Schrödinger's cat is quite possibly the most famous paradox in physics, and rudimentary real-world versions have even been built in the laboratory. Now a team of German physicists has set a new record by building a quantum "cat" out of 20 entangled qubits, as described in a recent paper in Science. A second group of Chinese researchers has successfully done the same with 18 qubits. The prior record, set in 2011, was 14 qubits.

Erwin Schrödinger originally proposed his thought experiment to illustrate the innate absurdity of quantum mechanics. He envisioned putting a hypothetical cat in a box containing a radioactive substance, a detector to sense any emitted radiation, and a lethal vial of poison. If the radioactive substance decays, the detector will trigger the release of the poison and the cat dies. If it doesn't, the cat remains alive. But a strict interpretation of quantum mechanics holds that until someone opens the box to see what happened, the cat exists in a superposition of states, both alive and dead.

It really is possible to build a version of Schrödinger's cat in the laboratory. It's more of a quantum "cat state," whereby two or more particles manage to be in two different states at the same time. In 2005, for instance, National Institute of Standards and Technology researchers created a quantum cat state out of six atoms in simultaneous "spin up" and "spin down" states—rather like spinning clockwise and counterclockwise at the same time. Other groups have done the same with photons.

In 2016, physicists at Yale University put a new twist on the quantum cat state, creating a system where photons were not just in a superposition of states, but also entangled—what Einstein famously called "spooky action at a distance," because changing the state of one will change the state of the other. In other words, the quantum "cat" both lives and dies in two boxes at once.

That has important implications for quantum computing, since it's the first step toward building a logical operation between two qubits. (It also would enable error correction.) But these entangled cat states are very fragile. The slightest bit of outside interference or interaction will cause the system to "decohere," and the entangled state will be lost.

The cat state

"Qubits in the cat state are considered extremely important for the development of quantum technologies," said co-author Jian Cui, a physicist at the Peter Grünberg Institute at Jülich (PGI-8). "The secret of the enormous efficiency and performance expected of future quantum computers is to be found in this superposition of states."

Naturally, he and his colleagues were keen to expand the number of possible entangled cat-state qubits. The team used a quantum simulator based on Rydberg atom arrays. "Rydberg atoms" are atoms that are in an especially excited state well above their ground state, such that the electrons are far away from the nucleus. They used optical tweezers to capture and hold rubidium atoms in place, then zapped the atoms with another laser to raise them to the Rydberg state.

"We practically inflated some atoms to such an extent that their atomic shells merge with the adjacent atoms to simultaneously form two opposite configurations, namely excitations occupying all even or odd sites," said Cui. "This goes so far that the wave functions overlap as in the analogy of Schrödinger's cat and we were able to create the superposition of the opposite configurations."

The trick to avoiding decoherence until they could measure the cat-state: switching the lasers off and on at just the right rate to speed up the preparation process. The advantage gained from the 20 entangled cat-state qubits they achieved translates into over a million superimposed states. Ultimately, physicists would like to achieve 300 entangled qubits, which could store more numbers than—well, pretty much every particle in the universe. They have a long way to go before hitting that goal, but this is a key step forward.

DOI: Science, 2019. 10.1126/science.aax9743 (About DOIs).