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There was a good article about how "strong" today's quantum computers are compared to what they should be in order to be a threat to Bitcoin at all, so about 2 years ago we were able to read the following:
Researchers at the University of Sussex estimated in February that a quantum computer with 1.9 billion qubits could essentially crack the encryption safeguarding Bitcoin within a mere 10 minutes. Just 13 million qubits could do the job in about a day. Fortunately, the ability to deploy quantum computers with so many qubits still seems many years away. IBM unveiled its 127-qubit processor just last year, while a unit sporting 1,000 qubits is set to be completed by the end of 2023.
The article says that something like that won't happen for at least another 10-20 years, but considering how today's technology is progressing, nothing can surprise me.
Three is an important thing here about that cracking, it's about cracking the private key associated with a public known key!
Those numbers in the article are for that kind of breaking, which can't work again an address with unspent coins, the difference between them is insane:
https://en.bitcoin.it/wiki/Quantum_computing_and_BitcoinOn traditional computers, it takes on the order of 2128 basic operations to get the Bitcoin private key associated with a Bitcoin public key, it is known for sure that it would take a sufficiently large quantum computer on the order of only 1283 basic quantum operations to be able to break a Bitcoin key.
So for public keys is goes 2
128 to 128
3, it does start to sound doable.
For a direct private keys attack, so this is lost coins like Satoshi's :
For example, finding some data which hashes to a specific SHA-256 hash requires 2256 basic operations on a traditional computer, but 2128 basic quantum operations.
although it does cut an enormous number by a huge margin, if we speak about that time in human life terms, no difference!
You want to feel safe, don't re-use your address!