A Tuneable Few Electron Triple Quantum Dot

In this paper we report on a tuneable few electron lateral triple quantum dot design. The quantum dot potentials are arranged in series. The device is aimed at studies of triple quantum dot properties where knowing the exact number of electrons is important as well as quantum information applications involving electron spin qubits. We demonstrate tuning strategies for achieving required resonant conditions such as quadruple points where all three quantum dots are on resonance. We find that in such a device resonant conditions at specific configurations are accompanied by novel charge transfer behaviour.Comment: 11 pages, 4 figure

New onset, transient and stable motoric cognitive risk syndrome: Clinical characteristics and association with incidence of probable dementia in the NuAge cohort.

BACKGROUND: Motoric cognitive risk syndrome (MCR) is a pre-dementia stage. The existence of stable and transient MCR, their related clinical characteristics and their association with incident dementia is a matter of debate. OBJECTIVE: This study aims to examine the clinical characteristics and the time course associated with new onset, transient and stable MCR, and their association with incidence of probable dementia in community-dwelling older adults living in the province of Quebec (Canada). DESIGN: Quebec elderly population-based observational cohort study with 3 years of follow-up. SETTING: Community dwellers. SUBJECTS: A subset of participants (n = 1,113) from the “Quebec Longitudinal Study on Nutrition and Successful Aging” (NuAge) cohort. METHODS: Participants with MCR were identified at baseline and after 1 year of follow-up. Socio-demographic characteristics, 30-item Geriatric depression scale (GDS) score, cardiovascular risk factors and diseases were recorded at baseline. Incidence of probable dementia was measured at annual follow-up visits over a 3-year period. RESULTS: Over the period of the first year of follow-up, the prevalence of MCR was 8.5% with 4.3% having new onset MCR, 2.8% transient MCR and 1.4% stable MCR. A higher 30-item GDS score was reported with new onset and transient MCR, and the highest prevalence of cerebrovascular diseases was shown with stable MCR compared to non-MCR participants (p < 0.05). MCR was associated with overall incidence of probable dementia, regardless of its status (Hazard Ratio ≥ 1.86, p ≤ 0.034). CONCLUSION: Greater prevalence of depressive symptoms and cerebrovascular diseases were reported, respectively, with new onset and transient MCR, and stable MCR. The association of MCR with incidence of probable dementia remains significant, regardless of MCR subtypes

Single-molecule study for a graphene-based nano-position sensor

In this study we lay the groundwork for a graphene-based fundamental ruler at the nanoscale. It relies on the efficient energy-transfer mechanism between single quantum emitters and low-doped graphene monolayers. Our experiments, conducted with dibenzoterrylene (DBT) molecules, allow going beyond ensemble analysis due to the emitter photo-stability and brightness. A quantitative characterization of the fluorescence decay-rate modification is presented and compared to a simple model, showing agreement with the $d^{-4}$ dependence, a genuine manifestation of a dipole interacting with a 2D material. With DBT molecules, we can estimate a potential uncertainty in position measurements as low as 5nm in the range below 30nm

Bipolar spin blockade and coherent state superpositions in a triple quantum dot

Spin qubits based on interacting spins in double quantum dots have been successfully demonstrated. Readout of the qubit state involves a conversion of spin to charge information, universally achieved by taking advantage of a spin blockade phenomenon resulting from Pauli's exclusion principle. The archetypal spin blockade transport signature in double quantum dots takes the form of a rectified current. Currently more complex spin qubit circuits including triple quantum dots are being developed. Here we show both experimentally and theoretically (a) that in a linear triple quantum dot circuit, the spin blockade becomes bipolar with current strongly suppressed in both bias directions and (b) that a new quantum coherent mechanism becomes relevant. Within this mechanism charge is transferred non-intuitively via coherent states from one end of the linear triple dot circuit to the other without involving the centre site. Our results have implications in future complex nano-spintronic circuits.Comment: 21 pages, 7 figure

Time Resolved Control of Electron Tunnelling Times and Single-shot Spin Readout in a Quantum Dot

We are pursuing a capability to perform time resolved manipulations of single spins in quantum dot circuits involving more than two quantum dots. In this paper, we demonstrate full counting statistics as well as averaging techniques we use to calibrate the tunnel barriers. We make use of this to implement the Delft protocol for single shot single spin readout in a device designed to form a triple quantum dot potential. We are able to tune the tunnelling times over around three orders of magnitude. We obtain a spin relaxation time of 300 microseconds at 10T.Comment: Submitted to EP2DS 2009 Conference Proceeding

An electrostatically defined serial triple quantum dot charged with few electrons

A serial triple quantum dot (TQD) electrostatically defined in a GaAs/AlGaAs heterostructure is characterized by using a nearby quantum point contact as charge detector. Ground state stability diagrams demonstrate control in the regime of few electrons charging the TQD. An electrostatic model is developed to determine the ground state charge configurations of the TQD. Numerical calculations are compared with experimental results. In addition, the tunneling conductance through all three quantum dots in series is studied. Quantum cellular automata processes are identified, which are where charge reconfiguration between two dots occurs in response to the addition of an electron in the third dot.Comment: 12 pages, 9 figure

Theory of electronic transport through a triple quantum dot in the presence of magnetic field

Theory of electronic transport through a triangular triple quantum dot subject to a perpendicular magnetic field is developed using a tight binding model. We show that magnetic field allows to engineer degeneracies in the triple quantum dot energy spectrum. The degeneracies lead to zero electronic transmission and sharp dips in the current whenever a pair of degenerate states lies between the chemical potential of the two leads. These dips can occur with a periodicity of one flux quantum if only two levels contribute to the current or with half flux quantum if the three levels of the triple dot contribute. The effect of strong bias voltage and different lead-to-dot connections on Aharonov-Bohm oscillations in the conductance is also discussed

Quantum interference and phonon-mediated back-action in lateral quantum dot circuits

Spin qubits have been successfully realized in electrostatically defined, lateral few-electron quantum dot circuits. Qubit readout typically involves spin to charge information conversion, followed by a charge measurement made using a nearby biased quantum point contact. It is critical to understand the back-action disturbances resulting from such a measurement approach. Previous studies have indicated that quantum point contact detectors emit phonons which are then absorbed by nearby qubits. We report here the observation of a pronounced back-action effect in multiple dot circuits where the absorption of detector-generated phonons is strongly modified by a quantum interference effect, and show that the phenomenon is well described by a theory incorporating both the quantum point contact and coherent phonon absorption. Our combined experimental and theoretical results suggest strategies to suppress back-action during the qubit readout procedure.Comment: 25 pages, 8 figure