  Broadband energy-time entanglement can be used to enhance the rate of
two-photon absorption (TPA) by combining a precise two-photon resonance with a
very short coincidence time. Because of this short coincidence time, broadband
TPA is not sensitive to the spectrum of intermediate levels, making it the
optimal choice when the intermediate transitions are entirely virtual. In the
case of distinct intermediate resonances, it is possible to enhance TPA by
introducing a phase dispersion that matches the intermediate resonances. Here,
we consider the effects of a phase flip in the single photon spectrum, where
the phases of all frequencies above a certain frequency are shifted by half a
wavelength relative to the frequencies below this frequency. The frequency at
which the phase is flipped can then be scanned to reveal the position of
intermediate resonances. We find that a resonant phase flip maximizes the
contributions of the asymmetric imaginary part of the dispersion that
characterizes a typical resonance, resulting in a considerable enhancement of
the TPA rate. Due to the bosonic symmetry of TPA, the enhancement is strongest
when the resonance occurs when the frequency difference of the two photons is
much higher than the linewidth of the resonance. Our results indicate that
broadband entangled TPA with spectral phase flips may be suitable for
phase-sensitive spectroscopy at the lower end of the spectrum where direct
photon detection is difficult.
