Making quantum computers error-proof is one of the biggest challenges currently facing the technology. Researchers at Google Quantum AI have now demonstrated one way to do so more efficiently. Their work suggests the smaller quantum computers that will be available in the next few years could be more impactful than expected.
Quantum computers can run mathematical operations, analogous to addition or multiplication, that conventional computers cannot. This makes them suitable for solving some problems that had previously been considered unsolvable, such as those that underpin cryptography. Most important among the quantum operations the computers run are “non-Clifford gates”, which are the key ingredients for algorithms that transcend conventional computation.
Quantum computers are very error-prone, and their building blocks, called qubits, must be encoded into special groups, or “logical qubits”, before they can execute operations reliably.Executing non-Clifford gates in an error-corrected way requires large numbers of qubits, which means having to build very large quantum computers – a formidable engineering challenge. Now, Emma Rosenfeld at Google Quantum AI and her colleagues have used the firm’s Willow quantum computer to show that the error-correction process can be made more efficient. Their new technique ultimately reduced Willow’s errors 40 times.
At the centre of their advance is the idea of a “magic state”, which is a special quantum state that qubits must assume before the quantum computer can run error-corrected non-Clifford operations. Creating these states had, so far, been a real bottleneck for quantum computing, says Rosenfeld. In 2024, her colleagues at Google Quantum AI proposed a more efficient method for creating magic states called “magic state cultivation”.
Here, instead of many imperfect qubits being repeatedly combined and manipulated into a magic state that is then put towards running an error-corrected operation, the magic state is “grown” within a smaller patch of qubits. The Google researchers showed that magic state cultivation could, in theory, enable quantum computers to run consequential programs with just one-tenth the number of qubits we had expected they would need. The new experiment by Rosenfeld and her colleagues proves that magic state cultivation works in practice.
This is not the first time that new methods for making magic states have lowered a quantum computer’s error rates. Advances in creating magic states and error-correcting non-Clifford gates have also been achieved in experiments with quantum computers that have qubits based on ions. Willow’s qubits, which are based on tiny superconducting circuits, achieved error rates similar to those past experiments, but Rosenfeld says that they work more quickly, potentially reducing the quantum computer’s overall runtime.
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