Single-electron memory (SEM) refers to experimental hardware technologies in which the position or presence of a single charge carrier, usually an electron, makes the difference between the logical low (0) and high (1) states in a digital system.? Primitive forms of SEM have been explored, and some engineers believe it is only a matter of time before it is put into use at the consumer level.? When this happens, computer memory chips will have far greater capacity than anything known today.? There are various ways in which SEM might work.? Two hypothetical schemes are outlined here: SEM might be used to detect the presence or absence of electrical charges in individual atoms. If the number of electrons in an atom is the same as the number of protons (the normal or usual case), that atom has no electrical charge. If there are fewer electrons than protons, the atom acquires a positive charge; if there are more electrons than protons, the atom acquires a negative charge. Charged atoms are known as ions. The charge (or absence thereof) can be detected, and electronic circuits designed to act in different ways depending on whether a given atom is negative, neutral, or positive. In binary logic, a neutral or negative ion might represent a 0 (low) bit, and a positive ion might represent 1 (high). Ions can also represent the states of trinary logic. A negative ion can represent logic -1; a neutral ion can represent logic 0; a positive ion can represent logic +1. The contents of an ionic SEM chip could be changed by means of precisely directed electrical currents. Another way in which SEM might operate involves the relative energy states of atoms depending on the positions, or orbits, of electrons within. Every atom has several different energy levels, called shells, at which electrons can exist. The shells resemble spheres surrounding the nucleus. The larger the diameter of the shell, the greater the energy contained in an electron orbiting in that shell. Low-energy shells in each atom of a given element might be assigned logic 0; high-energy shells could be assigned logic 1. An electron changes shells when a photon of a certain energy level (represented by a pulse of energy at a specific wavelength) is emitted or absorbed by the atom. When a photon having the correct wavelength is absorbed by the atom, the electron rises to a higher energy state; when the electron falls to a lower energy state, a photon of the same wavelength is emitted by the atom. The contents of such a memory could be changed using tiny electromagnetic pulses directed at specific atoms in a chip