  Quantum state tomography (QST) is a universal tool for the design and
optimization of entangled-photon sources. It typically requires single-photon
detectors and coincidence measurements. Recently, it was suggested that the
information provided by the QST of photon pairs generated by spontaneous
parametric down-conversion could be obtained by exploiting the stimulated
version of this process, namely difference frequency generation. In this
protocol, so-called "stimulated-emission tomography" (SET), a seed field is
injected along with the pump pulse, and the resulting stimulated emission is
measured. Since the intensity of the stimulated field can be several orders of
magnitude larger than the intensity of the corresponding spontaneous emission,
measurements can be made with simple classical detectors. Here, we
experimentally demonstrate SET and compare it with QST. We show that one can
accurately reconstruct the polarization density matrix, and predict the purity
and concurrence of the polarization state of photon pairs without performing
any single-photon measurements.
