Winning at Quantum Dice

The ability to control individual spins (the intrinsic units of angular momentum carried by electrons) in semiconductors is an important requirement for a new generation of devices based on spin rather than charge logic. Single magnetic ions are promising for this type of application because of their long spin coherence times. The difficulty lies in addressing these stable but isolated spins, a feat that can be achieved by placing a single magnetic ion like manganese (Mn) inside a semiconductor quantum dot [1]. This arrangement strongly mixes the states of the Mn spin and the charge carriers trapped inside the nanoobject. As a result, optical initialization and readout of an Mn spin state can be achieved [2] using resonant laser excitation, as employed previously in single-electron and hole spin-pumping schemes used in quantum dots [3, 4]. A key feature of these established spin-pumping techniques is a depletion of the spin level that is resonantly excited, which may at first seem counterintuitive.