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Broadband energy-time entanglement can be used to enhance the rate oftwo-photon absorption (TPA) by combining a precise two-photon resonance with avery short coincidence time. Because of this short coincidence time, broadbandTPA is not sensitive to the spectrum of intermediate levels, making it theoptimal choice when the intermediate transitions are entirely virtual. In thecase of distinct intermediate resonances, it is possible to enhance TPA byintroducing a phase dispersion that matches the intermediate resonances. Here,we consider the effects of a phase flip in the single photon spectrum, wherethe phases of all frequencies above a certain frequency are shifted by half awavelength relative to the frequencies below this frequency. The frequency atwhich the phase is flipped can then be scanned to reveal the position ofintermediate resonances. We find that a resonant phase flip maximizes thecontributions of the asymmetric imaginary part of the dispersion thatcharacterizes a typical resonance, resulting in a considerable enhancement ofthe TPA rate. Due to the bosonic symmetry of TPA, the enhancement is strongestwhen the resonance occurs when the frequency difference of the two photons ismuch higher than the linewidth of the resonance. Our results indicate thatbroadband entangled TPA with spectral phase flips may be suitable forphase-sensitive spectroscopy at the lower end of the spectrum where directphoton detection is difficult.

Enhancement of broadband entangled two-photon absorption by resonant spectral phase flips