Strain-Tunable GaAs Quantum dot: A Nearly Dephasing-Free Source of Entangled Photon Pairs on Demand

Entangled photon generation from semiconductor quantum dots via the biexciton-exciton cascade underlies various decoherence mechanisms related to the solid-state nature of the quantum emitters. So far, this has prevented the demonstration of nearly-maximally entangled photons without the aid of inefficient and complex post-selection techniques that are hardly suitable for quantum communication technologies. Here, we tackle this challenge using strain-tunable GaAs quantum dots driven under two-photon resonant excitation and with strictly-degenerate exciton states. We demonstrate experimentally that our on-demand source generates polarization-entangled photons with fidelity of 0.978(5) and concurrence of 0.97(1) without resorting to post-selection techniques. Moreover, we show that the remaining decoherence mechanisms can be overcome using a modest Purcell enhancement so as to achieve a degree of entanglement >0.99. Our results highlight that GaAs quantum dots can be readily used in advanced communication protocols relying on the non-local properties of quantum entanglement

Highly indistinguishable single photons from incoherently and coherently excited GaAs quantum dots

Semiconductor quantum dots are converging towards the demanding requirements of photonic quantum technologies. Among different systems, quantum dots with dimensions exceeding the free-exciton Bohr radius are appealing because of their high oscillator strengths. While this property has received much attention in the context of cavity quantum electrodynamics, little is known about the degree of indistinguishability of single photons consecutively emitted by such dots and on the proper excitation schemes to achieve high indistinguishability. A prominent example is represented by GaAs quantum dots obtained by local droplet etching, which recently outperformed other systems as triggered sources of entangled photon pairs. On these dots, we compare different single-photon excitation mechanisms, and we find (i) a "phonon bottleneck" and poor indistinguishability for conventional excitation via excited states and (ii) photon indistinguishablilities above 90% for both strictly resonant and for incoherent acoustic- and optical-phonon-assisted excitation. Among the excitation schemes, optical phonon-assisted excitation enables straightforward laser rejection without a compromise on the source brightness together with a high photon indistinguishability

Populist citizens in four European countries: Widespread dissatisfaction goes with contradictory but pro-democratic regime preferences

Are populist citizens a threat to democracy? Some philosophers view populism and democracy as irreconcilable conceptions of governing. Another line of thought describes populism as useful democratic corrective. Drawing on nationally representative surveys from four European countries, this study investigates how European populist citizens think about democracy. Descriptive analyses reveal that populist worldviews only weakly predict how people think about democracy. On average, populist and non-populist citizens aspire to similar kinds of political systems and both endorse liberal-democratic institutions. Yet, populists and non-populists differ in the degree to which they hold inconsistent beliefs. Citizens with populist outlook more frequently express contradictory demands that political institutions cannot possibly deliver. Based on these findings, we conclude that most populist citizens do not pursue an elaborate anti-democratic conception of governing. Rather, the widespread dissatisfaction among populists may create an indeterminate openness for institutional change that political elites could steer in different directions

Left behind and united by populism? Populism’s multiple roots in feelings of lacking societal recognition

A prominent but underspecified explanation for the rise of populism points to individuals’ feelings of being “left behind” by the development of society. At its core lies the claim that support for populism is driven by the feeling of lacking the societal recognition one deserves. Our contribution builds on the insight that individuals can feel they lack recognition in different ways and for different reasons. We argue that—because of this multifaceted character—the common perception of being neglected by society unites otherwise heterogeneous segments of the population in their support for populism. Relying on data from the German Longitudinal Election Study (GLES) Pre-Election Cross-Section 2021, our preregistered study investigated the multiple roots of populist attitudes in feelings of lacking societal recognition in two steps. First, our results indicate that, from rural residents to sociocultural conservatives or low-income citizens, seemingly unrelated segments of society harbor feelings of lacking recognition, but for distinct reasons. Second, as anticipated, each of the distinct feelings of lacking recognition are associated with populist attitudes. These findings underscore the relevance of seemingly unpolitical factors that are deeply ingrained in the human psyche for understanding current populist sentiment. Overall, by integrating previously disparate perspectives on the rise of populism, the study offers a novel conceptualization of “feeling left behind” and explains how populism can give rise to unusual alliances that cut across traditional cleavages

Entanglement swapping with photons generated on-demand by a quantum dot

Photonic entanglement swapping, the procedure of entangling photons without any direct interaction, is a fundamental test of quantum mechanics and an essential resource to the realization of quantum networks. Probabilistic sources of non-classical light can be used for entanglement swapping, but quantum communication technologies with device-independent functionalities demand for push-button operation that, in principle, can be implemented using single quantum emitters. This, however, turned out to be an extraordinary challenge due to the stringent requirements on the efficiency and purity of generation of entangled states. Here we tackle this challenge and show that pairs of polarization-entangled photons generated on-demand by a GaAs quantum dot can be used to successfully demonstrate all-photonic entanglement swapping. Moreover, we develop a theoretical model that provides quantitative insight on the critical figures of merit for the performance of the swapping procedure. This work shows that solid-state quantum emitters are mature for quantum networking and indicates a path for scaling up.Comment: The first four authors contributed equally to this work. 17 pages, 3 figure

Left Atrial Chamber and Appendage Function After Internal Atrial Defibrillation: A Prospective and Serial Transesophageal Echocardiographic Study

AbstractObjectives. The purpose of this prospective study was to assess left atrial chamber and appendage function after internal atrial defibrillation of atrial fibrillation and to evaluate the time course of recovery.Background. External cardioversion of atrial fibrillation may result in left atrial appendage dysfunction (“stunning”) and may promote thrombus formation. In contrast to external cardioversion, internal atrial defibrillation utilizes lower energies; however, it is unknown whether the use of lower energies may avoid stunning of the left atrial appendage.Methods. Transesophageal and transthoracic echocardiography were performed in 20 patients 24 h before and 1 and 7 days after internal atrial defibrillation to assess both left atrial chamber and appendage function. Transthoracic echocardiography was again performed 28 days after internal atrial defibrillation to assess left atrial function. The incidence and degree of spontaneous echo contrast accumulation (range 1+ to 4+) was noted, and peak emptying velocities of the left atrial appendage were measured before and after internal atrial defibrillation. To determine left atrial mechanical function, peak A wave velocities were obtained from transmitral flow velocity profiles.Results. Sinus rhythm was restored in all patients. The mean ± SD peak A wave velocities increased gradually after cardioversion, from 0.47 ± 0.16 m/s at 24 h to 0.61 ± 0.13 m/s after 7 days (p < 0.05) and 0.63 ± 0.13 m/s after 4 weeks. Peak emptying velocities of the left atrial appendage were 0.37 ± 0.16 m/s before internal atrial defibrillation, decreased significantly after internal atrial defibrillation to 0.23 ± 0.1 m/s at 24 h (p < 0.01) and then recovered to 0.49 ± 0.23 m/s (p < 0.01) after 7 days. The corresponding values for the degree of spontaneous echo contrast were 1.2 ± 1.2 before internal atrial defibrillation versus 2.0 ± 1.0 (p < 0.01) and 1.1 ± 1.3 (p < 0.01) 1 and 7 days after cardioversion, respectively. One patient developed a new thrombus in the left atrial appendage, and another had a thromboembolic event after internal atrial defibrillation.Conclusions. Internal atrial defibrillation causes depressed left atrial chamber and appendage function and may result in the subacute accumulation of spontaneous echo contrast and development of new thrombi after cardioversion. These findings have important clinical implications for anticoagulation therapy before and after low energy internal atrial defibrillation in patients with atrial fibrillation.(J Am Coll Cardiol 1997;29:131–8)

Разработка информационной системы формата "Вопрос-Ответ" (Q&A) с использованием Ruby on Rails

Объект исследования: системы формата "Вопрос-ответ" и использование фреймворка Ruby on Rails для разработки информационной системы. Целью работы является разработка системы формата "Вопрос-ответ" (Q&A), используя фреймворк Ruby on Rails.Object of study: questions and answers information systems and development information system using Ruby on Rails. Goal: development Q&A information system using Ruby on Rails framework