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.
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 dot, and it predated by several more years the first coherent manipulation of a single spin in a semiconductor. Semiconductor spin qubits now come in four distinct flavors, each of which was proposed by theory that set a target for experiments to pursue. Those experiments always brought surprises, and the interplay between theory and experiment makes semiconductor spin qubits a particularly vibrant field of study.
In this article, professor Mark Eriksson describes the experimental development and the current state of the art of semiconductor quantum-dot spin qubits.