Quantum Computers will have some applications in Cryptography, I hope.
There are already some Quantum Key Exchange algorithms which can detect wiretapping by a third party reliably without requiring a PKI. If any third party is listening, the connection will simply fail as a fundamental property of nature. (Though MitM is still possible, but you have to go full, a simple wiretap is not sufficient anymore)
Another possibility are quantum algorithms that allow one to perform cryptography without having to trust the underlying device, it could even be malicious, and everything would either work without leaking or just fail without leaking. (Though IIRC research in that area is still ongoing)
QKEs have nothing to do with quantum algorithms running on quantum computers.
Also they don't solve any real problem, but that's another topic. It's an extremely impractical and expensive way to implement a key exchange over short distances which, as you said, is vulnerable to mitm attacks. I don't know why anyone would want to have that.
There are already some Quantum Key Exchange algorithms which can detect wiretapping by a third party reliably without requiring a PKI. If any third party is listening, the connection will simply fail as a fundamental property of nature. (Though MitM is still possible, but you have to go full, a simple wiretap is not sufficient anymore)
Another possibility are quantum algorithms that allow one to perform cryptography without having to trust the underlying device, it could even be malicious, and everything would either work without leaking or just fail without leaking. (Though IIRC research in that area is still ongoing)