Weird Physics Can Make an Invisible Cat Visible

These cat pictures are brought to you by quantum entanglement as discussed by Einstein and Schrödinger

quantum cats
Gabriela Barreto Lemos

In a neat demonstration of one of physics’ most mind-boggling concepts, researchers have captured a image of a cat stencil using light that never bounced off or even near the stencil itself.

The image was generated using light particles—photons—that were quantum entangled. This means that a pair of particles share characteristics. Alter one, and it affects the other, even if the particles are separated by thousands of miles. 

In this demonstration, researchers split a green laser beam into two and adjusted the wavelengths so that one beam was red and the other yellow. (Laser light contains exactly one color of light with all the peaks and troughs of the lightwaves lined up.) Part of the set-up entangled particles in those two beams. Next, they sent the red photons to the cat stencil and the yellow photons to a camera.

The cat cut-outs were made of a material invisible to yellow light. Since the light particles were entangled, however, the yellow photons "showed" the camera the image of the cat simply because the red photons could "see" the stencil. 

"Even other physicists say 'you can't do that' at first, but that is quantum behavior for you, very strange," lead researcher Gabriela Barreto Lemos of the Institute for Quantum Optics and Quantum Information in Vienna, Austria, told National Geographic.

The researchers chose a cat outline to pay tribute to a thought experiment proposed by Austrian physicist Erwin Schrödinger, the scientist who came up with the term entanglement. His idea, the "Schrödinger’s cat" paradox, discusses the odd consequences of quantum physics: In his example, a cat is in a bunker with a flask of poisonous gas that has a 50/50 chance of leaking. The cat will, after some period of time, be dead. Or it will be alive. But until we look, we have no idea whether the cat is dead or alive. Therefore, the cat exists in a superposition of simultaneously dead and alive. Our observation forces the possibilities to collapse into one reality.

Science reporters cutely observed that with the new demonstration, "Schrödinger’s cat was caught on quantum film."

Imaging the cat stencils, while interesting, is not of immediate practical use. In their paper, however, the researchers write that the method might help medical technicians to take pictures of living tissues without damage.

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