Electrical control over single hole spins in nanowire quantum dots

Single electron spins in semiconductor quantum dots (QDs) are a versatile platform for quantum information processing, however controlling decoherence remains a considerable challenge. Recently, hole spins have emerged as a promising alternative. Holes in III-V semiconductors have unique properties, such as strong spin-orbit interaction and weak coupling to nuclear spins, and therefore have potential for enhanced spin control and longer coherence times. Weaker hyperfine interaction has already been reported in self-assembled quantum dots using quantum optics techniques. However, challenging fabrication has so far kept the promise of hole-spin-based electronic devices out of reach in conventional III-V heterostructures. Here, we report gate-tuneable hole quantum dots formed in InSb nanowires. Using these devices we demonstrate Pauli spin blockade and electrical control of single hole spins. The devices are fully tuneable between hole and electron QDs, enabling direct comparison between the hyperfine interaction strengths, g-factors and spin blockade anisotropies in the two regimes

Disentangling the effects of spin-orbit and hyperfine interactions on spin blockade

We have achieved the few-electron regime in InAs nanowire double quantum dots. Spin blockade is observed for the first two half-filled orbitals, where the transport cycle is interrupted by forbidden transitions between triplet and singlet states. Partial lifting of spin blockade is explained by spin-orbit and hyperfine mechanisms that enable triplet to singlet transitions. The measurements over a wide range of interdot coupling and tunneling rates to the leads are well reproduced by a simple transport model. This allows us to separate and quantify the contributions of the spin-orbit and hyperfine interactions.Comment: 5 pages, 4 figure

Demonstration of Entanglement of Electrostatically Coupled Singlet-Triplet Qubits

Quantum computers have the potential to solve certain interesting problems significantly faster than classical computers. To exploit the power of a quantum computation it is necessary to perform inter-qubit operations and generate entangled states. Spin qubits are a promising candidate for implementing a quantum processor due to their potential for scalability and miniaturization. However, their weak interactions with the environment, which leads to their long coherence times, makes inter-qubit operations challenging. We perform a controlled two-qubit operation between singlet-triplet qubits using a dynamically decoupled sequence that maintains the two-qubit coupling while decoupling each qubit from its fluctuating environment. Using state tomography we measure the full density matrix of the system and determine the concurrence and the fidelity of the generated state, providing proof of entanglement

Spectroscopy of spin-orbit quantum bits in indium antimonide nanowires

Double quantum dot in the few-electron regime is achieved using local gating in an InSb nanowire. The spectrum of two-electron eigenstates is investigated using electric dipole spin resonance. Singlet-triplet level repulsion caused by spin-orbit interaction is observed. The size and the anisotropy of singlet-triplet repulsion are used to determine the magnitude and the orientation of the spin-orbit effective field in an InSb nanowire double dot. The obtained results are confirmed using spin blockade leakage current anisotropy and transport spectroscopy of individual quantum dots.Comment: 5 pages, supplementary material available at http://link.aps.org/supplemental/10.1103/PhysRevLett.108.16680

Influence of Adsorbed and Nonadsorbed Polymer Additives on The Viscosity of Magnesium Oxide Suspensions

Adsorbed polymer additives have been employed to reduce water content and improve cement workability through lowering viscosity, but the influence of over-dosage and the presence of nonadsorbed chains have yet to be fully understood. Model magnesium oxide (MgO) suspensions were used to investigate the potential processing effect of “free” chain concentration on cementitious mixtures. The rheological impact of the free chains was measured through incorporation of nonadsorbing poly(ethylene glycol) (PEG) to suspensions stabilized with an adsorbed comb-polymer superplasticizer. Analyses of the rheological data, that showed viscosity-increases and viscosity-reduction due to free PEG concentrations revealed a transition from depletion flocculation to depletion stabilization that contributed to the flow properties of the suspensions. The viscosity-reduction observed for high concentrations of free chains may be useful for improved mixing of cements with free polymer in addition to the adsorbed polycarboxylate etherbased superplasticizer. Additionally, the influence of free PEG on the macroscale flow behavior was also examined through local velocity measurements under shear

Suppression of Zeeman gradients by nuclear polarization in double quantum dots

We use electric dipole spin resonance to measure dynamic nuclear polarization in InAs nanowire quantum dots. The resonance shifts in frequency when the system transitions between metastable high and low current states, indicating the presence of nuclear polarization. We propose that the low and the high current states correspond to different total Zeeman energy gradients between the two quantum dots. In the low current state, dynamic nuclear polarization efficiently compensates the Zeeman gradient due to the $g$-factor mismatch, resulting in a suppressed total Zeeman gradient. We present a theoretical model of electron-nuclear feedback that demonstrates a fixed point in nuclear polarization for nearly equal Zeeman splittings in the two dots and predicts a narrowed hyperfine gradient distribution

Nanoscale spin rectifiers controlled by the Stark effect

The control of orbital and spin state of single electrons is a key ingredient for quantum information processing, novel detection schemes, and, more generally, is of much relevance for spintronics. Coulomb and spin blockade (SB) in double quantum dots (DQDs) enable advanced single-spin operations that would be available even for room-temperature applications for sufficiently small devices. To date, however, spin operations in DQDs were observed at sub-Kelvin temperatures, a key reason being that scaling a DQD system while retaining an independent field-effect control on the individual dots is very challenging. Here we show that quantum-confined Stark effect allows an independent addressing of two dots only 5 nm apart with no need for aligned nanometer-size local gating. We thus demonstrate a scalable method to fully control a DQD device, regardless of its physical size. In the present implementation we show InAs/InP nanowire (NW) DQDs that display an experimentally detectable SB up to 10 K. We also report and discuss an unexpected re-entrant SB lifting as a function magnetic-field intensity

A systematic map of evidence on the contribution of forests to poverty alleviation

This is the final version. Available from the publisher via the DOI in this record.Background: Forests provide an essential resource to the livelihoods of an estimated 20% of the global population. The contribution of forest ecosystems and forest-based resources to poverty reduction is increasingly emphasized in international policy discourse and conservation and development investments. However, evidence measuring the effect of forest-based activities on poverty outcomes remains scattered and unclear. Lack of systematic understanding of forest-poverty relationships, in turn, inhibits research, policymaking, and efficient financial resource allocation. Methods: To identify relevant studies for inclusion in this systematic map we searched six bibliographic databases, 15 organizational websites, eight systematic evidence syntheses (reviews and maps), and solicited information from key informants. Search results were screened for relevance against predefined inclusion criteria at title, abstract, and full text levels, according to a published protocol. Included articles were coded using a predefined framework. Trends in the evidence, knowledge gaps and relatively well-researched sub-topics are reported in a narrative synthesis. Occurrence and extent of existing evidence about links between interventions and outcomes are presented in a visual heatmap. Data are available through the open access Evidence for Nature and People Data Portal (http://www.natureandpeopleevidence.org). Results: A total of 242 articles were included in the systematic map database. Included articles measured effects of 14 forest-based intervention types on 11 poverty dimensions. The majority of the evidence base (72%) examined links between productivity-enhancement strategies (e.g. forest management, agroforestry, and habitat management) and monetary income and/or social capital outcomes. Other areas with high occurrence of articles include linkages between interventions involving governance, individual rights/empowerment or linked enterprises/livelihood alternatives with impacts on monetary income from direct sale of goods. A key knowledge gap was on the impacts of investment-based interventions (i.e. enhancing produced, human, and social capitals). Another was the impacts of forest-based interventions on financial capital (savings, debt), non-monetary benefits, and health. Conclusions: The evidence base on forest-based productive activities and poverty alleviation is growing but displays a number of biases in the distribution of articles on key linkages. Priorities for future systematic reviews and evaluations include in-depth examinations into the impacts of rights-based activities (e.g. governance, empowerment) on poverty dimensions; and productivity-enhancing activities on social capital. More comprehensive and robust evidence is needed to better understand the synergies and trade-offs among the different objectives of forest conservation and management and variation in outcomes for different social groups in different social-ecological contexts