Google reports that its Willow quantum chip has reduced errors by approximately 40 times when preparing specialized quantum states required for complex calculations. The improvement addresses a significant hardware obstacle that must be overcome before quantum computers could potentially break into Bitcoin wallets.
Published in Science this week, the research examines an overlooked part of Bitcoin’s quantum security debate. Discussions often focus on the number of qubits required to crack wallet encryption, while paying less attention to the substantial computing resources needed to prepare and operate such a machine.
Why older Bitcoin holdings could face greater risks
Bitcoin wallets use private keys to authorize transactions and corresponding public keys to verify them. Spending transactions often reveal public keys, while certain Bitcoin address types expose them from the beginning.
Conventional computers cannot feasibly calculate a private key from its public counterpart. However, a sufficiently advanced quantum computer running Shor’s algorithm could theoretically perform this task, allowing an attacker to generate valid signatures and spend the associated bitcoin. Mathematician Peter Shor introduced the algorithm in 1994, but no successful attack of this kind has been demonstrated against Bitcoin.
Storing a recovery phrase offline would not eliminate this risk if an attacker could derive the private key from publicly available blockchain information.
Some Bitcoin transaction formats conceal public keys behind cryptographic hashes until the owner spends the funds. An attacker targeting these wallets would need to wait until the public key became visible and potentially recover the private key before the transaction was confirmed.
Lost bitcoin presents another challenge. Moving existing holdings to stronger cryptographic protections generally requires access to the original private keys. Owners who have lost those keys cannot migrate their funds, potentially leaving exposed public keys vulnerable to future quantum attacks.
Some of Bitcoin’s earliest holdings have had their public keys exposed on the blockchain since they were mined.
Long-inactive wallets have also begun moving funds amid growing discussions about quantum threats. On Oct. 7, 100.02 BTC that had remained untouched since July 2010 moved for the first time in more than 16 years, worth approximately $8.5 million at the time. In August, six wallets dormant since 2011–2014 transferred another 553.59 BTC, valued at around $40 million.
These movements do not necessarily indicate that investors are reacting to quantum concerns. They could also reflect wallet transfers, changes in custody or preparations to sell.
The hardware challenge behind quantum attacks
Quantum computers use quantum bits, or qubits, which are highly sensitive to environmental disturbances. Heat, vibrations and unwanted signals can introduce errors, while increasing the number of qubits creates additional opportunities for mistakes.
Engineers address this challenge by combining physical qubits into groups that detect and correct errors. These groups function as more reliable computational units called logical qubits.
Certain quantum operations also require specially prepared states known as magic states. Because these states are consumed during calculations, the computer must continuously produce new ones.
This creates a resource problem: hardware dedicated to generating magic states cannot be used for other computational tasks at the same time.
Google’s cultivation technique seeks to make this process more efficient. It starts with an imperfect quantum state, checks for errors and discards unsuccessful attempts, retaining only states that satisfy the required quality standards.
The company reported approximately 99.99% fidelity, meaning the states that passed inspection closely matched their intended forms. However, only about 8% of attempts survived the preparation and measurement process. Roughly eight out of every 100 attempts therefore produced states that passed the checks.
Researchers still need to demonstrate that the method can operate at a larger scale, maintain state quality during extended calculations and integrate with more advanced error-correction systems.
What the breakthrough means for Bitcoin
In March, Google estimated that a future superconducting quantum computer with fewer than 500,000 physical qubits could break the cryptographic protections used by Bitcoin and Ethereum under specific hardware assumptions.
Research reported by CoinDesk in September subsequently reduced the computational workload for one important part of the attack. However, that work did not cover the complete process or all the physical hardware required.
The studies address different obstacles. Earlier research focused on reducing the mathematical workload, while Google’s cultivation method targets the resources needed to prepare the quantum computer for those calculations.
The improvements cannot simply be multiplied together to claim that attacking Bitcoin has become proportionally easier. Google’s March estimate already assumed efficient magic-state production, so the Willow experiment evaluates an aspect of that approach rather than delivering an additional 40-fold reduction in the estimated cost of an attack.
Google’s earlier research also highlighted the demanding requirements of a potential attack. One modeled scenario, lasting approximately nine minutes after a public key became available, required an estimated 500,000 sufficiently reliable magic states every second.
While Willow represents progress in quantum computing, the findings do not mean Bitcoin wallets can currently be cracked using this technology. Building a machine capable of carrying out such an attack remains a substantial engineering challenge.
