Quantum process tomography of a single solid state qubit
  We present an example of quantum process tomography performed on a single solid state qubit. The qubit used is two energy levels of the triplet state in the Nitrogen-Vacancy defect in Diamond. Quantum process tomography is applied to a qubit which has been allowed to decohere for three different time periods. In each case the process is found in terms of the $\chi$ matrix representation and the affine map representation. The discrepancy between experimentally estimated process and the closest physically valid process is noted. 
M. Howard, J. Twamley, C. Wittmann, T. Gaebel, F. Jelezko and J.   Wrachtrup