[antiRepublicrat]

The good thing about password or access cracking is that there's no way for the quantum computer to know the right answer

For going through the interface it would never work, especially since accounts are usually locked out or logins suspended for a short time after only a few tries.

This comes into play when someone has the password file with the hashes of the passwords. A quantum computer can compute the correct inputs very quickly.

[TalonDJ]

True, but first they have to use some trick to get the password file (yeah, not hard on some systems). But there are ways to prevent that which don't require a Quantum computer.