Complete Characterization of a Quantum Process: the Two-Bit Quantum Gate
  We show how to fully characterize a quantum process in an open quantum system. We particularize the procedure to the case of a universal two-qubit gate in a quantum computer. We illustrate the method with a numerical simulation of a quantum gate in the ion trap quantum computer. 
J. F. Poyatos, J. I. Cirac (Departamento de Fisica Aplicada,   Universidad de Castilla--La Mancha, Ciudad Real, Spain), P. Z...