Google reported that its Willow quantum chip reduced errors approximately 40-fold when preparing "magic states," specialized quantum resources that could eventually be used in attempts to crack Bitcoin wallets. The research, published in Science this week, addresses a significant engineering bottleneck in quantum computing rather than making attacks immediately feasible.
The experiment achieved approximately 99.99 percent fidelity in the states that passed quality inspection, though only about 8 percent of preparation attempts succeeded overall. The findings do not make a Bitcoin attack 40 times cheaper, but they advance a method called cultivation that could help future quantum computers sustain the roughly 500,000 reliable magic states per second that one modeled attack would require.
Why Older Bitcoin Holdings Face Greater Risk
Bitcoin wallets use a secret key to approve payments and a matching public key that allows the network to verify those approvals. Working backward from the public key to recover the secret key is supposed to require impractical computing power. However, a sufficiently large quantum computer running Shor's algorithm, first published in 1994, could theoretically perform this calculation.
Some of Bitcoin's oldest holdings have had their public keys exposed since they were mined. Later payment formats can hide the public key behind a hash until the owner makes a transaction, offering protection that some older coins do not have. A recovery phrase provides no protection against a quantum attack since the attacker would work from information already on the blockchain.
Older Bitcoin holdings have recently moved on chain. In October, 100.02 BTC untouched since July 2010 moved after more than 16 years. In August, six wallets inactive since 2011 through 2014 moved 553.59 BTC. While such transfers do not necessarily indicate quantum concerns—holders may be reorganizing or preparing to sell—they coincide with increased discussion about quantum threats.
The Engineering Challenge Behind Quantum Attacks
Quantum computers store information in units called qubits, which are extremely fragile and prone to errors from heat, vibration, and stray signals. Engineers organize physical qubits into groups that detect and correct errors, creating more reliable logical qubits. These logical qubits require specially prepared quantum states, called magic states, to perform certain operations. Each state is consumed when used, requiring continuous production throughout a calculation.
A quantum attack spends much of its effort on preparation work. Google's cultivation method produces cleaner states using less hardware than older methods, addressing the cost of supplying a quantum machine during operation rather than reducing the mathematical workload of an attack itself.
Context Within Earlier Quantum Research
In March, Google estimated that a future superconducting quantum computer with fewer than 500,000 physical qubits could theoretically break the wallet cryptography used by Bitcoin and Ethereum under specific hardware assumptions. In September, researchers reported cutting the workload of a core calculation used in such an attack. The Willow results address a different component—the cost of producing the quantum states needed to run such calculations.
These gains cannot simply be multiplied together. Google's March estimate already assumed efficient magic-state production, and the Willow experiment tests part of that engineering approach rather than providing an additional 40-fold reduction in the overall cost of attacking Bitcoin. Substantial scaling challenges remain before such theoretical attacks become practical.
