Optimal quantum subtracting machine

The impossibility of undoing a mixing process is analyzed 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 parametrize 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 nontrivial cases, we computed the analytical solution, comparing the performance of the optimal machine with other protocols.