Based on 5 minutes of reading about Grover's algorithm, I don't see how a practical quantum computer (an analog system) can repeat an operation 2^64th times to break an 128 bit key without some bias towards interaction in the qubits involved. You'd have to have 1/(2^128) attenuation between each and every channel for that to even work reliably. That's 380 dB of signal to noise ratio. Typically 40 or 50 dB of isolation is all you get between channels in communications systems.
[Edit -- Additional considerations]
In order for Grover's algorithm to work, you have to implement your conventional algorithm in Quantum hardware, with perfect fidelity. Digital computers can do this just fine because every gate is also a comparator, so signal to noise ratio isn't an issue. I fail to see how a quantum gate can possibly operate with enough fidelity to even just copy the input state after 2^64 stages, let alone complex logic AND the Grover Diffusion Operator.
[1] https://en.wikipedia.org/wiki/Grover%27s_algorithm
