On Quantum Computing Quantum Computing with rf SQUIDs

Abstract

Quantum mechanics is one of the cornerstones of modern physi cs. It governs the behavior and the properties of matter in a fundamental wa y, in particular on the microscopic scale of atoms and molecules. Hence, what we may call a classical computer, i.e., one of those machines on or under the desktop s in our offices together with all their potential descendants, is itself following t he rules of quantum mechanics. However, such devices are no quantum computers in the se ns that all the inside information processing can perfectly be described within c lassical information theory. In fact, we do not need quantum mechanics in order to expl ain how the zeros and ones – the bits – inside a classical computer evolve. The r eason for this is that the architecture of classical computers does not make use of one of the most fundamental features of quantum mechanics, namely the possibi lity of superpositions. Throughout the entire processing of any program on a classic l computer, each of the involved bits takes on either the value zero or one. Quant um mechanics, however, would in addition allow superpositions of zeros one on s, that is, bits – now called qubits (quantum-bits) – which are somehow in the stat e zero and one at the same time. Computing devices which exploit this possibilit y, and with that all the essential features of quantum mechanics, are called quantu m computers [1]. Since they have an additional capability they are at least as power ful as classical computers: every problem that can be solved on a classical computer can be handled by a quantum computer just as well. The converse, however, is als o true since the dynamics of quantum systems is governed by linear differentia l equations, which can in turn be solved (at least approximately) on a classical com puter. Hence, classical and quantum computers could in principle emulate each other and quantum computers are thus no hypercomputers 1. So why quantum computing? And if there is any