Quantum imaging with undetected photons

• Published in: Nature (London) Vol. 512; no. 7515; pp. 409 - 412
• Main Authors: Lemos, Gabriela Barreto, Borish, Victoria, Cole, Garrett D., Ramelow, Sven, Lapkiewicz, Radek, Zeilinger, Anton
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 28-08-2014; Nature Publishing Group
• Subjects: 639/766/400/482; Atoms & subatomic particles; Cameras; Crystals; Experiments; Humanities and Social Sciences; letter; multidisciplinary; Quantum physics; Science; Scientific imaging; Sensors
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature13586

A new quantum imaging experiment demonstrates images made with light that does not encounter the object; one of a pair of photons created at two crystals illuminates the object but is never detected, and the other photon, which is in a joint quantum state with the first and does not interact with the object, forms an image of the object on a camera.
Phantom photon photography
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.
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 interference
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,
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. Here we introduce and experimentally demonstrate a quantum imaging concept based on induced coherence without induced emission
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. 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 imaging
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or ghost imaging
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, 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.