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 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]