  In virtual-state spectroscopy, information about the energy-level structure
of an arbitrary sample is retrieved by Fourier transforming sets of measured
two-photon absorption probabilities of entangled photon pairs where the degree
of entanglement and the delay time between the photons have been varied. This
works well for simple systems but quickly becomes rather difficult when many
intermediate states are involved. We propose and discuss an extension of
entangled two-photon absorption spectroscopy that solves this problem by means
of repeated measurements at different pump wavelengths. Specifically, we
demonstrate that our extension works well for a variety of realistic
experimental setups.
