Casper the Quantum Ghost

A strange kind of imaging relies on quantum mechanics

5:53pm, August 13, 2009
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Casper just got upgraded from the “Friendly Ghost” to the “Quantum Ghost.” In a spooky new study, researchers found telltale signs of quantum weirdness lurking in an optical trick called ghost imaging. Discovered over a decade ago, ghost imaging allows researchers to create an image of something using light that never bounced off the actual object. The new work adds to the debate over whether ghost imaging is quantum in nature, or if normal, everyday physics can explain the phenomenon.

To look for evidence of quantum behavior, Scottish and Austrian researchers designed a system to create a ghost image of an appropriately ghoulish object (see image above). In the experiment, an ultraviolet laser beam diverged into two beams inside a special crystal. One beam bounced off the image of a ghost, displayed on a small television screen, before going into a detector. The other beam either went straight to a detector or first bounced off a hologram that was not ghost-shaped but had contrasting light and dark areas. Data from the two detectors were combined to produce the final image.

When the hologram was included in the experimental setup, the clarity of the ghost image improved, says study coauthor Miles Padgett of the University of Glasgow in Scotland. The team concludes that photons in the two streams were intimately linked, a quantum property known as entanglement. This “spooky action at a distance” allowed the contrast information from the hologram to be applied specifically to the lines of the ghost. The results are “the clearest demonstration that at least some forms of ghost imaging are quantum,” Padgett says. “That’s not to say that all systems are quantum, but ours is.”

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