing the performance of the
optimal machine with other protocols.
