[mikewarot]

Because the algorithms that offer "quantum supremacy" all are effectively analog algorithms, and rely on phase shifts to do the heavy lifting. Any small phase shifts get amplified quickly and swamp the usefulness.

All known "quantum error correction" algorithms are focused on not flipping qubits, and ignore phase.

For instance, using Grover's algorithm to crack a 128 bit key requires 2^64 quantum operations in sequence. To make this happen your phase shift and interaction between the qubits has to be low enough that it doesn't accumulate to the level sufficient to flip a single qubit in all that time.

You'd need signal to noise ratios of about 380 dB to make this happen. This would be 220 dB more!!! than detecting the signal from the Voyager (10^-20 watts) probe right next to a SuperNova explosion (10^40 watts)