  Mid-infrared light scatters much less than shorter wavelengths, allowing
greatly enhanced penetration depths for optical imaging techniques such as
optical coherence tomography (OCT). However, both detection and broadband
sources in the mid-IR are technologically challenging. Interfering entangled
photons in a nonlinear interferometer enables sensing with undetected photons
making mid-IR sources and detectors obsolete. Here we implement mid-infrared
frequency-domain OCT based on ultra-broadband entangled photon pairs. We
demonstrate 10 ${\mu}$m axial and 20 ${\mu}$m lateral resolution 2D and 3D
imaging of strongly scattering ceramic and paint samples. Together with $10^6$
times less noise scaled for the same amount of probe light and also vastly
reduced footprint and technical complexity this technique can outperform
conventional approaches with classical mid-IR light.

Optics;Quantum Physics