Quantum engineers at UNSW Sydney have placed a ‘Schrödinger’s cat‘—the famous paradox of quantum superposition where a system exists simultaneously in a quantum superposition, in this case being both dead and alive—inside the nucleus of a single antimony atom on a silicon chip.
“Antimony is a heavy atom, which possesses a large nuclear spin—meaning a large magnetic dipole,” said researcher Xi Yu in a press release. “The spin of antimony can take eight different directions, instead of just two.” Yu went on to note that this fact may “not seem much,” but adds that it “completely changes the behavior of the system.” He explains: “A superposition of the antimony spin pointing in opposite directions is not just a superposition of ‘up’ and ‘down’, because there are multiple quantum states separating the two branches of the superposition.”
Why quantum error correction is critical and still unresolved
Inside the quantum collaboration
The silicon quantum chip development represents a significant international effort, with UNSW’s Danielle Holmes, Ph.D., leading fabrication while University of Melbourne colleagues handled antimony atom insertion.
“This work is a wonderful example of open-borders collaboration between world-leading teams with complementary expertise,” noted project leader Professor Andrea Morello, in the press release.
Key contributors included:
• UNSW Sydney: Device fabrication and operation
• University of Melbourne: Atomic integration
• Sandia National Laboratories and NASA Ames (USA): Theoretical framework
• University of Calgary (Canada): Quantum state assessment
The team’s next milestone targets what Morello calls “a ‘Holy Grail’ in quantum computing,”- the demonstration of quantum error detection and correction.
Source: UNSW Newsroom, January 2025
This extra complexity offers a built-in defense against errors, which are one of the biggest obstacles in quantum computing development. Current quantum computers fall into the category of Noisy Intermediate-Scale Quantum (NISQ) devices, meaning they are limited by relatively high error rates and a small number of qubits, making the development of error-resistant quantum digits (qudits) especially valuable for clearing the way for future progress.
In their research, the scientists show that flipping just one or two spin orientations will not immediately change a logical 0 (dead cat) into a 1 (alive cat). Instead, the team found it takes seven consecutive spin-flip errors before the “cat” state collapses. “As the proverb goes, a cat has nine lives. One little scratch is not enough to kill it. Our metaphorical ‘cat’ has seven lives: it would take seven consecutive errors to turn the ‘0’ into a ‘1’,” said Yu, the lead author of the paper.
By integrating this multi-level “cat” directly into silicon, the team taps into established semiconductor technology while enabling more resilient quantum operations. They propose that this approach will allow engineers to spot and correct small errors in real time, preventing them from cascading into fatal mistakes. The work is published in Nature Physics.
Toward multi‐error‐correcting code directly within a single atom
The authors show that the multilevel nature of the Sb nucleus (with its spin of 7/2) can naturally implement a multi‐error‐correcting code directly within a single atom. Unlike a standard spin‐1/2 qubit (which would lose its logical state if just one spin flip error occurred), their so‐called “spin‐cat” encoding requires up to seven consecutive spin flips before the cat state collapses. This hardware‐efficient approach significantly bolsters resistance to phase‐flip errors, taking advantage of the long nuclear coherence times in isotopically pure silicon. As a result, small errors remain correctable in real time rather than accumulating into a fatal logical error—paving the way toward fault‐tolerant quantum processors that rely on scalable semiconductor manufacturing methods.
Quantum research team (from left) Benjamin Wilhelm, Xi Yu, Andrea Morello and Danielle Holmes demonstrate their work on superposition states with a feline companion—a playful nod to the quantum paradox central to their breakthrough. [Image: Lee Henderson]
Looking ahead, the team envisions coupling multiple high-spin nuclei for scalable logic operations using well-established techniques of donor placement and silicon processing. As the authors explain in their paper: “In the near term, multi-qubit logic gates can be realized if two or more 123Sb nuclei are hyperfine-coupled to a common electron. This would permit the implementation of a geometric CZ gate by imparting a 2π rotation to the electron, conditional on some specific state or states of the two nuclei.” By integrating multiple donors in close proximity, the spins can be collectively manipulated and read out with shared electronic ancillas, providing a path to fault-tolerant quantum computing in silicon. Further refinements—such as optimizing inter-donor spacing, minimizing residual strain, and engineering robust electron couplings—could enable long-lived, large-scale quantum processors that harness these robust “cat” encodings.
The antimony breakthrough comes just weeks after Google’s landmark announcement of the Willow quantum processor, which demonstrated the first large-scale achievement of quantum error correction below threshold. Google’s 105-qubit processor showed that adding more qubits could actually reduce errors rather than increase them. As we noted, the processor completed a specialized computation in five minutes that would theoretically take today’s fastest supercomputers 10 septillion years.
