An asymmetric pair of coupled InAs quantum dots is tuned into resonance by applying an electric field so that a single hole forms a coherent molecular wave function. The optical spectrum shows a rich pattern of level anticrossings and crossings that can be understood as a superposition of charge and spin configurations of the two dots. Coulomb interactions shift the molecular resonance of the optically excited state (charged exciton) with respect to the ground state (single charge), enabling light-induced coupling of the quantum dots. This result demonstrates the possibility of optically coupling quantum dots for application in quantum information processing.

Low-temperature electron-spin relaxation is studied by the optical orientation method in bulk n-GaAs with donor concentrations from 10 14 cm Ϫ3 to 5ϫ10 17 cm Ϫ3 . A peculiarity related to the metal-to-insulator transition is observed in the dependence of the spin lifetime on doping near n D ϭ2ϫ10 16 cm Ϫ3 . In the metallic phase, spin relaxation is governed by the Dyakonov-Perel mechanism, while in the insulator phase it is due to anisotropic exchange interaction and hyperfine interaction

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.

Fine and hyperfine splittings arising from electron, hole, and nuclear spin interactions in the magnetooptical spectra of individual localized excitons are studied. We explain the magnetic field dependence of the energy splitting through competition between Zeeman, exchange, and hyperfine interactions. An unexpectedly small hyperfine contribution to the splitting close to zero applied field is described well by the interplay between fluctuations of the hyperfine field experienced by the nuclear spin and nuclear dipole/dipole interactions. DOI: 10.1103/PhysRevLett.86.5176 PACS numbers: 78.67.Hc, 71.70.Ej The spin of an electron in a 10 nm GaAs quantum dot (QD) interacts with ϳ10 5 nuclear spins. This hyperfine interaction, though relatively weak, may ultimately limit spin coherence of localized electrons in QDs or shallow impurities -a concern that strongly influences developing visions of information technologies based on spin [1][2][3]. Nevertheless, there may be ways around even this intrinsic scattering process; for example, by optically polarizing all nuclear spin and thereby dramatically reducing phase space [1]. Furthermore, one could imagine using the nuclear spin for information storage or to control the electronic spin [4]. However, it is necessary to develop a more precise understanding of spin interactions in nanostructures in the presence of external magnetic and optical fields before such creative ideas can be explored.In this Letter, we present and analyze spectroscopic signatures of spin via fine and hyperfine structure splittings in the magneto-optical spectra of individual GaAs QDs under polarized and nearly resonant laser excitation. We find it necessary to consider the interaction of the electron spin with an external magnetic field (Zeeman interaction), exchange Coulomb interactions between the electron and hole, and hyperfine interactions between the electronic spin and the spins of the nuclei. Because of the hyperfine interaction, it is necessary to consider also the nuclear spin system. We are led then to consider, for the nuclei, the Zeeman interaction, dipole-dipole interactions between neighboring nuclear spins [5], and the hyperfine interaction. We quantify in experiment and theory how interactions manifest themselves in the spectral fine structure of a single exciton, discovering and explaining a remarkably complex dependence on magnetic field arising from competition between these various spin interactions.We have studied QDs formed by monolayer-high interface islands in a 4.2 nm GaAs quantum well with 25 nm Al 0.3 Ga 0.7 As barriers. The quantum wells were grown using molecular beam epitaxy with two-minute growth interrupts at the interfaces to allow large interface islands to develop. Individual QDs were excited and detected through 1 2 micron diameter apertures in an aluminum shadow mask patterned on the sample surface. A split-coil superconducting magnet was used in backscattering Faraday geometry. Previous studies have demonstrated that optical pumping could lead to large nu...