However, for using the method in practice, the entire tomographic procedure must be scalable. Thus, we need to store the state in the computer in a scalable way. Clearly, the straightforward way of storing the N -qubit state in a 2 N × × 2 N density matrix is not scalable. However, ϱ ϱ P I is a blockdiagonal matrix due to its permutational invariance and thus it can be stored much more efficiently.