Theoretically, if you intercept in the middle, you destroy the pattern that you observe. This is a physical quantum effect, and will happen no matter what hardware you use
Since the intended use is key distribution, a MITM is fine as long as you can detect it reliably: you can keep sending new keys until one isn't eavesdropped upon, and then use that key.
I'm not talking about eavesdropping, I'm talking full on MITM. Cut the connection and insert a middle man. Both sides think they're communicating with their intended target, but they're communicating with you. How does quantum crypto protect you from that?
If someone intercepts the quantum key, it will modify it 25% of the time. If you randomly measure (and verify publicly with the sender) a fraction of your total key and find it unmodified, it means the rest of the key probably is too, up to a certain security factor. By starting with a longer key and measuring more of it (or doing privacy amplification, for example xor-ing multiple keys together), you can get as much security as you want. It also means the security is everlasting, meaning someone cannot retroactively break your key in 100 years using some mega-computer.
