SQPT approaches this using d 2 linearly independent inputs ρ ρ j , where d is the dimension of the Hilbert space H . For each of these input states ρ ρ j , sending it through the process gives an output state E ( ρ ρ ) which can be written as a linear combination of the ρ ρ k , i.e. E ( ρ ρ j ) = ∑ ∑ k c j k ρ ρ k . By sending each ρ ρ j through many times, quantum state tomography can be used to determine the coefficients c j k experimentally.