Bright single-photon sources in bottom-up tailored nanowires

The ability to achieve near-unity light extraction efficiency is necessary for a truly deterministic single photon source. The most promising method to reach such high efficiencies is based on embedding single photon emitters in tapered photonic waveguides defined by top-down etching techniques. However, light extraction efficiencies in current top-down approaches are limited by fabrication imperfections and etching induced defects. The efficiency is further tempered by randomly positioned off-axis quantum emitters. Here, we present perfectly positioned single quantum dots on the axis of a tailored nanowire waveguide using bottom-up growth. In comparison to quantum dots in nanowires without waveguide, we demonstrate a 24-fold enhancement in the single photon flux, corresponding to a light extraction efficiency of 42 %. Such high efficiencies in one-dimensional nanowires are promising to transfer quantum information over large distances between remote stationary qubits using flying qubits within the same nanowire p-n junction.Comment: 19 pages, 6 figure

Spin-orbit qubit in a semiconductor nanowire

Motion of electrons can influence their spins through a fundamental effect called spin-orbit interaction. This interaction provides a way to electrically control spins and as such lies at the foundation of spintronics. Even at the level of single electrons, spin-orbit interaction has proven promising for coherent spin rotations. Here we report a spin-orbit quantum bit implemented in an InAs nanowire, where spin-orbit interaction is so strong that spin and motion can no longer be separated. In this regime we realize fast qubit rotations and universal single qubit control using only electric fields. We enhance coherence by dynamically decoupling the qubit from the environment. Our qubits are individually addressable: they are hosted in single-electron quantum dots, each of which has a different Land\'e g-factor. The demonstration of a nanowire qubit opens ways to harness the advantages of nanowires for use in quantum computing. Nanowires can serve as one-dimensional templates for scalable qubit registers. Unique to nanowires is the possibility to easily vary the material even during wire growth. Such flexibility can be used to design wires with suppressed decoherence and push semiconductor qubit fidelities towards error-correction levels. Furthermore, electrical dots can be integrated with optical dots in p-n junction nanowires. The coherence times achieved here are sufficient for the conversion of an electronic qubit into a photon, the flying qubit, for long-distance quantum communication

Glass transition behavior of polymer films of nanoscopic dimensions

Glass transition behavior of nanoscopically thin polymer films is investigated by means of molecular dynamics simulations. We study thin polymer films composed of bead-spring model chains and supported on an idealized fcc lattice substrate surface. The impact on the glass transition temperature of the strength of polymer-surface interaction and of chain grafting to the surface is investigated. Three different methods-volumetric, energetic, and dynamic-are used to determine the glass transition temperature of the films. On the basis of these, we are able to identify two different transition temperatures. When the temperature is lowered, a first transition occurs that is characterized by an anomaly in the heat capacity. Upon decreasing the temperature further, a point is reached at which the internal relaxation times diverge, as calculated, using for instance, the mode coupling theory. In qualitative agreement with the experiments, the former temperature depends on the characteristics of the polymer-surface interaction. By contrast, the latter temperature is independent of these

Selective excitation and detection of spin states in a single nanowire dot

We report exciton spin memory in a single InAs0.25P0.75 quantum dot embedded in an InP nanowire. By synthesizing clean quantum dots with linewidths as narrow as about 30 µeV, we are able to resolve individual spin states at magnetic fields on the order of 1 T. We can prepare a given spin state by tuning excitation polarization or excitation energy. These experiments demonstrate the potential of this system to form a quantum interface between photons and electrons

Orientation-dependent optical-polarization properties of single quantum dots in nanowires

The correlation of the polarization of light absorbed that are emitted by a QD embedded in a nanowire with its propagation direction with respect to the nanowire axis are discussed. The study has also demonstrated that by directing light along the nanowne axis we can access the intrinsic polarization properties of an exciting confined in a QD in a nano wire, thereby introducing a theoretical model that intuitively explains the experimental lickings and shows how the polarization dependence in absorption is affected by various parameters such as nanowire diameter. The experiments revealed that the polarization of the absorbed light when directed perpendicular to the nanowire axis, is affected hI the nanowire geometer and is strongly linearly polarized along the nanowire, in consideration with the intrinsic polarization of the nanowire OD. The results also demonstrates the polarization of photons radiated from a Zeeman split exciting in a nanowire QD with the optical path aligned along the nanowire axis as well as perpendicular to the nanowire axis

Self-assembled quantum dots in a nanowire system for quantum photonics

Quantum dots embedded within nanowires represent one of the most promising technologies for applications in quantum photonics. Whereas the top-down fabrication of such structures remains a technological challenge, their bottom-up fabrication through self-assembly is a potentially more powerful strategy. However, present approaches often yield quantum dots with large optical linewidths, making reproducibility of their physical properties difficult. We present a versatile quantum-dot-innanowire system that reproducibly self-assembles in core-shell GaAs/AlGaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface, are highly stable, and can be positioned with nanometre precision relative to the nanowire centre. Unusually, their emission is blue-shifted relative to the lowest energy continuum states of the GaAs core. Large-scale electronic structure calculations show that the origin of the optical transitions lies in quantum confinement due to Al-rich barriers. By emitting in the red and self-assembling on silicon substrates, these quantum dots could therefore become building blocks for solid-state lighting devices and third-generation solar cells