Dead and Alive: Quantum Quirk Sheds Light on Schroedinger's Cat

Image: Cat pictures
These cat pictures represent the famous Schrödinger cat paradox, in which a quantum cat closed in a box can be dead and alive at the same time. The dark and light cat bodies are images of a cardboard cut-out. They arise due to destructive and constructive quantum interference, respectively. In this experiment the photons that interact with the cardboard cut-out are not detected, while the images are obtained by detecting only photons that never interact with the object.Gabriela Barreto Lemos

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By sending green, red and yellow laser beams down a path to a detector, researchers have shed light on the famous physics idea known as the "Schrödinger's cat" thought experiment.

Schrödinger's cat involves picturing a cat, in a box, with a vial of poison that can kill the cat if it opens. Over any given period there's a 50-50 chance the poison vial will open. A person who opens the box and looks at the cat will then observe that it is either dead or alive.

Most people would say that even before you open the box, before you can see the cat, it's in either one state or the other, either dead or alive. [The 9 Biggest Unsolved Mysteries in Physics]

But according to quantum mechanics, that's not true. In this theory, until you open the box, the cat is both dead and alive, in a state called superposition. It's only when the cat is observed that it takes on a certain value (dead or alive). The thought experiment has spawned numerous studies to find out, among other things, when exactly superposed states "collapse" into a single state.

Now, scientists at the Austrian Academy of Sciences have found a way to "look" at the cat without actually looking at it. Postdoctoral researcher Gabriela Lemos and her colleagues tested whether one could use particles of light called photons that never touch an object to see that object anyway, using an effect called entanglement.

They found that they could indeed see the cat in the box. Their findings were detailed in Thursday's issue of the journal Nature.

The work might help physicists solve what they call the measurement problem, which is the question of why quantum states take on definite values only when they are observed.

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Entanglement happens when two particles — atoms, photons or anything else — interact in some way. For example, if two photons are entangled, then photon A will have some state that reflects the state of its entangled partner, photon B.

You can't know the exact state of photon B merely by looking at photon A. Photon B could still exist in any of several different states until it is looked at. However, researchers can take measurements from several entangled pairs of photons, and then check to see if their correlation — the entanglement — holds by looking at their partners.

Splitting beams

In the new experiment, the scientists fired a laser through a series of beam splitters, ultimately to a detector. The laser they fired was a green beam, and at the first beam splitter, it was split into two green beams. One of the beams went to a crystal that changed the green light into a combination of red and yellow photons. Then, the beam of red and yellow light was split into its components, with the red photons landing on a cardboard cutout of a cat while the yellow light continued on to a last beam splitter. This splitter sent half of the yellow photons to one detector, and half to another detector.

Meanwhile, the second half of the green laser beam was sent to meet up with the red beam after it passed through the cutout of the cat. The combined red and green photons hit a second crystal, which converted the green and red beam into red and yellow. This went to a beam splitter that took out the red photons, and sent this second yellow beam to meet up with the other yellow beam at the final beam splitter, which divided the photons between the two detectors.

All tangled up

At the detectors, the researchers saw the image of the cat. This was noteworthy because the photons that made the image had never hit the cardboard cutout. Remember, the red photons that hit the cutout never made it to the detector, and the yellow ones that did travel to the detector never hit the cutout.

The cat image was made from the yellow photons because they were entangled with the red photons. You can't say that this yellow photon went to the first crystal and another photon went to the second — so the cat is still both dead and alive. [Twisted Physics: 7 Mind-Blowing Findings]

However, the red photons did hit the cardboard, and they were entangled with some of the yellow photons. We know which path they took: They hit the cutout. Thus, their states take on a definite value. The yellow photons that are entangled take on definite complementary values.

The results show that the researchers measured the entangled state of a pair of photons using only one photon of the entangled pair. In previous work, in order to see the entangled state, researchers had to measure both photons.

"What's most intriguing thing about this experiment is how information is contained in twin photons," Lemos told LiveScience. "How, in an entangled state, [this information] can be accessed by one photon."

— Jesse Emspak, LiveScience

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