The pertinent discovery dates to February 1990. To pick up a xenon atom, one holds the tip close to it and applies a pulse of +1 volts. That causes the atom to jump from the surface and attach itself to the tip. The tip then moves to a new location, and one applies a pulse of −1 volts. The atom then jumps back to the surface.

Having discovered how to do this, Eigler's group has gone on to use this arrangement as a switch of ultimately small size in which the active element consists of no more than a single atom. When the atom lies on the surface, beneath the tip, the arrangement is in a low-conductance mode. When the atom jumps to the tip, the system switches into high conductance. In a representative experiment these two states, respectively, passed a current of 1.2 × 10⁻⁸ and 9 × 10⁻⁸ amperes, repeatedly turning the high current on and off as the single xenon atom jumped back and forth between tip and surface in response to the voltage pulses (see Figure 2.6).

Eigler regards this as a proof of concept for an atom switch. Such switches, in turn, offer a possible route whereby the manipulation of atoms could win a niche in the world of technology. As people at IBM generally appreciate, miniature switches are the basic elements of computers. Transistors serve this role in today's designs, but there are

FIGURE 2.6 An atomic switch. Left, the switch operates by transferring an atom back and forth between surface and tip, through application of voltage pulses. Right, plot of current through the switch. A, low-conductance state with xenon atom on surface. B, +0.8-volt pulse transfers atom to the tip. C, resulting high-conductance state with greatly increased current flow. D, −0.8-volt pulse returns atom to the surface and reestablishes the low-conductance state. (Courtesy of Donald Eigler, IBM.)