Selective and Efficient Quantum State Tomography and its Application to   Quantum Process Tomography
  We present a method for quantum state tomography that enables the efficient estimation, with fixed precision, of any of the matrix elements of the density matrix of a state, provided that the states from the basis in which the matrix is written can be efficiently prepared in a controlled manner. Furthermore, we show how this algorithm is well suited for quantum process tomography, enabling to perform selective and efficient quantum process tomography. 
Ariel Bendersky, Juan Pablo Paz
quant-ph