  We present an experimental characterization of the statistics of multiple
photon pairs produced by spontaneous parametric down-conversion realized in a
nonlinear medium pumped by high-energy ultrashort pulses from a regenerative
amplifier. The photon number resolved measurement has been implemented with the
help of a fiber loop detector. We introduce an effective theoretical
description of the observed statistics based on parameters that can be assigned
direct physical nterpretation. These parameters, determined for our source from
the collected experimental data, characterize the usefulness of down-conversion
sources in multiphoton interference schemes that underlie protocols for quantum
information processing and communication.
