Google Willow Accelerates Quantum Threat to Bitcoin, 500,000 Qubits Enough to Crack Wallets

Google announced that its Willow quantum chip managed to suppress error rates by about 40-fold when preparing the special quantum states that fuel computation. The results, published in the journal Science this week, highlight a side often overlooked in the debate over the quantum threat to bitcoin: not just the size of the attacking machine, but also the resource burden of preparing an operation before the calculation begins.

In bitcoin's architecture, wallets store private keys to authorize transactions, while public keys allow the network to verify the validity of signatures. Some types of transactions expose the public key from the outset, so anyone able to reverse a public key into a private key can take over ownership of the assets. Shor's algorithm, introduced by mathematician Peter Shor in 1994, theoretically allows a quantum computer to perform that reversal, though no real demonstration has been made to date.

In March, Google estimated that a superconducting quantum computer with fewer than 500,000 physical qubits—under certain hardware assumptions—could break the cryptography of bitcoin and ethereum wallets. For one attack scenario lasting about nine minutes after the public key is exposed, the machine is estimated to need about 500,000 sufficiently reliable magic states every second. A magic state is a processed quantum state consumed in certain operations, akin to fuel that must be continuously supplied during the computation.

The Willow breakthrough touches on a point rarely accounted for in popular literature: the cost of preparing clean quantum states. The research team used a method called cultivation, which starts with a rough version of the target state, checks for errors, and discards flawed attempts. The approach is claimed to produce clean states with lower hardware requirements than previous methods. However, the 99.99 percent fidelity figure applies only to states that passed inspection, not to all attempts.

What needs to be underlined is that this result does not automatically cut the estimated cost of a quantum attack on bitcoin by 40-fold. That is because Google's March estimate already assumed efficient magic state production. The Willow experiment tested one engineering component of that assumption, rather than directly delivering a reduction in attack cost. Another study in September also cut the core calculation burden, but it covered only one component, not a full attack including its hardware costs.

On the other hand, the quantum debate is beginning to intersect with on-chain movements. On October 7, 100.02 BTC that had been inactive since July 2010 changed hands after more than 16 years, worth about US$8.5 million. Earlier, in August, six wallets inactive since 2011–2014 moved 553.59 BTC, worth about US$40 million at the time. These movements do not necessarily mean the owners were reacting to the quantum threat, since they could also be wallet consolidation, asset reorganization, or preparation for a sale.

“Fragile quantum states mean that adding qubits actually increases the chance of errors. That is why the ability to correct errors efficiently determines how quickly the quantum threat to cryptography becomes real,” reads the core of the findings published in Science.

For Indonesia, the biggest implication is not direct bitcoin ownership, but the readiness of domestic crypto infrastructure. Domestic crypto asset exchanges registered with OJK need to start mapping quantum risk, especially for custodial wallets that hold users' public keys. Protecting old wallets whose public keys have been exposed from the outset is a shared task: quantum-resistant protocols can be introduced, but asset migration still requires valid old keys.

Going forward, the question is no longer whether quantum computers will become powerful enough, but how quickly the cost of preparing magic states falls to a level that makes an attack economical. As long as magic state supply remains a bottleneck, bitcoin has room to breathe. However, every breakthrough like Willow narrows that room, and the crypto industry—including in Indonesia—is required to prepare a migration path before the threat shifts from theoretical to operational.