Shadow play may be a source of entertainment for children, but it can also be considered an imaging technique. The shape of an object can be determined by shining light on it, and then looking at where the light was blocked — a strategy extensively used in radiography. But Gabriela Barreto Lemos and colleagues have now demonstrated that an object can be imaged by looking at light that never even interacted with it (Nature http://dx.doi.org/10.1038/nature13586; 2014) — all by exploiting the laws of quantum physics.

In their experiment, the researchers generated two pairs of entangled photons using two equal spontaneous parametric down-conversion events. One of the four produced photons interacted with the object to be imaged, and was subsequently aligned and made indistinguishable from its counterpart in the other pair. The image was then reconstructed by making the two remaining photons, which never touched the object, interfere with each other. The two original photons, including the only photon that physically interacted with the sample, were simply discarded.

Credit: NPG

The principle works because quantum interference between two alternatives emerges only if there is no information available to distinguish them. And what matters is not whether this information is actually gathered, but merely the possibility of it being acquired. In the experiment, if the first photon had interacted with the object, in principle it would have been distinguishable from its counterpart in the other pair — ruling out interference between the remaining two photons. Thus, the presence or absence of interference is exactly what permits the observation.

This basic scheme can be turned into an imaging technique by repeating the four-photon experiment for every spatial point in the object plane. The area in which interference takes place delineates the object — a pattern etched on a silica plate — which is placed in the path of one of the discarded photons.

This is more than simply an intriguing test of quantum theory. In standard imaging techniques, one needs to tune the frequency of radiation such that it optimizes the contrast for both the object and the detector. Here, this is no longer the case as the two are decoupled; the detected photon need not 'see' the object and the photon interacting with the object need not be detected. Indeed, the silica plate in the experiment was completely transparent to the wavelength of light forming the image.