Google says its Willow quantum processor has reduced errors by roughly 40 times when generating specialized quantum states needed for advanced calculations. The improvement addresses a major hardware challenge that would need to be overcome before a future quantum computer could potentially compromise Bitcoin wallets.
The findings, published in Science this week, focus on a less-discussed aspect of the quantum threat to Bitcoin. Much of the debate centers on how large a quantum computer would need to be to break wallet encryption, while giving less attention to the hardware resources required to prepare the machine for those calculations.
Why Bitcoin’s oldest coins face greater exposure
Bitcoin wallets rely on a private key to authorize transactions and a corresponding public key that allows the network to verify them. In many cases, spending bitcoin reveals the public key, while certain transaction formats make it visible from the outset.
Deriving a private key from its public counterpart is considered computationally impractical for conventional computers, helping protect cryptocurrency wallets from unauthorized access.
A sufficiently powerful quantum computer running Shor’s algorithm could theoretically reverse that relationship. An attacker who recovered the private key could create valid transaction signatures and spend the bitcoin associated with it. Mathematician Peter Shor introduced the algorithm in 1994, but no one has yet demonstrated this type of attack against Bitcoin.
Keeping a recovery phrase stored securely offline would not prevent such an attack if the attacker could derive the private key directly from public information available on the blockchain.
Some newer Bitcoin transaction formats conceal public keys behind cryptographic hashes until the owner spends the funds. In those cases, an attacker would need to wait until the key became visible and potentially recover the corresponding private key before the transaction could be confirmed.
Lost bitcoin presents a separate challenge. Although the network could adopt stronger cryptographic protections, owners would generally need access to their existing private keys to transfer their holdings into a more secure format. Coins whose keys have been lost cannot be migrated by their owners, leaving exposed public keys potentially vulnerable to future quantum attacks.
Some of Bitcoin’s oldest holdings have had their public keys visible on the blockchain since the coins were originally mined.
Several long-dormant wallets have also become active as discussions about quantum computing intensify. On Oct. 7, 100.02 BTC that had remained untouched since July 2010 moved for the first time in more than 16 years, representing 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.
However, these transactions do not necessarily indicate that holders are concerned about quantum threats. Moving bitcoin can also reflect wallet reorganizations, changes in custody arrangements or plans to sell.
The hidden cost of quantum computing
Quantum computers process information using quantum bits, or qubits, which are highly susceptible to environmental disturbances. Heat, vibration and unwanted signals can introduce errors, and increasing the number of qubits creates additional opportunities for those errors to occur.
To address this problem, engineers combine multiple physical qubits into groups that detect and correct mistakes. These groups form more reliable computational units known as logical qubits.
Quantum computers also require specially prepared states called magic states to perform certain operations. Each state is consumed during use, meaning the system must continually generate replacements throughout a calculation.
This creates a hardware allocation problem: resources used to prepare magic states cannot simultaneously be dedicated to other computational tasks.
Google’s technique, known as cultivation, aims to make this process more efficient. It begins with an imperfect quantum state, checks for errors and discards unsuccessful attempts before retaining states that meet the required standards.
The company reported approximately 99.99% fidelity, indicating that the states that passed the checks closely matched their intended quantum states.
However, only around 8% of attempts survived the preparation and measurement process. In other words, approximately eight out of every 100 attempts produced states that passed inspection.
Further research is needed to scale up the process, maintain the quality of the states during longer calculations and integrate them into more advanced error-correction systems. Researchers must also address the demands of real-time operations.
How Google’s findings relate to earlier quantum research
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, CoinDesk reported on research that reduced the computational workload associated with a key operation in such an attack. That work focused on one part of the calculation rather than the complete attack or the full physical infrastructure required.
These developments address different parts of the same challenge. Earlier research reduced the mathematical work involved, while Google’s cultivation technique targets the hardware overhead required to supply the quantum computer with usable magic states.
The improvements cannot simply be combined to claim an equivalent reduction in the cost of attacking Bitcoin. Google’s March estimate already assumed an efficient method of producing magic states. The Willow experiment evaluates part of that engineering approach rather than delivering an additional 40-fold reduction in the estimated cost of breaking Bitcoin’s cryptography.
The March research also illustrated the scale of the task. For one modeled attack lasting approximately nine minutes after a public key became available, Google estimated that the quantum computer would need around 500,000 sufficiently reliable magic states per second.
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