One item on the long list of strange facts about quantum mechanics is that the mere possibility of something happening is often just as good as it actually happening. For example, the fact that a photon could potentially travel down a given path can be enough to create an interference pattern that requires the photon to take that path.

Something similar is true regarding a phenomenon called quantum interference. A team of researchers from the University of Vienna has now taken advantage of this idea to create a bizarre imaging technique where the photons that actually strike the object being imaged are discarded. The image itself is then built other with photons that were entangled with the discarded ones.

Interference is the ability of two waves, such as photons, to interact either additively or destructively. In the quantum world, whether or not interference occurs depends on the ability to distinguish the two things that are interfering. If they are distinguishable, interference cannot occur. But you don't have to actually distinguish between them in order to block interference. As the authors of the new paper write, "The mere possibility of obtaining information that could distinguish between overlapping states inhibits quantum interference."

The device the team set up would create a pair of entangled photons and send one to the target to be imaged. That target was the equivalent of a cardboard cutout; made of silicon, it had an area that was transparent to photons of the wavelength used in the experiment. Naturally, the researchers set up the transparent area so it was shaped like a cat.

If the photon struck the non-transparent area, it was absorbed. If it passed through the transparent area, however, it went on to interfere with one of a second pair of photons. So the question of whether interference occurs or not tells us about the shape of the transparent areas. You could potentially detect this interference using the photons that went through the transparent image. But the authors discarded these photons and instead created an image using the remaining, entangled photons, which were influenced by the presence or absence of interference.

To ensure this was the case, the authors set the experiment up so that the photons that went through the transparency had a wavelength that the detector was incapable of detecting—and then imaged the object using the wavelength of the entangled photons.

Their concluding sentence is a bit of an homage to Watson and Crick, discoverers of the structure of DNA, who made a not-very-subtle hint that they recognized many of the implications of their work. Using similar phrasing to the biologists, the authors note, "It has not escaped our attention that, on the other hand, by knowing the object, one could obtain information about the quantum state without detecting it."

That's probably a bigger deal than it might sound like. Directly measuring a quantum state tends to destroy it; in this case, the authors could potentially gain some information about the state while leaving the photons available for use.

Nature, 2014. DOI: 10.1038/nature13586 (About DOIs).