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photonics

The light that sees what it couldn’t see

The cat silhouette on the right is a result of detected photons that had never interacted with the cut-out (left)

GABRIELA BARRETO LEMOS E COLABORADORES / UNIVERSIty of VIEnNAThe cat silhouette on the right is a result of detected photons that had never interacted with the cut-out (left)GABRIELA BARRETO LEMOS E COLABORADORES / UNIVERSIty of VIEnNA

An international team of physicists has achieved a seemingly impossible feat: photographing objects without using any photons (light particles) that come into contact with them. They produced images using a camera that captures special photons that are connected only remotely with those that actually illuminate the target objects (Nature, August 28, 2014). “We were able to transmit all the information from the photons that interacted with the objects to the photons that we detected,” explains Brazilian physicist Gabriela Barreto Lemos, who conducted the experiment and has been working since 2012 at the University of Vienna in the laboratory of Anton Zeilinger, nicknamed “Mr. Beam” for his experiments showing the peculiar workings of quantum mechanics. Lemos says she was delighted by the idea of the experiment. “But sometimes I got really mad, because it’s not easy to play with photons you can’t detect.” The experiment was conducted using a circuit of mirrors targeted by laser beams. Two crystals created pairs of “sibling” photons: one with an infrared wavelength and the other with a wavelength corresponding to red visible light. Even after taking different paths, the photons from a single pair shared information, in a phenomenon called quantum entangling. Only the infrared photons actually came into contact with the target object – a piece of cardboard with a cutout in the shape of a cat, in honor of physicist Erwin Schrödinger, who called attention to the absurd consequences of quantum mechanics in 1935 when he proposed a mental experiment in which a cat would be simultaneously alive and dead. Yet the cameras at the end of the circuit detected only the red photons. The circuit is set up in such a way that when a photon strikes the cardboard, it is possible to know which crystal produced it. But when a photon passes through the cat-shaped cut-out, it’s as if it had been emitted by both crystals at the same time. “The image is created by a quantum juxtaposition of both possibilities,” Lemos explains. “What we see on camera exists only because we don’t know which crystal created the photons.” The method, patented by the researchers, may be useful for imaging biological tissues and live cells.

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