Google says its Willow quantum chip has achieved an approximately 40-fold reduction in errors when producing specialized quantum states needed for complex calculations. The advance tackles a major hardware challenge that must be addressed before future quantum computers could potentially compromise Bitcoin wallets.
The research, published in Science this week, examines a frequently overlooked aspect of Bitcoin’s quantum vulnerability. While much of the discussion focuses on how many qubits would be needed to break wallet security, the substantial resources required to prepare and operate such a machine receive less attention.
Why older Bitcoin wallets may face greater risks
Bitcoin wallets use private keys to authorize transactions and matching public keys to verify them. Spending bitcoin often reveals the public key, while some transaction formats make it visible from the outset.
Deriving a private key from a public key is considered impractical for conventional computers. However, a sufficiently advanced quantum computer running Shor’s algorithm could theoretically reverse this process, allowing an attacker to produce valid transaction signatures and steal the associated funds. Mathematician Peter Shor introduced the algorithm in 1994, but no one has demonstrated a successful attack of this kind against Bitcoin.
Keeping a recovery phrase offline would not prevent an attack that relies on a public key already exposed on the blockchain.
Some Bitcoin transaction formats conceal public keys behind cryptographic hashes until the owner spends the funds. In such cases, an attacker would need to wait for the public key to become visible and recover the corresponding private key quickly enough to potentially interfere with the transaction.
Lost bitcoin presents an additional challenge. Transferring funds to stronger cryptographic protections generally requires access to the original private keys. If those keys are unavailable, owners cannot migrate their holdings, potentially leaving exposed public keys vulnerable to future quantum attacks.
Some of Bitcoin’s oldest holdings have had their public keys visible on the blockchain since they were mined.
Long-inactive wallets have also transferred funds as concerns about quantum computing have grown. On Oct. 7, a wallet moved 100.02 BTC that had remained untouched since July 2010, worth approximately $8.5 million at the time. In August, six wallets inactive since 2011–2014 transferred another 553.59 BTC, valued at roughly $40 million.
These movements do not necessarily indicate that their owners are preparing for quantum threats. They could also reflect wallet reorganizations, custody changes or plans to sell.
The cost of preparing a quantum computer
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 failures.
To improve reliability, engineers group physical qubits into systems that detect and correct errors. These systems function as more dependable computational units known as logical qubits.
Certain quantum operations also require specially prepared states called magic states. Each state is consumed during use, forcing the computer to generate replacements continuously throughout a calculation.
This creates a resource bottleneck because hardware dedicated to producing magic states cannot simultaneously handle other computational tasks.
Google’s cultivation technique aims to improve the efficiency of this process. It begins with an imperfect quantum state, checks for errors and discards unsuccessful attempts, retaining only states that meet the required quality standards.
The company reported approximately 99.99% fidelity, indicating that the states passing inspection closely matched their intended forms. However, only about 8% of attempts survived the preparation and measurement process, meaning roughly eight out of every 100 attempts produced acceptable results.
Researchers must still demonstrate that the technique 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.
In September, research reported by CoinDesk described a reduction in the computational workload for one part of a potential attack. However, that work did not cover the entire attack or account for all the physical hardware needed to carry it out.
The two lines of research address different challenges. Earlier work focuses on reducing the mathematical calculations required, while cultivation targets the resources needed to prepare quantum states for those operations.
The improvements cannot simply be combined to suggest that the cost of attacking Bitcoin has fallen by an equivalent multiple. Google’s March estimate already assumed efficient magic-state production. The Willow experiment evaluates part of that engineering approach rather than demonstrating an additional 40-fold reduction in the estimated cost of breaking Bitcoin’s cryptography.
Google’s earlier research also illustrated the scale of the challenge. One modeled attack, lasting approximately nine minutes after a public key became available, would require an estimated 500,000 sufficiently reliable magic states every second.
Although Willow represents progress in quantum computing, the findings do not mean Bitcoin wallets can currently be cracked with this technology. Building a machine capable of carrying out such an attack remains a substantial engineering challenge.
