Can quantum computers break quantum cryptography? I happened to learn that encryption by quantum cryptography would be impossible to break as it's state changes the instant of an eavesdropping event(by non-quantum systems) occurs. But I'm not sure if it's still held in the case of the eavesdropper using a quantum computer(not sure how that works either) as well. Couldn't find the answer on Google so felt the community could shed some light on this.
I happened to learn that encryption by quantum cryptography would be impossible to break as it's state changes the instant an eavesdropping event (by non-quantum systems) occurs.


Actually, the QKD system doesn't assume that the eavesdropper is nonquantum; he is allowed to attempt to generate an entangled state with the qubit being transmitted.  However, even the act of generating such an entangled state would alter the received bit (with a probability that is bounded away from zero), hence any such eavesdropper is detectable, even if the eavesdropper is allowed a quantum computer.

[Insert standard skepticism about whether QKD is a practical system, compared to the alternatives]