Complementary classical fidelities as an efficient criterion for the   evaluation of experimentally realized quantum operations
  It is shown that a good estimate of the fidelity of an experimentally realized quantum process can be obtained by measuring the outputs for only two complementary sets of input states. The number of measurements required to test a quantum network operation is therefore only twice as high as the number of measurements required to test a corresponding classical system. 
Holger F. Hofmann