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.
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.
Old coins have also been moving as the quantum debate grows louder. On Oct. 7, 100.02 BTC untouched since July 2010 moved after more than 16 years, carrying a value of about $8.5 million. In August, six wallets inactive since 2011–2014 moved another 553.59 BTC, worth roughly $40 million at the time.
Those transfers do not singularly establish that holders are responding to quantum fears, as moving coins can even mean changing wallets, reorganizing holdings or preparing to sell.
Quantum computers store information in units called qubits, which are extremely fragile. Heat, vibration and stray signals can disturb them and introduce errors, and adding more qubits also adds more opportunities for mistakes.
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