Molecular vibration spectroscopy with undetected photons

We present a novel technique of molecular vibration spectroscopy with undetected photons (MSUP) with dual stimulated parametric down-conversion crystals. In our MSUP, quantum coherence between independent signal photons is induced by the path indistinguishability of the conjugate idler single-photon-added coherent states and results in perfect visibility under the same pump and seed beam intensities. If the seed beam intensity is imbalanced by the sample absorption, the modulated interference fringe reveals the absorption spectrum as a function of seed beam frequency. As a proof-of-principle experiment, a rovibrational absorption spectrum of hydrogen cyanide (H¹³C¹⁴N) molecules in a gas cell at approximately 1550 nm is successfully measured by analyzing the single-photon interference fringe of the signal field at 807 nm.