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Section 1: Introduction

Thirty-two ions, fluorescing under illumination by laser light in an electrodynamic trap.

Figure 1: Thirty-two ions, fluorescing under illumination by laser light in an electrodynamic trap.

Source: © R. Blatt at Institut f. Experimentalphysik, Universtaet Innsbruck, Austria. More info

The creation of quantum mechanics in the 1920s broke open the gates to understanding atoms, molecules, and the structure of materials. This new knowledge transformed our world. Within two decades, quantum mechanics led to the invention of the transistor to be followed by the invention of the laser, revolutionary advances in semiconductor electronics, integrated circuits, medical diagnostics, and optical communications. Quantum mechanics also transformed physics because it profoundly changed our understanding of how to ask questions of nature and how to interpret the answers. An intellectual change of this magnitude does not come easily. The founders of quantum mechanics struggled with its concepts and passionately debated them. We are the beneficiaries of that struggle and quantum mechanics has now been developed into an elegant and coherent discipline. Nevertheless, quantum mechanics always seems strange on first acquaintance and certain aspects of it continue to generate debate today. We hope that this unit provides insight into how quantum mechanics works and why people find it so strange at first. We will also sketch some of the recent developments that have enormously enhanced our powers for working in the quantum world. These advances make it possible to manipulate and study quantum systems with a clarity previously achieved only in hypothetical thought experiments. They are so dramatic that some physicists have described them as a second quantum revolution.

Neutral rubidium atoms in an optical lattice trap.

Figure 2: Neutral rubidium atoms in an optical lattice trap.

Source: © M. Greiner. More info

An early step in the second quantum revolution was the discovery of how to capture and manipulate a single ion in an electromagnetic trap, reduce its energy to the quantum limit, and even watch the ion by eye as it fluoresces. Figure 1 shows an array of fluorescing ions in a trap. Then methods were discovered for cooling atoms to microkelvin temperatures (a microkelvin is a millionth of a degree) and trapping them in magnetic fields or with light waves (Figure 2). These advances opened the way to stunning advances such as the observation of Bose-Einstein condensation of atoms, to be discussed in Unit 6, and the creation of a new discipline that straddles atomic and condensed matter physics.

The goal of this unit is to convey the spirit of life in the quantum world—that is, to give an idea of what quantum mechanics is and how it works—and to describe two events in the second quantum revolution: atom cooling and atomic clocks.