Real-world "Schrödinger's cat" brings quantum computing breakthrough (2025)

Physicists have brought the well-known thought experiment of Schrödinger's cat to life in a breakthrough that could help weed out errors in future quantum computers.

The team from Australia has demonstrated that an atom of antimony can be used to store data for use in quantum computations in such a way that it is better protected from errors than in a standard quantum bit, "or qubit," of data.

The impressive step brings us one step closer to realizing error detection and correction in quantum systems—a major obstacle to producing practical quantum computers.

Quantum computers rely on a quirk of physics in which a system can exist in a "superposition" of multiple states at once. This allows a quantum computer to explore multiple possibilities at the same time, radically speeding up processing times.

First proposed in 1935 by the Austrian physicist Erwin Schrödinger, "Schrödinger's cat" was designed to highlight the physicist's concerns with one interpretation of quantum mechanics—the then relatively new theory that explains how matter behaves at the atomic (and smaller) scale.

Under quantum mechanics, particles can exist in a superposition of multiple states at once, before collapsing into a single state upon interactions with other particles. In the 1930s, proponents of the so-called Copenhagen interpretation suggested that superpositions collapse only when observed by a conscious mind.

Schrödinger mused that, were this correct, you could place a cat in a box with a poison whose release was triggered by a radioactive decay governed by quantum mechanics—and, until the box was opened and observed, the atom would both have and have not decayed and the cat would be both dead and alive. The superposition would have been writ large.

As it was "quite ridiculous" to imagine this, Schrödinger argued that superpositions must be collapsed by more than just conscious observers.

"No one has ever seen an actual cat in a state of being both dead and alive at the same time, but people use the Schrödinger's cat metaphor to describe a superposition of quantum states that differ by a large amount," said physics professor Andrea Morello of Australia's University of New South Wales (UNSW) in a statement.

The key building block of a basic quantum computer is the so-called qubit. These are analogous to the regular bits in a conventional computer, in that they can represent a value of "0" or a value of "1."

But at the same time (and this is where the quantum aspect creeps in), they can represent both values—existing in a superposition of "0" and "1" just like Schrödinger's cat was both "dead" and "alive."

In their study, Morello and his colleagues go a step further, for their "cat" is an atom of antimony. This heavy metal has a large nuclear spin that can take eight different directions. This means that, if the nuclear spin is used to encode data, an antimony atom can store eight different values—"0," "1" and six other stops in between.

This has profound consequences for error-proofing quantum computers, the team explained in its paper.

"Normally people use a quantum bit, or 'qubit'—an object described by only two quantum states—as the basic unit of quantum information," said paper author and UNSW quantum information researcher Benjamin Wilhelm in a statement.

"If the qubit is a spin, we can call 'spin down' the '0' state, and 'spin up' the '1' state. But if the direction of the spin suddenly changes, we have immediately a logical error: 0 turns to 1 or vice versa, in just one go. This is why quantum information is so fragile," he said.

With the eight possible states of antimony, however, a single error is not enough to mess up the quantum encoding and flip "0" to "1"—or, if you prefer, "alive" to "dead."

"As the proverb goes, a cat has nine lives; one little scratch is not enough to kill it," said lead author Xi Yu, also of UNSW, in a statement.

He continued: "Our metaphorical 'cat' has seven lives; it would take seven consecutive errors to turn the '0' into a '1'! This is the sense in which the superposition of antimony spin states in opposite directions is 'macroscopic'—because it's happening on a larger scale, and realises a Schrödinger cat."

Morello said: "A single or even a few errors do not immediately scramble the information. If an error occurs, we detect it straight away, and we can correct it before further errors accumulate. To continue the 'Schrödinger cat' metaphor, it's as if we saw our cat coming home with a big scratch on his face.

"He's far from dead, but we know that he got into a fight; we can go and find who caused the fight, before it happens again and our cat gets further injuries," he said.

Reference

Yu, X., Wilhelm, B., Holmes, D., Vaartjes, A., Schwienbacher, D., Nurizzo, M., Kringhøj, A., Blankenstein, M. R. van, Jakob, A. M., Gupta, P., Hudson, F. E., Itoh, K. M., Murray, R. J., Blume-Kohout, R., Ladd, T. D., Anand, N., Dzurak, A. S., Sanders, B. C., Jamieson, D. N., & Morello, A. (2025). Schrödinger cat states of a nuclear spin qudit in silicon. Nature Physics. https://doi.org/10.1038/s41567-024-02745-0

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