Will We Ever Have a Quantum Computer?
M. I. Dyakonov · Advances in parallel computing · 2020
In the hypothetical quantum computing one replaces the classical twostate 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 2 N 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.