A PERIODIC TABLE FOR ARTIFICIAL ATOMS. At the atomic level, confinement leads to quantization: electrons trapped in atoms possess only certain discrete energies. All of this applies as well to quantum dots, which are sculpted from tiny layers of semiconductor and metal. By modulating an external electric field, electrons can be added to or subtracted from the dot one by one. In effect the dots are artificial atoms and, like real atoms, can be sorted into a Periodic Table according to their complement of electrons. The electrons reside in two- dimensional orbits. This and the fact that the dots (small as they are) are so much bigger than regular atoms permits the study of quantum effects not seen before. For one thing, more magnetic flux (from an external magnet) can fit inside a micron-sized dot than in an angstrom- sized atom; this can make magnetic interactions much more prominent. Physicists suspect that quantum dot molecules and even crystals will be a novel arena for building tailored materials and for exploring new quantum physics. (New Scientist, 29 August 1998.)