Spin Phenomena in Self-assembled Quantum Dots

Abstract

The precise coherent manipulation of quantum mechanical degrees of freedom in nanostructured solids is currently one of the most fascinating and highly sought after goals in condensed matter science. This ability is not only of strong fundamental interest but is likely to lead to a breakthrough in information technologies that exploit the quantum mechanical nature of nanostructured materials. In particular, the spin of isolated charges (electrons or holes) trapped in nanometer sized semiconductor quantum dots (QDs) has recently emerged as one of the most promising solid-state systems for the implementation of a quantum bit (qubit) [1–4]. The major advantages of implementing the spin-based systems are: (a) spin couples much more weakly to the solid-state environment when compared to charge and (b) localization of spins in quantum dot nanostructures is known to dramatically prolong their quantum phase coherence time when compared with nanostructures with higher dimensionality. This arises due to the full motional quantization in QDs, which effectively decouples spin from the orbital motion. This property in turn suppresses the spin decoherence rate due to scattering processes (phonons, charge fluctuations) which couple to the spin via the spin-orbit interaction. The spin flip time in self-assembled QDs was measured to lie in the millisecond range [5], and coherence time is expected to be similarly long. However, in order to reach this theoretical limit other possible decoherence mechanisms need to be controlled. Most important amongst these is the hyperfine coupling between the confined electron spin and the ensemble of nuclear spins in the QD. Combination of robustness of spin-based approach with newly developed methods for the controlled fabrication of few-dot nanostructures that can be optically addressed over ultrafast timescales, opens a new window of opportunity to develop a class of fully coherent, spin based opto-electronic devices that may facilitate future information technologies.