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"Breeding" Schrödinger's Cat Could Unlock The Boundary Between The Quantum And Classical Worlds

Here's a sentence that's just as exciting as it is nonsensical: physicists are figuring out how to "breed" larger versions of Schrödinger's cat. Clear as mud? We'll explain, but suffice it to say, this could one day erase the imaginary line that separates the everyday behavior of macroscopic objects from the bizarre behavior of quantum particles.

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Don't Neuter This Cat

By Schrödinger's cat, we don't actually mean a feline animal. Schrödinger's cat is a metaphor inspired by Erwin Schrödinger's famous thought experiment, which says that if you put a cat in a box with a vial of poison that could release at random, you don't know whether the poison has released and killed the cat until you open the box. As a result, as long as the box is shut, the cat is both alive and dead.

This phenomenon of being in two states at once is called superposition, and it's a fundamental property of quantum physics. Just as Schrödinger's cat occupies both states of "alive" and "dead" until you open the box, quantum particles occupy multiple states until they're measured. That's what we mean when we say that physicists are "breeding" Schrödinger's cat: they're turning one particle in superposition into two particles in superposition that are linked together, or "entangled," which physicists did successfully in 2016. In the real world, "breeding" Schrödinger's cat could be an important step in developing better quantum computers.

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Schrödinger's Cat Gets Bigger

In 2017, physicists took the breeding process one step further. One peculiar thing about quantum physics is that the way quantum particles behave doesn't seem to have anything to do with the classical physics of the macroscopic world we interact with every day. That much is clear: how often have you seen a pendulum that swings right and left at the same time? What isn't clear is where the universe draws the line between the quantum and the classical—or if the line exists at all.

It seems Alexander Lvovsky and a team of scientists from the Russian Quantum Center and the University of Calgary have gotten closer to discovering that line. As laid out in a study published in Nature Photonics, the team used a beam splitter to cause an interference of two "cats"—light waves whose fields point in two directions at once, in this case—then placed a detector on one side, which would act as the measurement that "opens the box," so to speak. If the detector showed a specific result, it "bred" a cat with a higher amplitude on the other side. They were able to do this over and over, creating ever larger "cats." In a news release, co-author Demid Sychev put the breakthrough simply: "It is important that the procedure can be repeated: new 'cats' can, in turn, be overlapped on a beam splitter, producing one with even higher energy, and so on. Thus, it is possible to push the boundaries of the quantum world step by step, and eventually to understand whether it has a limit." With enough cats, we might someday know.

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