We demonstrate theoretically that it is possible to use Rabi oscillations to coherently control the electron tunneling in an asymmetric double quantum dot system, a quantum dot molecule. By applying an optical pump pulse we can excite an electron in one of the dots, which can in turn tunnel to the second dot, as controlled by an external voltage. Varying the intensity of the pulse one can suppress or enhance the tunneling between the dots for given level resonance conditions. This approach allows substantial flexibility in the control of the quantum mechanical state of the system.Comment: 4 pages, 5 figures, to appear in PR

The controlled creation, manipulation and detection of spin-polarized currents by purely electrical means remains a central challenge of spintronics. Efforts to meet this challenge by exploiting the coupling of the electron orbital motion to its spin, in particular Rashba spin-orbit coupling, have so far been unsuccessful. Recently, it has been shown theoretically that the confining potential of a small current-carrying wire with high intrinsic spin-orbit coupling leads to the accumulation of opposite spins at opposite edges of the wire, though not to a spin-polarized current. Here, we present experimental evidence that a quantum point contact -- a short wire -- made from a semiconductor with high intrinsic spin-orbit coupling can generate a completely spin-polarized current when its lateral confinement is made highly asymmetric. By avoiding the use of ferromagnetic contacts or external magnetic fields, such quantum point contacts may make feasible the development of a variety of semiconductor spintronic devices.

The quantum nature of matter lies in the wave function phases that accumulate while particles move along their trajectories. A prominent example is the Aharonov-Bohm phase, which has been studied in connection with the conductance of nanostructures. However, optical response in solids is determined by neutral excitations, for which no sensitivity to magnetic flux would be expected. We propose a new mechanism for the topological phase of a neutral particle, a polarized exciton confined to a semiconductor quantum ring. We predict that this magnetic-field induced phase may strongly affect excitons in a system with cylindrical symmetry, resulting in switching between 'bright' exciton ground states and novel 'dark' states with nearly infinite lifetimes. Since excitons determine the optical response of semiconductors, the predicted phase can be used to tailor photon emission from quantum nanostructures.It is known that the quantum mechanical phase of a state wave function is not a physical observable. This understanding, true in its absolute form, does not preclude the important possibility of observing relative phases in a suitably prepared system. In fact, much of the physics in meso-and nanoscopic systems is intrinsically connected to interference or phase-shift phenomena that manifest themselves in a number of experimentally measurable quantities. Prominent examples include the superconducting quantum interference devices (SQUIDs) [1] the quantum-phase factors induced by adiabatic changes (known as geometric Berry phases) [2], their generalization to non-adiabatic changes due to Aharonov and Anandan [3], and the well known Aharonov-Bohm (AB) effect [4]. Such a case appears naturally in systems with ring geometry in the presence of a magnetic field. In fact, recently available semiconductor rings ≃ 10-100nm in diameter allow one to explore this interesting physics in readily attainable magnetic fields. We report here on a new mechanism of phase difference acquired in a magnetic field by a composite and polarizable object with overall zero charge. Such neutral particles, excitons, are bound states of an electron and a hole in semiconductors, and are responsible for optical emission of crystals at low temperatures. The predicted interference effect has important observable consequences, as it affects the exciton emission and lifetime in nanoscopic semiconducting rings, and provides a novel phase interference phenomenon, the 'optical' AB effect. In particular, we predict a striking effect: the exciton emission can be strongly suppressed in certain magnetic-field windows.The AB phase is most simply described as the phase acquired by a charge as it traverses a region where a magnetic flux exists, while no effects of the classical Lorentz force are present. The AB effect has been verified in a number of beautiful experiments using superconducting rings [5], where electrons move in the regions with zero magnetic field. In semiconductors, the AB effect has been used in fascinating experiments to measure the relative phases ...

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We study a one-orbital Anderson impurity in a two-dimensional electron bath with Rashba spin-orbit interactions in the Kondo regime. The spin SU(2) symmetry breaking term couples the impurity to a two-band electron gas. A Schrieffer-Wolff transformation shows the existence of the Dzyaloshinsky-Moriya (DM) interaction away from the particle-hole symmetric impurity state. A renormalization group analysis reveals a two-channel Kondo model with ferro-and antiferromagnetic couplings. The parity breaking DM term renormalizes the antiferromagnetic Kondo coupling with an exponential enhancement of the Kondo temperature.