Google Says “Magic State Cultivation” Cut Quantum Errors by 40×

October 16 — On 15 October local time, Google announced that its quantum AI team has experimentally confirmed and overcome a core bottleneck in quantum computing: on the Willow quantum chip it validated the practical feasibility of a technique called “Magic State Cultivation,” cutting the quantum computing error rate by about 40 times compared with not using the technique. The results were published in the journal Science.

To achieve “universal quantum computing” and run real programs, a system must be able to carry out a specific set of quantum operations and perform quantum error correction. That goal can be reached by preparing so-called “magic states” in advance; they are called magic states precisely because they help build the core operations that go beyond the limits of classical computing.

But preparing high-quality, error-corrected magic states is extremely costly in computing resources. By some estimates, as much as 95% of a quantum computer’s total capacity may have to be devoted to preparing magic states, leaving very little for actual computation. More awkward still, magic states are single-use resources that cannot be reused, which makes them extremely scarce.

In 2024, Google Quantum AI researchers Craig Gidney, Noah Shutty and Cody Jones drew on ideas in the academic literature and developed them further, pioneering the new method now known as “magic state cultivation.”

The approach shows great potential for substantially reducing resource overhead. Its core mechanism works like a quantum filter: a noisy quantum state is first injected into a tiny region of the quantum chip, and then a series of highly coordinated measurements detects errors and discards flawed attempts. Gidney and his team showed that this process can “cultivate” quantum states of extremely high purity and very low error rates, while adaptively fitting them into larger usable quantum codes.

Building on these advances, together with many algorithmic innovations and engineering optimisations, Google has set a new state of the art in quantum metrology. On Google Quantum AI’s new Willow chip, the protocol cut the error rate by a factor of 40 compared with uncultivated states, and the logical fidelity of the resulting T states set a record of 99.99%.

Google says that for magic state cultivation to truly serve practical quantum computing, it still has to be validated on larger-scale systems, and many engineering and technical challenges remain to be solved.