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Optics in Computing

Quantum Computing with Atoms

Christopher R. Monroe, Dietrich G. Leibfried, B. E. King, D. M. Meekhof, Wayne M. Itano, David J. Wineland

Published Jan 1, 1997• 0 citations
Publisher

Abstract

One of the fundamental tenets of quantum mechanics is the existence of superposition states, or states whose properties simultaneously possess two or more distinct values. Although quantum superpositions and entanglements seldom appear outside of the microscopic quantum world, there is growing interest in the creation of “big” superpositions and massively entangled states for use in applications such as a quantum computer.1 We report first steps toward this goal by demonstrating a fondamental two-bit quantum logic gate and a “Schrödinger cat”-like state of motion with a single trapped beryllium atom. Both experiments allow sensitive measurements of decoherence mechanisms which will play an important role in the feasibility of quantum computation.

DOI
10.1364/oc.1997.owc.1

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One of the fundamental tenets of quantum mechanics is the existence of superposition states, or states whose properties simultaneously possess two or more distinct values. Although quantum superpositions and entanglements seldom appear outside of the microscopic quantum world, there is growing interest in the creation of “big” superpositions and massively entangled states for use in applications such as a quantum computer.1 We report first steps toward this goal by demonstrating a fondamental two-bit quantum logic gate and a “Schrödinger cat”-like state of motion with a single trapped beryllium atom. Both experiments allow sensitive measurements of decoherence mechanisms which will play an important role in the feasibility of quantum computation.
Summary form only given. A quantum computer is composed of quantum bits that can support superposition states and hence allow parallel processing for certain problems such as factorization. All quantum computations can, in principle, be reduced to a series of two-bit quantum logic gates accompanied by simple single-bit rotations. An example of a two-bit quantum logic gate is the controlled-NOT (CN) gate, which rotates one quantum bit by 180 dependent on the state of a second quantum bit. We demonstrate operation of a two bit CN quantum logic gate on a trapped ion.
Introduction: about quanta, atoms, photons, and cats Experiments which manipulate and study isolated quantum systems have come of age. We can now trap single atoms or photons in a box, entangle them together, observe directly their quantum jumps, and realize in this way some of the thought-experiments imagined by the founding fathers of quantum physics. Schrödinger, who believed that observing an atom so to speak in vivo would remain forever impossible (Schrödinger 1952), would have been amazed, could he have seen what experimenters now achieve by manipulating atoms with lasers. These experiments are not just textbook illustrations of quantum concepts. They are considered by many as first steps towards harnessing the quantum world and realizing classically impossible tasks A quantum computer, for instance, would be a machine using quantum interference effects at a macroscopic scale in order to perform massive parallelism in computation (Nielsen and Chuang 2000). It would achieve an exponential speed-up to solve some problems such as the factoring of large numbers (Shor 1994). Such a machine would manipulate large ensembles of “quantum bits” made of atoms, molecules, or photons. Each bit would evolve in a superposition of two states labeled as “0” and “1”. These bits would be entangled together by quantum gates exploiting electromagnetic interactions between them. The behavior of this machine would be strange and counterintuitive. It would be a system made of thousands of two-level particles following during the calculation a huge number of different routes among which it remains coherently suspended.
Abstract The introduction to quantum computing foundations and practice motivates why quantum mechanics provides a model of computation that one should care about, and outlines the most important results in the field of quantum computing. In particular, the introduction gives details on the scope of the book, which is to provide a suitable and up-to-date canon of quantum algorithms along with important results establishing the real-world feasibility of building quantum computers. The introduction also discusses the double-slit experiment and the celebrated thought experiment Schrödinger’s cat to introduce the important quantum mechanical concepts of superposition, interferences, quantum measurement, and entanglement.
Today's computers work on bits that exist as either 0 or 1. Quantum computers aren't limited to two states; they encode information as quantum bits, or qubits, which can exist in superposition. Qubits represent atoms, ions, photons or electrons and their respective control devices that are working together to act as computer memory and a processor. Because a quantum computer can contain these multiple states simultaneously, it has the potential to be millions of times more powerful than today's most powerful supercomputers. A processor that can use registers of qubits will be able to perform calculations using all the possible values of the input registers simultaneously. This superposition causes a phenomenon called quantum parallelism, and is the motivating force behind the research being carried out in quantum computing. Due to technical obstacles, till date, a quantum computer has not yet been realized. But the concepts and ideas of quantum computing has been demonstrated using various methods like NMR, Ion Trap, Quantum Dot, Optical Methods, etc. A quantum computer manipulates qubits by executing a series of quantum gates, each a unitary transformation acting on a single qubit or pair of qubits. In applying these gates in succession, a quantum computer can perform a complicated unitary transformation to a set of qubits in some initial state. The qubits can then be measured, with this measurement serving as the final computational result. Research must devise a way to maintain decoherence and other potential sources of error at an acceptable level. Probably the most important idea in this field is the application of error correction in phase coherence as a means to extract information and reduce error in a quantum system without actually measuring that system. Thereby, quantum computers will emerge as the superior computational devices and perhaps one day make today's modern computer obsolete.