Quantum process tomography of a controlled-NOT gate
  We demonstrate complete characterization of a two-qubit entangling process - a linear optics controlled-NOT gate operating with coincident detection - by quantum process tomography. We use maximum-likelihood estimation to convert the experimental data into a physical process matrix. The process matrix allows accurate prediction of the operation of the gate for arbitrary input states, and calculation of gate performance measures such as the average gate fidelity, average purity and entangling capability of our gate, which are 0.90, 0.83 and 0.73, respectively. 
J. L. O'Brien, G. J. Pryde, A. Gilchrist, D. F. V. James, N. K.   Langford, T. C. Ralph and A. G. White