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Photo of the red outlines of 2 cats made from cardboard cut-outs.

These cat outline etchings are normally invisible to the wavelength of light that made the pictures.

Photograph by Gabriela Barreto Lemos

Dan Vergano

National Geographic

Published August 27, 2014

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.")

Photo of a blue cat and a yellow cat cardboard cut-outs.
These cat cutout pictures were created from light particles that never saw the shape.
Photograph by Gabriela Barreto Lemos

Undetected Photons

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."

Although the experiment team has applied for a patent, Lemos acknowledges that practical applications may take awhile.

"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."

Follow Dan Vergano on Twitter.

S Gull
S Gull

Could this effect be applied at the LHC? Or, in astrophysics like dark matter research?

Bob Scwairpants
Bob Scwairpants

hello, what about using this technique in the double-slit experiment.  Using this technique to view the wave/particle the light required to observe the wave/particle would have less chance of interfering with the experiment, since it would not know it was being observed what would the outcome be???

A. Morales
A. Morales





Jason MacNaughton
Jason MacNaughton

superimposing the two images should give you an added visual field. IR.  cool.

Mehrdad Nabaie
Mehrdad Nabaie

Wonderful !
This magical manner goes to prove the " technology of thought " & " The Secret " & similar projects of how creation is possible for human in miracle but still unknown ways .

James   Mitchell
James Mitchell

Does light travel as a wave or a particle, or both?

Tuck Neilson
Tuck Neilson

As a physician I can envision fantastic applications for imaging here that are likely without risk to the patient if they can be properly developed.  It's exciting to anticipate.

Edmond Cohen
Edmond Cohen

TI MI N G... I.S. clever demonstration of quantum mechanics.

Bernard Dunne
Bernard Dunne

If ever there was an experiment that could do with an illustration of the setup! ... but sounds like the Yellow laser picked up attributes from the Red laser simply by being in proximity to each other, which is cool.  I wonder would it have been possible with frequencies further apart in the spectrum, and just how close to each other were the laser beams.

n. Pdiddly
n. Pdiddly

it is spooky because they see a cat, and i see an owl

charles wright
charles wright

@Bernard Dunne  I think the point here is that the lasers are not necessarily in proximity to each other - but are entangled.  I believe the effect is independent of the physical distance between the split laser beams.


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