I'm not nearly well versed enough to speak intelligently on the subject, but my understanding is that only quantum computers can do a "perfect" key exchange, because the detection of interference by a third party can only be detected by a quantum computer.
Kind of, quantum key exchange is based on the uncertainty principle. By encoding the key exchange in quantum properties of particles, any mitm observer will change those properties, thus breaking the exchange. You don't need a quantum computer to do QKE, just the right kind of detector. There's already a bunch of people that do this without quantum computers.
