Coulomb blockade in an atomically thin quantum dot coupled to a tunable Fermi reservoir

Gate-tunable quantum-mechanical tunnelling of particles between a quantum confined state and a nearby Fermi reservoir of delocalized states has underpinned many advances in spintronics and solid-state quantum optics. The prototypical example is a semiconductor quantum dot separated from a gated contact by a tunnel barrier. This enables Coulomb blockade, the phenomenon whereby electrons or holes can be loaded one-by-one into a quantum dot. Depending on the tunnel-coupling strength, this capability facilitates single spin quantum bits or coherent many-body interactions between the confined spin and the Fermi reservoir. Van der Waals (vdW) heterostructures, in which a wide range of unique atomic layers can easily be combined, offer novel prospects to engineer coherent quantum confined spins, tunnel barriers down to the atomic limit or a Fermi reservoir beyond the conventional flat density of states. However, gate-control of vdW nanostructures at the single particle level is needed to unlock their potential. Here we report Coulomb blockade in a vdW heterostructure consisting of a transition metal dichalcogenide quantum dot coupled to a graphene contact through an atomically thin hexagonal boron nitride (hBN) tunnel barrier. Thanks to a tunable Fermi reservoir, we can deterministically load either a single electron or a single hole into the quantum dot. We observe hybrid excitons, composed of localized quantum dot states and delocalized continuum states, arising from ultra-strong spin-conserving tunnel coupling through the atomically thin tunnel barrier. Probing the charged excitons in applied magnetic fields, we observe large gyromagnetic ratios (~8). Our results establish a foundation for engineering next-generation devices to investigate either novel regimes of Kondo physics or isolated quantum bits in a vdW heterostructure platform.Comment: Published in Nature Nanotechnology. 7 pages + 14 supplementary information pages. 14 figure

Atomically-thin quantum dots integrated with lithium niobate photonic chips

The electro-optic, acousto-optic and nonlinear properties of lithium niobate make it a highly versatile material platform for integrated quantum photonic circuits. A prerequisite for quantum technology applications is the ability to efficiently integrate single photon sources, and to guide the generated photons through ad-hoc circuits. Here we report the integration of quantum dots in monolayer WSe2 into a Ti in-diffused lithium niobate directional coupler. We investigate the coupling of individual quantum dots to the waveguide mode, their spatial overlap, and the overall efficiency of the hybrid-integrated photonic circuit

Out-of-plane orientation of luminescent excitons in atomically thin indium selenide flakes

Van der Waals materials offer a wide range of atomic layers with unique properties that can be easily combined to engineer novel electronic and photonic devices. A missing ingredient of the van der Waals platform is a two-dimensional crystal with naturally occurring out-of-plane luminescent dipole orientation. Here we measure the far-field photoluminescence intensity distribution of bulk InSe and two-dimensional InSe, WSe$_2$ and MoSe$_2$. We demonstrate, with the support of ab-initio calculations, that layered InSe flakes sustain luminescent excitons with an intrinsic out-of-plane orientation, in contrast with the in-plane orientation of dipoles we find in two-dimensional WSe$_2$ and MoSe$_2$ at room-temperature. These results, combined with the high tunability of the optical response and outstanding transport properties, position layered InSe as a promising semiconductor for novel optoelectronic devices, in particular for hybrid integrated photonic chips which exploit the out-of-plane dipole orientation.Comment: Published in Nature Communications. 12 pages, 5 figures. Link: https://www.nature.com/articles/s41467-019-11920-

Resonance fluorescence from waveguide-coupled strain-localized two-dimensional quantum emitters

Efficient on-chip integration of single-photon emitters imposes a major bottleneck for applications of photonic integrated circuits in quantum technologies. Resonantly excited solid-state emitters are emerging as near-optimal quantum light sources, if not for the lack of scalability of current devices. Current integration approaches rely on cost-inefficient individual emitter placement in photonic integrated circuits, rendering applications impossible. A promising scalable platform is based on two-dimensional (2D) semiconductors. However, resonant excitation and single-photon emission of waveguide-coupled 2D emitters have proven to be elusive. Here, we show a scalable approach using a silicon nitride photonic waveguide to simultaneously strain-localize single-photon emitters from a tungsten diselenide (WSe2) monolayer and to couple them into a waveguide mode. We demonstrate the guiding of single photons in the photonic circuit by measuring second-order autocorrelation of g$^{(2)}(0)=0.150\pm0.093$ and perform on-chip resonant excitation yielding a g$^{(2)}(0)=0.377\pm0.081$. Our results are an important step to enable coherent control of quantum states and multiplexing of high-quality single photons in a scalable photonic quantum circuit

Resonance fluorescence from waveguide-coupled strain-localized two-dimensional quantum emitters

Efficient on-chip integration of single-photon emitters imposes a major bottleneck for applications of photonic integrated circuits in quantum technologies. Resonantly excited solid-state emitters are emerging as near-optimal quantum light sources, if not for the lack of scalability of current devices. Current integration approaches rely on cost-inefficient individual emitter placement in photonic integrated circuits, rendering applications impossible. A promising scalable platform is based on two-dimensional (2D) semiconductors. However, resonant excitation and single-photon emission of waveguide-coupled 2D emitters have proven to be elusive. Here, we show a scalable approach using a silicon nitride photonic waveguide to simultaneously strain-localize single-photon emitters from a tungsten diselenide (WSe2) monolayer and to couple them into a waveguide mode. We demonstrate the guiding of single photons in the photonic circuit by measuring second-order autocorrelation of g$^{(2)}(0)=0.150\pm0.093$ and perform on-chip resonant excitation yielding a g$^{(2)}(0)=0.377\pm0.081$. Our results are an important step to enable coherent control of quantum states and multiplexing of high-quality single photons in a scalable photonic quantum circuit

COVID-19 symptoms at hospital admission vary with age and sex: results from the ISARIC prospective multinational observational study

Background: The ISARIC prospective multinational observational study is the largest cohort of hospitalized patients with COVID-19. We present relationships of age, sex, and nationality to presenting symptoms. Methods: International, prospective observational study of 60 109 hospitalized symptomatic patients with laboratory-confirmed COVID-19 recruited from 43 countries between 30 January and 3 August 2020. Logistic regression was performed to evaluate relationships of age and sex to published COVID-19 case definitions and the most commonly reported symptoms. Results: ‘Typical’ symptoms of fever (69%), cough (68%) and shortness of breath (66%) were the most commonly reported. 92% of patients experienced at least one of these. Prevalence of typical symptoms was greatest in 30- to 60-year-olds (respectively 80, 79, 69%; at least one 95%). They were reported less frequently in children (≤ 18 years: 69, 48, 23; 85%), older adults (≥ 70 years: 61, 62, 65; 90%), and women (66, 66, 64; 90%; vs. men 71, 70, 67; 93%, each P < 0.001). The most common atypical presentations under 60 years of age were nausea and vomiting and abdominal pain, and over 60 years was confusion. Regression models showed significant differences in symptoms with sex, age and country. Interpretation: This international collaboration has allowed us to report reliable symptom data from the largest cohort of patients admitted to hospital with COVID-19. Adults over 60 and children admitted to hospital with COVID-19 are less likely to present with typical symptoms. Nausea and vomiting are common atypical presentations under 30 years. Confusion is a frequent atypical presentation of COVID-19 in adults over 60 years. Women are less likely to experience typical symptoms than men

Traditions et cultures des conseils de Premières Nations : analyse d'un cas de politique publique sous l'angle identitaire

Ce mémoire est une reprise d'un mémoire déposé en juin 2002 dont l'évaluation n'a pu être terminée. Cette recherche opérationnelle sur l'élaboration d'une politique publique sur la sécurité du revenu des Premières Nations était une expérience d'exécution de mandats et un suivi intégral d'un processus de partenariat entre APNQL et le gouvernement du Canada sur une démarche de conciliation des positions stratégiques entre les deux parties. Le ministère responsable canadien des Autochtones soutenait des exigences de financement sur obligation de l'application du régime québécois de sécurité du revenu comme condition de programme. De l'autre côté, les représentants autochtones développaient une thèse dans laquelle les modalités universelles de la politique québécoise ne pouvaient s'appliquer dans des communautés où il existait des écarts de réalités culturelles et communautaires et de besoins avec le profil socioéconomique de l'ensemble des Québécois. Cette dualité de positions stratégiques provoquait l'apparition d'une avenue d'autonomie, soit celle de politiques publiques. La négociation de cette avenue a été conduite par deux éléments : la rigueur de l'autorité gouvernementale et l'approche identitaire de l'autonomie. L'utilisation nouvelle de méthodes de travail dites scientifiques et empiriques et un regard culturel sur les débats a provoqué une série d'interactions utiles, difficiles et efficaces pour en venir à une formule adaptée d'une politique-cadre de la sécurité du revenu. Les Premières Nations ont posé le problème de l'inadéquation du régime proprement québécois et le Canada, la réglementation obligatoire en ces matières pour avoir du financement. Négocier un programme adapté en suivant le principe de fond de l'universalité et bâtir un schéma identitaire d'un programme tout en se donnant une autonomie de conception des modalités d'un programme et en respectant le principe de l'universalité ont été des tâches de négociations très ardues. Les commentaires de la politique-cadre sur la sécurité du revenu des Premières Nations a été divers ; la juridiction en cette matière n'a pas été obtenue, les acquis de la politique-cadre permettaient d'adapter la livraison des services ou une certaine autonomie de programme a été atteinte. Toutefois, 80% de Premières Nations a adhéré à la politique-cadre