Artificial Atoms

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.