  At a given moment, the state of the hypothetical quantum computer with N
qubits is characterized by 2^N quantum amplitudes, which are complex continuous
variables restricted by the normalization condition only. Their values cannot
be arbitrary, they must be under our control. For moderate N = 1000, the number
of quantum amplitudes greatly exceeds the number of particles in the Universe.
Thus the answer to the question in title is: When physicists and engineers will
learn to keep under control this number of continuous parameters.
