Distinguishability and "which pathway" information in multidimensional interferometric spectroscopy with a single entangled photon-pair.

Correlated photons inspire abundance of metrology-related platforms, which benefit from quantum (anti-) correlations and outperform their classical-light counterparts. While such demonstrations mainly focus on entanglement, the role of photon exchange-phase and degree of distinguishability have not been widely utilized in quantum-enhanced applications. Using an interferometric setup we show that even at low degree entanglement, when a two-photon wave-function is coupled to matter, it is encoded with a reliable "which pathway?" information. An interferometric exchange-phase-cycling protocol is developed, which enables phase-sensitive discrimination between microscopic interaction histories (pathways). We find that quantum-light interferometry facilitates utterly different set of time-delay variables, which are unbound by uncertainty to the inverse bandwidth of the wave-packet. We illustrate our findings on an exciton model-system, and demonstrate how to probe intraband dephasing in time-domain without temporal resolution at the detection. The exotic scaling of multiphoton coincidence with respect to the applied intensity is discussed.