I've read on the Bitcoin wiki that Bitcoin private key is usually a 256 bit number, but it can also be between 128 to 512 bits. Does this mean that someone with a quantum computer can theoretically generate all 128 bit long private keys in 2^64 time using Grover's algorithm? Also, is there any way to check if an address corresponds to a key of certain length? So, if such attack is possible, how likely it is to be executed on practice (how likely early quantum computers will be able to break 64 bits of security) and what can be done to prevent it?