Like twins separated at birth who are later reunited, two laser beams revealed invisible objects in a display of their weird quantum connection, researchers reported on Wednesday.
The images, of tiny cats and a trident, are an advance for quantum optics, an emerging physics discipline built on surprising interactions among subatomic particles that Einstein famously called "spooky." (Related: "Teleportation: Behind the Science of Quantum Computing.")
A conventional camera captures light that bounces back from an object. But in the experiment reported in the journal Nature, light particles, or photons, that never strike an object are the ones that produce its picture.
"Even other physicists say 'you can't do that' at first, but that is quantum behavior for you, very strange," says Gabriela Barreto Lemos of the Institute for Quantum Optics and Quantum Information in Vienna, Austria, who led the study.
A 2009 University of Glasgow experiment with a divided laser beam first demonstrated such "ghost imaging." But experts say the new technique, which uses two laser beams of different colors, offers new visualization advantages.
The two laser beams are "entangled" in quantum physics terms, meaning their photons share characteristics even when far apart. So broadly speaking, altering one alters the other.
"What they've done is a very clever trick. In some ways it is magical," says quantum optics expert Paul Lett of the National Institute of Standards and Technology in Gaithersburg, Maryland, who was not part of the experiment team. "There is not new physics here, though, but a neat demonstration of physics."
Optics Goes Quantum
The new imaging technique may allow for improved medical imaging or silicon chip lithography in hard-to-see situations, the team suggests.
In medicine, for instance, doctors might probe tissues using invisible wavelengths of light that won't damage cells, while simultaneously using entangled visible light beams to create clear images of the tissues.
"The two-color advantage is a cool idea," Lett says. "It happens a lot in imaging that the best wavelength of light for a probe is not the one that makes for the best picture. You can imagine tuning light colors like this to get the best advantages of both."
In particular, the experiment's approach could create images in visible light of objects that normally can be seen only under infrared light, says quantum optics expert Miles Padgett of Scotland's University of Glasgow, who headed the 2009 "ghost imaging" experiment.
Ironically, the idea of entanglement owes something to Einstein, who in 1935 criticized it as an unlikely (in his view) mathematical shortcoming of quantum physics, which treats subatomic particles as both point-like and as waves.
Manipulating these wavy particles, quantum physics predicted, would alter other seemingly unconnected particles far away. Einstein called this interference (in translation), "spooky action at a distance," which he saw as unlikely. But it turns out to work. (Related: "The Tragic Story of How Einstein's Brain Was Stolen.")
In the new experiment, the physicists entangled photons in two separate laser beams with different wavelengths, and hence color: one yellow and one red. (Watch: "None of the Above: Fun With Laser Beams.")
The team passed the red light beam through etched stencils and into cutouts of tiny cats and a trident, about 0.12 inches (3 millimeters) tall. The yellow beam traveled on a separate line, never hitting the objects. What's more, the etched shapes were designed to be invisible to yellow light.
The cat shape is a nod to physicist Erwin Schrödinger, who invented the famous "Schrödinger's cat" paradox, a thought experiment in which a notional cat is simultaneously dead and alive. Subatomic particles do seem to behave in this peculiar way sometimes, occupying many places at once.
After the red light passed by the objects, the physicists ran it together with the yellow laser beam at both parallel and right angles.
The red light was then discarded, and the yellow light headed for a camera. There, that yellow light revealed a picture of the object. And a negative of the picture emerged from the light that had interfered at a right angle.
"The phenomena really arises from the interference of the photons together," Lemos says. "It's not that the red photons have changed the yellow ones, it's that quantum mechanics says they have to share [wavelength] phases which we can detect to see a picture."
"This is a long-standing, really neat experimental idea," says Lett. "Now we have to see whether or not it will lead to something practical, or will remain just a clever demonstration of quantum mechanics."
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