[exdsq]

There's a lot of research and practical work on quantum-proof cryptography which is already in use in some cryptocurrencies - 'just' need to hardfork and update it when it's ready for Bitcoin

[anonporridge]

No need for a hard fork. A soft fork like Taproot is doing this year would be sufficient.

[21eleven]

What cryptocurrencies are currently using post-quantum cryptography?

[DennisP]

Only one I'm aware of is QRL ("quantum-resistant ledger").

https://www.theqrl.org/