Information is central to quantum mechanics. In particular, quantum interference occurs only if there exists no information to distinguish between the superposed states. The mere possibility of obtaining information that could distinguish between overlapping states inhibits quantum interference1,2. Here we introduce and experimentally demonstrate a quantum imaging concept based on induced coherence without induced emission3,4. Our experiment uses two separate down-conversion nonlinear crystals (numbered NL1 and NL2), each illuminated by the same pump laser, creating one pair of photons (denoted idler and signal). If the photon pair is created in NL1, one photon (the idler) passes through the object to be imaged and is overlapped with the idler amplitude created in NL2, its source thus being undefined. Interference of the signal amplitudes coming from the two crystals then reveals the image of the object. The photons that pass through the imaged object (idler photons from NL1) are never detected, while we obtain images exclusively with the signal photons (from NL1 and NL2), which do not interact with the object. Our experiment is fundamentally different from previous quantum imaging techniques, such as interaction-free imaging5 or ghost imaging6,7,8,9, because now the photons used to illuminate the object do not have to be detected at all and no coincidence detection is necessary. This enables the probe wavelength to be chosen in a range for which suitable detectors are not available. To illustrate this, we show images of objects that are either opaque or invisible to the detected photons. Our experiment is a prototype in quantum information—knowledge can be extracted by, and about, a photon that is never detected.
We thank M. Horne for reading the manuscript, clarifying suggestions and many discussions, P. Enigl for designing the figures for the objects, D. Greenberger and S. von Egan-Krieger for discussions, and C. Schaeff for equipment loans. Microfabrication was carried out at the Center for Micro- and Nanostructures (ZMNS) of the Vienna University of Technology. We acknowledge D. Ristanic for assistance with cryogenic Si etching and M. Schinnerl for contact mask production. G.B.L. was funded by the Austrian Academy of Sciences (ÖAW) through a fellowship from the Vienna Center for Science and Technology (VCQ). S.R. is funded by an EU Marie Curie Fellowship (PIOF-GA-2012-329851). This project was supported by ÖAW, the European Research Council (ERC Advanced grant no. 227844 ‘QIT4QAD’, and SIQS grant no. 600645 EU-FP7-ICT), and the Austrian Science Fund (FWF) with SFB F40 (FOQUS) and W1210-2 (CoQus).