Open any textbook on quantum mechanics, and the two-state system of choice is likely to be a spin-½ particle, such as an electron. The corresponding states, spin up and spin down, form the prototypical quantum bit (qubit), and rotations of the spin state constitute the simplest quantum logic gates. Because of their negative charge, electrons can be manipulated with voltages applied to nanoscale electrodes, or gates. And the application of appropriate voltages can confine the electrons to small islands called quantum dots (see the article by Marc Kastner, Physics Today, January 1993, page 24).
Twenty years ago Daniel Loss and David DiVincenzo proposed that the spin of a single electron in a semiconductor quantum dot could form not just a model but also a real, physical qubit.1 Their theoretical work predated by four years the first experiments to successfully trap a single electron in a gate-defined quantum...
