Satoshi's Bitcoin Faces Bigger Quantum Threat After Google's New Willow Success

Google says its Willow chip can prepare the special quantum states needed for advanced computing with about 40 times less error, alleviating one of the hardware bottlenecks behind any future attempt to crack a Bitcoin wallet.

Google says its Willow quantum chip has reduced errors by about 40 times when preparing special modes used in quantum computing, taking a costly step toward building machines powerful enough to crack a Bitcoin wallet.

The result, published this week in Science, addresses something that rarely comes up in the quantum Bitcoin debate — which typically focuses on how much of a machine an attacker needs, while ignoring how much of that machine it takes to be ready to compute.

A Bitcoin wallet uses a secret key to confirm payments and a matching public key that allows the network to verify those confirmations. Spending usually exposes the public key, while some payment types expose it from scratch.

Working from that to the secret is expected to require an impractical amount of computation, making reverse engineering of crypto-wallets virtually impossible.

However, a large enough quantum computer running Shor's algorithm could in theory do this math. Anyone who retrieves the secret key can sign transactions as the owner, and the network accepts them as genuine. Mathematician Peter Shore first published this method in 1994, but no one has yet succeeded in proving it.

A recovery phrase locked in a physical drawer offers no protection against that, since an attacker is working from information on the blockchain.

Some later payment formats hide the public key behind a hash, a digital fingerprint, until the owner pays. An attacker targeting a previously secret key must wait for it to be exposed and may need to recover the secret quickly enough to interfere with the payment.

This is also why lost coins are such a difficult problem. Bitcoin can provide stronger protections for spending, but someone still needs the keys to transfer old assets to them. If those keys are lost, the owner can't move. The revealed public key remains available to a future attacker.

Some of the oldest Bitcoin assets have had their public keys exposed since they were mined.

As the quantum debate gets louder, so do the old coins. On October 7, Bitcoin moved past 100.02 untouched since July 2010 after more than 16 years and was worth around $8.5 million. In August, six inactive wallets from 2011 to 2014 transferred another 553.59 bitcoins, worth approximately $40 million at the time.

These transfers alone do not indicate that holders are responding to quantum fears, as coin movements could even mean changing wallets, reorganizing holdings or preparing to sell.

Quantum computers store information in units called qubits, which are very fragile. Heat, vibration, and stray signals can disrupt them and cause errors, and adding more qubits also creates more opportunities for mistakes.

Engineers combat this by organizing many physical qubits into groups that detect and correct errors. Each group behaves like a more reliable unit known as a logical qubit.

Those units then need the digital equivalent of fuel to perform specific operations, specially prepared quantum states that researchers call "magic states." Each is consumed when used, so the computer must continue to generate them during a calculation.

Consider a restaurant kitchen where each dish requires a batch of fresh, non-reusable sauces, where some staff must prepare sauces while others cook. In a quantum computer, the hardware dedicated to generating magic states is hardware that cannot be dedicated to any other task.

Google's method, called cultivation, starts with a rough version of the state, checks it for errors, and discards incomplete attempts. The goal is to produce clean modes using less hardware than older methods.

The company reported about 99.99 percent fidelity, a measure of how closely the end states match what the researchers intended.

This number only applies to states that have passed the inspection. Willow retained about 8% of her efforts after checks during preparation and measurement. For every 100 attempts, about 8 succeed.

The next steps involve running the process on a much larger scale and keeping these modes useful as they feed into longer computations. The researchers say that more work is needed on real-time operations and transitions to more robust error-correction settings.

In March, Google estimated that a future superconducting quantum computer with fewer than 500,000 physical qubits could crack the wallet encryption used by Bitcoin and Ethereum, under a specific set of hardware assumptions.

In September, CoinDesk reported that researchers had scaled down the main computing workload used in that attack. That would cover the work of one component, skipping the full attack and physical hardware costs of running it.

These studies reduce the amount of math an attacker has to do. Cultivation pays for the cost of providing the machine while doing this calculation.

Profits cannot simply be multiplied together. Google's March estimate already assumed efficient production of Magic mode. Rather than another 40-fold reduction in the estimated cost of attacking Bitcoin, this experiment tests part of this engineering approach.

The March paper also shows how much fuel an attack burns. For a modeled attack that takes about 9 minutes after the public key is available, Google estimates that the device needs 500,000 sufficiently reliable magic states per second.

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