Will We Ever Have a Quantum Computer? M.I. Dyakonov 1 Laboratoire Charles Coulomb, Université Montpellier, France Abstract. 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 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. Keywords. Quantum computing, qubits 1. Introduction The idea of quantum computing was first put forward in a rather vague form by the Russian mathematician Yuri Manin in 1980. In 1981 it was independently proposed (also in a vague form) by Richard Feynman. Realizing that (because of the exponential increase of the number of quantum states) computer simulations of quantum systems become impossible when the system is large enough, he advanced the idea that to make them efficient the computer itself should operate in the quantum mode: “Nature isn’t classical and if you want to make a simulation of Nature, you’d better make it quantum mechanical, and by golly it’s a wonderful problem, because it doesn’t look so easy”. David Deutsch in 1985, formally described the universal quantum computer, as a quantum analog of the Universal Turing machine. The subject did not attract much attention until Peter Shor in 1994 proposed an algorithm allowing to factor very large numbers on an ideal quantum computer much faster compared to the conventional (classical) computer. This outstanding theoretical result has triggered an explosion of general interest in quantum computing and many thousands of research papers, mostly theoretical, have been and still continue to be published at an increasing rate. 1 Laboratoire Charles Coulomb, Université Montpellier, cc 070, 34095 Montpellier, France michel.dyakonov@gmail.com Parallel Computing: Technology Trends I. Foster et al. (Eds.) © 2020 The authors and IOS Press. This article is published online with Open Access by IOS Press and distributed under the terms of the Creative Commons Attribution Non-Commercial License 4.0 (CC BY-NC 4.0). doi:10.3233/APC200019 11