When will we have a quantum computer?
Abstract
At a given moment, the state of the hypothetical quantum computer with N qubits is characterized by 2N 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.
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