Entangled cats? Stranger things could happen if quantum rules scaled up to the everyday world.

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What is the limit to self-contradiction? The question arises in politics and quantum physics alike.

A team of Russian and Canadian physicists have figured out how to push the limits of self-contradicting quantum states, by breeding Schrdingers cats.

Their experiment, which involves sending cat-state photons through a hall of mirrors which multiplies their number, is described in Nature Photonics today.

Using the new method, the authors hope to help answer a fundamental question, namely: at what scale does the absurdity of quantum mechanics end and common-sense reality begin?

In the microscopic world of quantum mechanics, particles can do seemingly impossible things: such as being simultaneously in two contradictory states at once. For the Austrian physicist Erwin Schrdinger, who helped put quantum mechanics on firm foundations in 1926 with his Nobel- winning equation, this idea was too crazy to be believed.

In 1935, to illustrate how absurd quantum ideas had become, Schrdinger came up with a scenario involving a cat which, according to quantum theory, is both alive and dead at the same time.

The way he did it was to link the fate of a cat to a specific quantum event.

With ingenuity more typical of a Bond villain than a physicist, Schrdinger imagined a cat trapped inside a steel box along with some radioactive material, a Geiger counter, a hammer and a vial of hydrogen cyanide. If one of the radioactive atoms decays a chance quantum event it would trigger the hammer to smash the vial of poisonous gas, and farewell Felix.

Before you open the box to check, says quantum theory, the radioactive atom is both decayed and not-decayed. By extension, said Schrdinger, the cat is both alive and deadthe distinction between them blurry and smeared out.

But what seemed impossible to Schrdinger, is a commonplace for modern day physicists, who have worked out how to produce various analogues of Schrdingers cat in real physical systems. They are used in many quantum technologies including quantum computation, teleportation, and cryptography.

In essence, a particle in a Schrdingers cat state is one that is holding two contradictory states at once. For example, an electron could be simultaneously spin up and spin down. Or, a photon of light could be simultaneously waving in two opposite directions.

Until now, experimenters have only managed to muster small groups of Schrdingers cat photons with limited energies, but the new work creates any number by breeding them.

The method works by taking two photons, already in cat states, and firing them simultaneously through the same beam-splitter, which gets the two photons entangled. After some more beam-splitting the arrangement spits out more cat states than went in a bit like if Felix hopped through a cat-flap and two cats appeared on the other side.

The snag is, the process only works about one fifth of the time. (The rest of the time, there's no entanglement, and no breeding of cats.)

And running the photons through the ring again would increase the amplitude even further. Using this iterative approach could potentially produce as many quantum cat states as you like.

Thus, it is possible to push the boundaries of the quantum world step by step, and eventually to understand whether it has a limit, says Demid Sychev, of the Russian Quantum Center and the Moscow State Pedagogical University, and lead author of the study.

Meanwhile, the debate which originated with Schrdinger, Bohr and Einstein continues today: the question of whether the universe is innately fuzzy or whether it is just the way we see it. As Schrdinger eloquently put it in 1935: There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.

Producing quantum phenomena with more particles, and in larger scales, might just help us spot the difference between these two pictures, and finally get to grips with reality.

Even if our politicians still struggle with it.

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Physicists breed Schrdinger's cats to find boundaries of the | Cosmos - Cosmos