Google Quantum AI reported on Thursday that its Willow quantum processor cut errors in a key quantum operation by a factor of 40 while achieving 99.99% logical fidelity, according to a study published in the journal Science on October 15. The result has renewed debate over how long Bitcoin's current digital signatures can withstand future quantum attacks. The findings arrive as Coinbase, Starknet and other projects weigh quantum-resistant upgrades.

Willow Reaches 99.99% Fidelity in Magic State Test

The study tested a process called magic state cultivation, in which Willow produced special quantum states with 99.99% logical fidelity. Quantum computers require these states to perform complex calculations, and earlier estimates suggested that their production could consume up to 95% of a machine's capacity. Taming such errors is widely viewed as the central obstacle between today's compact processors and the large fault-tolerant machines that researchers say would be needed to threaten conventional cryptography.

Google filtered out faulty states through repeated checks on the processor, and only 8% of attempts passed. Researchers successfully transferred the remaining states between two error-correction methods, although larger tests are still needed.

Bitcoin's Longer-Term Quantum Risk

In March, Google Quantum AI researchers estimated that a future machine could break Bitcoin's current signature protection using fewer than 500,000 physical qubits. The calculation assumed future hardware rather than a successful attack. Willow, by comparison, operates 105 qubits today, a gauge of the gap between current hardware and the scale such estimates describe. The researchers described a possible nine-minute attack against transactions with exposed public keys, and about 6.9 million BTC may face exposure through already visible public keys. Bitcoin's signatures rely on elliptic-curve cryptography, which researchers have long known a sufficiently large quantum computer could break — the assumption that motivates the migration planning now underway across the industry.

Other networks are also debating protections. Ethereum co-founder Vitalik Buterin had raised concerns about AI-assisted attacks on lattice cryptography, the mathematical family behind several leading post-quantum encryption standards, and Cardano founder Charles Hoskinson challenged them in an October 9 post on X, arguing that security choices require evidence.

Coinbase Calls for Early Bitcoin Preparation

Cryptocurrency exchange Coinbase published a security paper on October 13 urging developers to plan before quantum attacks become practical, describing Bitcoin's eventual transition as a multiyear effort. Its recommendations include examining BIP-360, which changes how certain Bitcoin transactions reveal keys by introducing post-quantum signature options, as well as wallets capable of supporting both existing and newer signature methods during a transition.

In its Bitcoin quantum security plan, Coinbase cited limited block space, custody systems and dormant coins as unresolved problems. Owners who have lost access to their wallets may be unable to move funds once upgrades become available.

Other Networks Review Their Security Plans

Starknet said in a post on X, where its account has more than 350,000 followers, that it was considering becoming an independent blockchain with a goal of reaching quantum resistance in 2027. On October 15, the project opened a nonbinding governance vote on whether to study the change. The vote runs through October 22 and does not authorize a network move.

Google has separately set a 2029 target for completing its own transition to post-quantum cryptography, a timeline that follows the first post-quantum encryption standards finalized by the U.S. National Institute of Standards and Technology in 2024.