[wcoenen]

If all else fails, banks can generate terabytes of random one-time pad bytes, and then physically transport those on tape to other banks to set up provably secure communication channels that still go over the internet.

It would be a pain to manage but it would be safe from quantum computing.

[SAI_Peregrinus]

They could also use pre-shared keys with symmetric cryptography. AES-256-GCM is secure against quantum attack, no need to bother with one-time pads.