Quantum computation

Article By:

Pfeifer, Peter Department of Physics and Astronomy, University of Missouri, Columbia, Missouri; Center for Nonlinear Studies, Los Alamos National Laboratory, Los Alamos, New Mexico.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.801870

A mode of computation based upon quantum bits, which can be put into any superposition of two quantum states, designated |0&x3009 x232A; and |1&x3009 x232A;. Whereas classical computers perform operations on classical bits, which can be in only one of two discrete states, 0 or 1, quantum computers perform operations on quantum bits, or qubits. Quantum computers hold the promise to perform classically intractable tasks—computations that require astronomical time or hardware resources on the fastest classical computer—in minuscule time with minuscule resources. The building blocks of a quantum computer are atomic particles which, following the laws of quantum physics, behave like waves and exhibit interference phenomena as if they were in several locations at the same time. By equating different locations—for example, an electron in the lowest orbit or in an excited orbit of an atom—to binary digits 0 or 1, one may interpret the time-evolving state of the particles as executing several computations at the same time. One set of locations at a given time describes the result of one computation. Thus one atom can do two computations at once; two atoms can do four; three atoms can do eight. The challenge is to coerce the atoms to follow trajectories that amount to meaningful computations and to read out a definite result from the multitude of computations occurring in parallel. The control of trajectories is the hardware part of the challenge; the design of useful trajectories—algorithms that are superior to classical algorithms—is the software part of the challenge. See also: Digital computer; Supercomputer

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