In the hypothetical quantum computing one replaces the classical two-state bit by a quantum element (qubit) with two basic states, ↑ and ↓. Its arbitrary state is described by the wave function ψ = a↑ + b↓, where a and b are complex amplitudes, satisfying the normalization condition. Unlike the classical bit, that can be only in one of the two states, ↑ or ↓, the qubit can be in a continuum of states defined by the quantum amplitudes a and b. The qubit is a continuous object. At a given moment, the state of a quantum computer with N qubits is characterized by 2N quantum amplitudes, which are continuous variables restricted by the normalization condition only. Thus, the hypothetical quantum computer is an analog machine characterized by a super-astronomical number of continuous variables (even for N∼100÷1000). Their values cannot be arbitrary, they must be under our control. Thus the answer to the question in title is: When physicists and engineers will learn to keep under control this number of continuous parameters, which means – never.
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