Nature 512, 409–412 (2014)

Scientists in Austria have demonstrated quantum imaging with photons that never actually interact with the imaged object. The technique is based on a quantum interference experiment with two separate down-conversion nonlinear crystals (labelled NL1 and NL2). The crystals are illuminated by the same continuous-wave 532 nm laser, and create 810 nm and 1,550 nm photons through parametric down-conversion processes. The 1,550 nm photons from NL1 are separated by a dichroic mirror, transmitted through the object, and collinearly sent to NL2 with the 532 nm laser. The 1,550 nm photons from NL1 and NL2 are then discarded and only 810 nm photons are combined at a beam splitter to cause interference. A cardboard cut-out or an etched 500-μm-thick silicon plate, which is opaque to illumination at 810 nm and highly transparent at 1,550 nm, is used as an object. The images are obtained with an electron-multiplying charge-coupled device camera with single-photon sensitivity at 810 nm. The demonstrated imaging is fundamentally different to interaction-free imaging or ghost imaging because the photons used to illuminate the object do not have to be detected at all and coincidence detection is not necessary. This enables the probe wavelength to be chosen in a range for which suitable detectors are not available.