Fig. 1: Conceptual scheme and setup of interaction-free, single-pixel quantum imaging with undetected photons.

a Classical imaging. Classical imaging requires a light source that emits light, which in turn interacts with the object of interest; a detector with spatial resolution directly detects light scattered from the object. b In our imaging scheme, all the above conditions are alleviated, which means imaging can be realized without direct interaction, direct detection, and a charge-coupled device (CCD) camera. c Conceptual diagram. Our imaging protocol is realized by integrating two interferometers based on the induced coherence (IC) and the interaction-free measurement (IFM) with the single-pixel imaging (SPI). d Experimental setup. A folded version of panel (c) realizes IC. The double-pass spontaneous parametric down-conversion processes correspond to source-1 and source-2 in panel (c). The IFM is realized with a single-photon Michelson interferometer. The object is placed into arm ③ of the IFM. Vac denotes the vacuum port of the IFM module. The idler photon is filtered out by dichroic mirror DM₃ and remains undetected throughout the entire imaging process. The circled numbers represent different arms of the interferometer. \(\theta\) and \(\phi\) represent the phase of the signal photon and the relative phase of the IFM. We also perform interaction-free quantum imaging with an intensified CCD (ICCD) camera (shown in inset f of panel d), instead of using SPI (shown in inset e of panel d). IC induced coherence, IFM interaction-free measurement, SPI single-pixel imaging, SLM spatial light modulator, DM dichroic mirror, NC nonlinear crystal, BS beam splitter, R reflector, SPD single-photon detector. Note: This figure is created by authors and does not contain any third-party materials or previously-created elements.