Constructing valid density matrices on an NMR quantum information processor via maximum likelihood estimation

• State estimation using maximum likelihood method was performed on an NMR quantum information processor.
• Physically valid density matrices were obtained every time in contrast to standard quantum state tomography.
• Density matrices of several different entangled and separable states were reconstructed for two and three qubits.

Estimation of quantum states is an important step in any quantum information processing experiment. A naive reconstruction of the density matrix from experimental measurements can often give density matrices which are not positive, and hence not physically acceptable. How do we ensure that at all stages of reconstruction, we keep the density matrix positive? Recently a method has been suggested based on maximum likelihood estimation, wherein the density matrix is guaranteed to be positive definite. We experimentally implement this protocol on an NMR quantum information processor. We discuss several examples and compare with the standard method of state estimation.