That depends on whether the public key has been exposed.
Bitcoin addresses encode the ripemd160 hash of the public key, so by default when payments are made to new addresses they are not quantum crackable.
But when someone spends from an address they publish the public key to the chain as part of the spend. From then on, any new deposits sent to the same address are at risk of quantum attack
Suppose I make a paper wallet on an offline PC, write down the address and discard both keys. If I now send some BTC to this address, how does the client figure out the public key?
That depends on whether the public key has been exposed.
Bitcoin addresses encode the ripemd160 hash of the public key, so by default when payments are made to new addresses they are not quantum crackable.
But when someone spends from an address they publish the public key to the chain as part of the spend. From then on, any new deposits sent to the same address are at risk of quantum attack
The hash is merely a convenience, the _actual_ public key used in transactions is present in the ledger and available to anybody who wants it.
How does that work?
Suppose I make a paper wallet on an offline PC, write down the address and discard both keys. If I now send some BTC to this address, how does the client figure out the public key?
Technically it might be more profitable to mine extraterrestrial diamonds.
at this point bitcoin will be worthless