Entangled biphoton virtual-state spectroscopy of the A2Σ+–X2Π system of OH

This Letter describes the first application of entanglement-induced virtual-state spectroscopy to a molecular system. Non-classical, non-monotonic behavior in the two-photon absorption cross-section of the OH A–X system, induced by entangled photon pairs is theoretically demonstrated. A Fourier transform analysis of the biphoton cross-section permits access to the energy eigenvalues of intermediate rovibronic states with a fixed excitation energy. Our analysis reveals that the implementation of virtual-state spectroscopy of the OH A–X system requires tuning of the relative path delay τe over a picosecond range with femtosecond resolution for a certain entanglement time Te.