Electrically controlling single spin coherence in semiconductor nanostructures

Introduction In 1998, Daniel Loss and David DiVincenzo published a seminal paper describing how semiconductor quantum dots could be used to create spin qubits for quantum information processing [28]. They recognized that a single spin in a magnetic field forms a natural two-level system that can serve as a quantum bit. Moreover, owing to the weak magnetic moment of the electron, the spin is relatively well isolated from the environment leading to long coherence times. To confine single spins, Loss and DiVincenzo envisioned the quantum dot architecture shown in Fig. 15.1. A GaAs/AlGaAs heterostructure confines electrons to a two-dimensional electron gas (2DEG). Depletion gates are fabricated on top of the structure to provide a tunable confinement potential, trapping a single electron in each quantum dot. Neighboring quantum dots are tunnel coupled, with the coupling strength controlled by the electrostatic potential. The orientation of a single spin can be controlled by using electron spin resonance (ESR), while nearest-neighbor coupling is mediated by the depletion gate tunable exchange interaction. It is fair to say that in 1998 many of the requirements of the Loss-DiVincenzo proposal had not been implemented, starting with the most basic necessity of a single electron lateral quantum dot [8]. The purpose of this chapter is to describe several experiments inspired by the Loss-DiVincenzo proposal. Many powerful experiments have been performed since 1998 and, given the space constraints here, we cannot give each experiment the attention it deserves.