Google Quantum Breakthrough: Why Old Bitcoin Wallets Face More Risk

TLDR

• Google says its Willow quantum chip cut errors about 40-fold when preparing “magic states,” a resource quantum computers need for advanced calculations.
• The method, called cultivation, reached about 99.99% fidelity, but Willow kept only about 8% of its attempts after error checks.
• A large enough quantum computer could, in theory, work out bitcoin secret keys, and some of the oldest coins have exposed public keys.
• Nic Carter says no threat is imminent, while Project Eleven’s Alex Pruden says the result makes current Q-day estimates more credible.
• Coinbase published technical priorities for Bitcoin’s post-quantum transition this week.

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Google says its Willow quantum chip cut errors about 40-fold when preparing special quantum states. The result was published in Science this week.

🚨JUST IN: Google may bring Bitcoin’s Q-Day sooner than expected after its Willow chip cuts quantum errors 40x.

Google says a technique called “magic state cultivation” now works on its Willow chip, cutting errors about 40x with a record 99.99% accuracy.

Q-Day is the moment a… pic.twitter.com/6fquMJURPH

— Coin Bureau (@coinbureau) October 15, 2026

These states are called “magic states.” Quantum computers need them to run certain advanced calculations.

Some estimates say up to 95% of a quantum machine’s capacity could go toward making them. Google says its new method lowers that cost.

How Google’s Cultivation Method Works

Cultivation starts with a rough, noisy state on a small part of the chip. Researchers then run measurements to check for errors and throw out flawed attempts.

Google reported about 99.99% fidelity, which shows how closely the finished states matched the target. That number applies only to the states that passed inspection.

Willow kept about 8% of its attempts after the checks. For every 100 tries, about eight made it through.

Google researchers Craig Gidney, Noah Shutty and Cody Jones began developing the approach in 2024. The team says larger demonstrations are still needed before it can support practical quantum computing.

What It Means for Bitcoin Wallets

A large quantum computer running Shor’s algorithm could, in theory, work out a secret key from a public key. Peter Shor published the method in 1994, but nobody has demonstrated it at that scale.

Some of bitcoin’s oldest holdings have had their public keys exposed since they were mined. Lost coins add to the problem because their owners cannot move them to stronger protections.

In March, Google estimated that a quantum computer with fewer than 500,000 physical qubits could break the wallet cryptography used by Bitcoin and Ethereum. That estimate already assumed efficient magic state production.

Experts gave different views on the new result. Nic Carter of Castle Island Ventures said there is no imminent threat, but that AI is speeding up progress on error correction.

Project Eleven CEO Alex Pruden said the result does not move Q-day timelines because earlier estimates assumed the technique. He said it makes those estimates more credible.

Old coins have also been moving. On Oct. 7, 100.02 BTC untouched since July 2010 moved, worth about $8.5 million. Such transfers do not prove holders are reacting to quantum fears, since coins can move for wallet changes or sales.

Earlier this week, Coinbase published technical priorities for Bitcoin’s post-quantum transition. It called the process complex and multi-year, and Google has set an internal goal of moving to post-quantum cryptography by 2029.