Quantum optical induced-coherence tomography by a hybrid interferometer

Quantum interferometry based on induced-coherence phenomena has demonstrated the possibility of undetected-photon measurements. Perturbation in the optical path of probe photons can be detected by interference signals generated by quantum mechanically correlated twin photons propagating through a different path, possibly at a different wavelength. To the best of our knowledge, this work demonstrates for the first time a hybrid-type induced-coherence interferometer that incorporates a Mach-Zehnder-type interferometer for near-visible photons and a Michelson-type interferometer for infrared (IR) photons, based on double-pass-pumped spontaneous parametric down-conversion. This configuration enables IR optical measurements via the detection of near-visible photons and provides methods for optimizing the quality of measurements by identifying photon pairs of different origins. We theoretically identify that the induced-coherence interference visibility is approximately the same as the heralding efficiency between twin photons along the relevant spatial modes, and experimentally maximize the visibility by setting up a common reference spatial mode for IR photons. Applications to both time-domain and frequency-domain quantum optical induced-coherence tomography for three-dimensional test structures are demonstrated. The results prove the feasibility of practical undetected-photon sensing and imaging techniques based on the presented structure.