Google has demonstrated on its Willow processor a method that greatly reduces errors when creating a component essential to universal quantum computing. This advance brings the search for useful quantum computers closer, but it does not yet make it possible to break the signatures that secure bitcoins.
In brief
- Google achieves a major milestone in quantum computing with Willow, with an error reduction of approximately 40 times.
- Bitcoin remains beyond the reach of current quantum computers, despite progress made by Google.
- Q-Day poses a long-term threat to the cryptographic signatures that secure bitcoins.
- Some bitcoins are more exposed than others, especially those whose public keys are already visible.
- The transition to post-quantum cryptography becomes essential to protect Bitcoin before the arrival of sufficiently powerful machines.
Google reduces major quantum error by 40 times
As quantum computing targets bitcoin, the Google Quantum AI team announced on October 15 that it had successfully tested “culture of magical states” on real hardware. This technique aims to produce very reliable quantum states, necessary for the execution of certain complex operations on an error-corrected quantum computer.
The results present several specific advances:
- The error rate was reduced by approximately 40 compared to untreated states;
- The researchers obtained a logical fidelity of 99.99%;
- The experiment was carried out on Willow, Google’s quantum processor;
- Only 8% of trials were retained after error detection;
- The method has yet to work on a much larger scale.
Magical states make it possible to perform operations essential to universal quantum calculation. Their production, however, represents a considerable cost. According to some estimates, it could mobilize up to 95% of the capabilities of a quantum machine, to the detriment of the calculations themselves.
Google’s technique acts as a filter. It first places an imperfect state in a small area of the processor, then performs several coordinated measurements to rule out faulty tests. The retained states are then transferred to a larger correction code.
Google presents this experiment as the first hardware demonstration of a practical solution to this bottleneck. The company, while recognizing that many technical obstacles remain, explain :
This success transforms a major theoretical idea into an experimentally demonstrated reality.
BTC remains out of reach of current machines
The announcement does not mean that Willow can find the private keys of Bitcoin wallets. It improves the quality of a resource needed for future quantum calculations, but provides neither the number of qubits nor the stability required to execute a full cryptographic attack.
Bitcoin notably uses the ECDSA signature algorithm on the secp256k1 curve. A sufficiently powerful quantum computer could apply Shor’s algorithm to calculate a private key from a public key. He could then produce a valid signature and transfer the funds without the authorization of their owner.
In a study published in 2026, Google researchers estimated that an attack against this cryptography could require a maximum of 1,450 logical qubits and around 70 million Toffoli-type operations. With certain assumptions about hardware and error correction, fewer than 500,000 physical qubits could be enough to complete the calculation in a few minutes.
These estimates remain far from current capacities. Corrected logical qubits require many physical qubits, because quantum systems remain sensitive to noise and quickly lose the information they manipulate. Google’s demonstration concerns an element of this future architecture, not a cryptographically operational machine.
The 99.99% fidelity also doesn’t match the processor’s overall accuracy. It measures the quality of a magical state produced during the experience. Presenting this figure as proof that a quantum computer would be almost ready to attack Bitcoin would therefore be misleading.
Q-Day nevertheless requires preparing Bitcoin
“Q-Day” designates the moment when a quantum computer will be able to compromise current cryptographic systems. There is no reliable date to predict this event, but a bitcoin migration would require several years of research, development and coordination.
Not all bitcoins have exactly the same exposure. Funds placed in addresses whose public key has never been revealed benefit from additional protection through hashing. Reused old addresses, pay-to-public-key outputs, and transactions broadcast to the mempool further expose their public key.
A fast enough machine could theoretically calculate a private key between the broadcast of a transaction and its confirmation. Old bitcoins attributed to Satoshi Nakamoto or long lost raise another problem: their owners may no longer be able to migrate their funds to a post-quantum system.
The answer lies in new signatures resistant to quantum calculation. Their integration, however, would require a protocol upgrade, new address formats and a method of transferring existing funds. The community should also decide the fate of parts that never migrate.
Google’s progress therefore does not trigger Q-Day. Rather, it confirms that several formerly theoretical obstacles are beginning to receive experimental solutions. For Bitcoin, the urgency is not to give in to fear, but to prepare a transition before the threat becomes concrete.
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