A new quantum imaging experiment demonstrates the seemingly impossible: pictures made with light that is never measured. The experiment involves the creation of pairs of photons that are indistinguishable. One member of the pair follows a path where it illuminates an object, but this remains undetected. The second photon, which does not interact with the object at all, is the one that is collected, and remarkably it can be used to form an image of the object. This phenomenon is a product of the quantum principle that indistinguishable photons will interfere, but interference is inhibited by the mere possibility of obtaining information that could distinguish between them. The presence of interference fringes is used to form the picture. As well as demonstrating a fascinating aspect of fundamental physics, this observation can be of practical relevance for a wide range of imaging applications since it allows the use of a detector in a different wavelength range than the one illuminating the object.
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