“Creating the Heaviest Schrödinger Cat: Researchers Place Crystal in Superposition and Push Quantum Limits”

Even if you’re not a quantum physicist, you’ve probably heard of Schrödinger’s famous cat. Erwin Schrödinger discovered a cat that could be both live and dead at the same time in a thought experiment in 1935. The apparent contradiction – after all, in everyday life we ​​only see cats live or die – has prompted scientists to try to understand similar situations in vitro. So far, they’ve been able to do this by using, for example, atoms or molecules in a state of quantum mechanical superposition that are in two places at the same time.

At ETH, a research team led by Yiwen Chu, a professor at the Solid State Physics Laboratory, created a much heavier Schrödinger cat by placing tiny crystals in a superposition of two oscillatory states. The results were published this week in the journal Knowledgecould produce more powerful qubits and explain the mystery of why quantum superpositions are not observed in the macroscopic world.

cat in the box

In Schrödinger’s original thought experiment, a cat was locked in a metal box with radioactive material, a Geiger counter, and a vial of poison. Over a period of time—an hour, for example—an atom in matter may or may not decay through a quantum mechanical process with a certain probability, and the decay products can cause the Geiger counter to explode and trigger a mechanism that destroys the vial of poison that will eventually kill the cat. .

Since an outside observer cannot know whether the atom is actually decaying, neither can he know whether the cat is alive or dead – according to quantum mechanics, which governs atomic decay, it must be in an on/off superposition. (Schrodinger’s idea is commemorated by a life-size cat figure outside his former home at Huttenstrasse 9 in Zurich.)

“Of course, in the laboratory we couldn’t achieve such an experiment with a real cat weighing several kilograms,” said Zhu. Instead, he and his colleagues managed to create what’s called a cat state using oscillating crystals, which represent cats, with superconducting circuits representing real atoms. These circuits are essentially qubits or qubits that can take on the logical state “0” or “1” or a superposition of the two states, “0 + 1”.

The link between the qubit and the “cat” crystal is not a Geiger counter and poison, but rather a layer of piezoelectric material that creates an electric field when the crystal deforms as it oscillates. This electric field can be combined with the electric field of the qubit, and thus the superposition of qubit states can be transferred to the crystal.

Simultaneous vibration in the opposite direction

As a result, the crystal can now swing in two directions at the same time – up/down and down/up, for example. These two directions represent the cat’s “on” or “off” state. “By superimposing two oscillatory states in the crystal, we have effectively created a 16 microgram Schrödinger cat,” explains Zhou. That’s roughly the mass of a fine grain of sand and not as big as a cat, but still billions of times heavier than an atom or molecule, making it the fattest quantum cat.

In order for the wiggle to be a real cat condition, it is important that it is distinguishable to the naked eye. This means that the separation between the “up” and “down” states must be greater than any thermal or quantitative fluctuation of the positions of the atoms in the crystal. Zhou and colleagues examined this by measuring the spatial separation of the two states using superconducting qubits. Even though the distance measured is only a millionth of a billionth of a meter—smaller even than an atom—it is large enough to clearly distinguish states.

Measure small disturbances with cat cases

In the future, Chu wanted to push the limits of his crystal cat block even further. “This is exciting because it allows us to better understand why quantum effects disappear in the macroscopic world of real cats,” he said.

In addition to this academic interest, there are also potential applications in quantum technology. For example, the quantum information stored in qubits can be made more robust by using cat states consisting of a large number of atoms in a crystal rather than relying on single atoms or ions, as is currently practiced. In addition, the extreme sensitivity of massive objects in superposition to external noise can be exploited to make precise measurements of small perturbations such as gravitational waves or to detect dark matter.