# Physicists breed Schrödinger’s cats to find boundaries of the quantum realm

## Scientists hope to find the dividing line between quantum mechanics and common-sense reality. Cathal O'Connell reports.

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’ Schrödinger’s 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 Schrödinger, 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, Schrödinger 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, Schrödinger 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 Schrödinger, the cat is both alive and dead—the distinction between them blurry and “smeared out”.

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

In essence, a particle in a ‘Schrödinger’s 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.

“It is possible to push the boundaries of the quantum world step by step, and eventually to understand whether it has a limit.”

Until now, experimenters have only managed to muster small groups of ‘Schrödinger’s 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 Schrödinger, 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 Schrödinger 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.

*Cathal O'Connell is a science writer based in Melbourne.*