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The wizardry of modern semiconductor technology makes it possible to fabricate particles of metal or “pools” of electrons in a semiconductor that are only a few hundred angstroms in size. Electrons in these structures can display astounding behavior. Such structures, coupled to electrical leads through tunnel junctions, have been given various names: single‐electron transistors,quantum dots, zero‐dimensional electron gases and Coulomb islands. In my own mind, however, I regard all of these as artificial atoms—atoms whose effective nuclear charge is controlled by metallic electrodes. Like natural atoms, these small electronic sytems contain a discrete number of electrons and have a discrete spectrum of energy levels. Artificial atoms, however, have a unique and spectacular property: The current through such an atom or the capacitance between its leads can vary by many orders of magnitude when its charge is changed by a single electron. Why this is so, and how we can use this property to measure the level spectrum of an artificial atom, is the subject of this article.
The charge and energy of a sufficiently small particle of metal or semiconductor are quantized just like those of an atom. The current through such a quantum dot or one‐electron transistor reveals atom‐like features in a spectacular way.