From CoinDesk · By Shaurya Malwa
Google said its Willow quantum chip reduced errors about 40-fold when preparing “magic states,” a resource needed for quantum calculations that could eventually threaten Bitcoin wallets.
The experiment achieved about 99.99 percent fidelity among states that passed inspection, though only about 8 percent of preparation attempts succeeded and substantial scaling challenges remain.
The findings do not make a Bitcoin attack 40 times cheaper, but they advance an engineering method that could help future quantum computers sustain the roughly 500,000 reliable magic states per second required under one modeled attack.
Google says its Willow quantum chip has cut errors about 40-fold when preparing special states used in quantum calculations, tackling a costly step toward building machines powerful enough to crack bitcoin wallets.
The result, published in Science this week, deals with something that rarely comes up in the bitcoin quantum debate — which usually focuses on how big a machine an attacker would need while underplaying how much of that machine would be occupied getting ready to calculate.
Why the oldest coins give an attacker more time
A bitcoin wallet uses a secret key to approve payments and a matching public key that lets the network check those approvals. Spending typically reveals the public key, while some payment types expose it from the start.
Working backward from it to the secret is supposed to take an impractical amount of computing, which makes it practically impossible to reverse-engineer crypto wallets.
However, a large enough quantum computer running Shor’s algorithm could, in theory, do that math. Whoever recovered the secret key could sign transactions as the owner, and the network would accept them as genuine. Mathematician Peter Shor first published the method in 1994, but nobody has yet managed to demonstrate it.
A recovery phrase locked in a physical drawer offers no protection against that, as the attacker would be working from information already sitting on the blockchain.
Some later payment formats hide the public key behind a hash, a digital fingerprint, until the owner spends. An attacker targeting a previously hidden key would have to wait for that exposure and might need to recover the secret quickly enough to interfere with the payment.
That is also why lost coins present a difficult problem. Bitcoin can introduce stronger spending protections, but someone still needs the existing keys to move old holdings into them. If those keys are lost, the owner cannot make the move. The exposed public key remains available to a future attacker.
Some of bitcoin’s oldest holdings have had their public keys exposed since they were mined.