Single spins in self-assembled quantum dots

The superposition principle is at the heart of quantum mechanics: a spin, for instance, can simultaneously be in the ‘up’ and ‘down’ states. As a consequence, two spins can be entangled, the entanglement representing an intimate link even though the spins may be far apart. Exploring these concepts experimentally is challenging as it is difficult to shield the quantum system from the deleterious effects of the environment. Most of the early successes involving light quanta (photons) were made in the context of atomic physics. However, a quiet revolution has taken place in the past decade or so: materials advances in both semiconductors and diamond have enabled experiments to be performed in the solid state with single spins and single photons. As semiconductors are so important for real-world devices, these experiments may lead to applications. In particular, a qubit in a semiconductor has potential applications in the areas of quantum metrology, quantum communication and quantum information processing. A qubit is a two-level quantum system that can be initialized, manipulated and read out. The manipulation in particular should be predictable: it should be carried out before the qubit loses information encoded in its phase, an issue known as the decoherence problem. Implementing the qubit concept in a semiconductor leads naturally to the electron spin 1 , which is a natural two-level system. An electron spin interacts only weakly (through the spin–orbit interaction) with the main source of decoherence in bulk semiconductors, the lattice vibrations (phonons) 2 , and has a strong interaction with experimental probes (unlike a nuclear spin for instance). The search for a coherent electron spin in the solid state has led most spectacularly to the nitrogen–vacancy centre in diamond. The atomic-like confinement, along with the low-mass carbon atoms, suppresses the spin–orbit interaction. Furthermore, spin dephasing arising from spin noise in the host nuclear spins is small and can be suppressed even further with isotopically pure material such that the spin coherence time can reach a millisecond even at room temperature 3