  By utilizing single particle interferometry, the fidelity or coherence of a
pair of quantum states is identified with their capacity for interference. We
consider processes acting on the internal degree of freedom (e.g., spin or
polarization) of the interfering particle, preparing it in states \rho_{A} or
\rho_{B} in the respective path of the interferometer. The maximal visibility
depends on the choice of interferometer, as well as the locality or
non-locality of the preparations, but otherwise depends only on the states
\rho_{A} and \rho_{B} and not the individual preparation processes themselves.
This allows us to define interferometric measures which probe locality and
correlation properties of spatially or temporally separated processes, and can
be used to differentiate between processes that cannot be distinguished by
direct process tomography using only the internal state of the particle.
