Separation of neutral and charge modes in one dimensional chiral edge channels

Coulomb interactions have a major role in one-dimensional electronic transport. They modify the nature of the elementary excitations from Landau quasiparticles in higher dimensions to collective excitations in one dimension. Here we report the direct observation of the collective neutral and charge modes of the two chiral co-propagating edge channels of opposite spins of the quantum Hall effect at filling factor 2. Generating a charge density wave at frequency f in the outer channel, we measure the current induced by inter-channel Coulomb interaction in the inner channel after a 3-mm propagation length. Varying the driving frequency from 0.7 to 11 GHz, we observe damped oscillations in the induced current that result from the phase shift between the fast charge and slow neutral eigenmodes. We measure the dispersion relation and dissipation of the neutral mode from which we deduce quantitative information on the interaction range and parameters.Comment: 23 pages, 6 figure

Electron quantum optics : partitioning electrons one by one

We have realized a quantum optics like Hanbury Brown and Twiss (HBT) experiment by partitioning, on an electronic beam-splitter, single elementary electronic excitations produced one by one by an on-demand emitter. We show that the measurement of the output currents correlations in the HBT geometry provides a direct counting, at the single charge level, of the elementary excitations (electron/hole pairs) generated by the emitter at each cycle. We observe the antibunching of low energy excitations emitted by the source with thermal excitations of the Fermi sea already present in the input leads of the splitter, which suppresses their contribution to the partition noise. This effect is used to probe the energy distribution of the emitted wave-packets.Comment: 5 pages, 4 figure

Partitioning of on-demand electron pairs

We demonstrate the high fidelity splitting of electron pairs emitted on demand from a dynamic quantum dot by an electronic beam splitter. The fidelity of pair splitting is inferred from the coincidence of arrival in two detector paths probed by a measurement of the partitioning noise. The emission characteristic of the on-demand electron source is tunable from electrons being partitioned equally and independently to electron pairs being split with a fidelity of 90%. For low beam splitter transmittance we further find evidence of pair bunching violating statistical expectations for independent fermions

The Effects of Wars: Lessons from Ukraine

The devastating war in Ukraine has transformed our understanding of military combat and international politics in the 21st Century. Until now, analyses of the effects of wars – including this one – have tended to be compartmentalised by sub-disciplines. These include International Relations (IR), Security Studies, International Political Economy (IPE), History, Historical Sociology, Public Administration and Psychology, among many others. In contrast, taken together, and addressed from an interdisciplinary perspective, a new set of studies shows how the war has had deep and complex “ripple” and “washback” effects. These are affecting human life; human development; economies; values and attitudes; policy and governance; and power distribution and relations around the world. This complexity, therefore, derives from both the number of actors involved, areas of human society affected and their interconnections. In other words, each of these layers of society have scope to feedback on one another and feedback loops may in turn affect the evolution of conflict and the possibility of its resolution in the future

Single electron quantum tomography in quantum Hall edge channels

We propose a quantum tomography protocol to measure single electron coherence in quantum Hall edge channels and therefore access for the first time the wave function of single electron excitations propagating in ballistic quantum conductors. Its implementation would open the way to quantitative studies of single electron decoherence and would provide a quantitative tool for analyzing single to few electron sources. We show how this protocol could be implemented using ultrahigh sensitivity noise measurement schemes.Comment: Version 3: long version (7 figures): corrections performed and references have been added. Figures reprocessed for better readabilit

Observation of edge magnetoplasmon squeezing in a quantum Hall conductor

Squeezing of the quadratures of the electromagnetic field has been extensively studied in optics and microwaves. However, previous works focused on the generation of squeezed states in a low impedance ($Z_0 \approx 50 \Omega$) environment. We report here on the demonstration of the squeezing of bosonic edge magnetoplasmon modes in a quantum Hall conductor whose characteristic impedance is set by the quantum of resistance ($R_K \approx 25 k \Omega$), offering the possibility of an enhanced coupling to low-dimensional quantum conductors. By applying a combination of dc and ac drives to a quantum point contact, we demonstrate squeezing and observe a noise reduction 18\% below the vacuum fluctuations. This level of squeezing can be improved by using more complex conductors, such as ac driven quantum dots or mesoscopic capacitors.Comment: 6+2 pages, 3+1 figure

System complexity and policy integration challenges: The Brazilian Energy- Water-Food Nexus

The Energy-Water-Food Nexus is one of the most complex sustainability challenges faced by the world. This is particularly true in Brazil, where insufficiently understood interactions within the Nexus are contributing to large-scale deforestation and land-use change, water and energy scarcity, and increased vulnerability to climate change. The reason is a combination of global environmental change and global economic change, putting unprecedented pressures on the Brazilian environment and ecosystems. In this paper, we identify and discuss the main Nexus challenges faced by Brazil across sectors (e.g. energy, agriculture, water) and scales (e.g. federal, state, municipal). We use four case studies to explore all nodes of the Nexus. For each, we analyse data from economic and biophysical modelling sources in combination with an overview of the legislative and policy landscape, in order to identify governance shortcomings in the context of growing challenges. We analyse the complex interdependence of developments at the global and local (Brazilian) levels, highlighting the impact of global environmental and economic change on Brazil and, conversely, that of developments in Brazil for other countries and the world. We conclude that there is a need to adjust the scientific approach to these challenges as an enabling condition for stronger science-policy bridges for sustainability policy-making. © 2019 The Author(s

Mesoscopic Klein-Schwinger effect in graphene

Strong electric field annihilation by particle-antiparticle pair creation, described in detail by Sauter and Schwinger, is a basic non-perturbative prediction of quantum electrodynamics. Its experimental demonstration remains elusive as Schwinger fields $E_S$ are beyond reach even for the light electron-positron pairs. Here we put forward a mesoscopic variant of the Schwinger effect in graphene, which hosts Dirac fermions with electron-hole symmetry. Using DC transport and RF noise, we report on universal 1d-Schwinger conductance at the pinch-off of ballistic graphene transistors. Strong pinch-off electric fields are concentrated in a length $\Lambda\gtrsim 0.1\;\mathrm{\mu m}$ at the transistor drain, and induce Schwinger e-h pair creation at saturation, for a Schwinger voltage $V_S=E_S\Lambda$ on the order of the pinch-off voltage. This Klein-Schwinger effect (KSE) precedes an instability toward an ohmic Zener regime, which is rejected at twice the pinch-off voltage in long devices. The KSE not only gives clues to current saturation limits in ballistic graphene, but also opens new routes for quantum electrodynamic experiments in the laboratory.Comment: 32 pages, 11 figures, updated to include the link to the set of experimental data on the Zenodo deposit at DOI 10.5281/zenodo.710463

Microwave studies of the fractional Josephson effect in HgTe-based Josephson junctions

The rise of topological phases of matter is strongly connected to their potential to host Majorana bound states, a powerful ingredient in the search for a robust, topologically protected, quantum information processing. In order to produce such states, a method of choice is to induce superconductivity in topological insulators. The engineering of the interplay between superconductivity and the electronic properties of a topological insulator is a challenging task and it is consequently very important to understand the physics of simple superconducting devices such as Josephson junctions, in which new topological properties are expected to emerge. In this article, we review recent experiments investigating topological superconductivity in topological insulators, using microwave excitation and detection techniques. More precisely, we have fabricated and studied topological Josephson junctions made of HgTe weak links in contact with two Al or Nb contacts. In such devices, we have observed two signatures of the fractional Josephson effect, which is expected to emerge from topologically-protected gapless Andreev bound states. We first recall the theoretical background on topological Josephson junctions, then move to the experimental observations. Then, we assess the topological origin of the observed features and conclude with an outlook towards more advanced microwave spectroscopy experiments, currently under development.Comment: Lectures given at the San Sebastian Topological Matter School 2017, published in "Topological Matter. Springer Series in Solid-State Sciences, vol 190. Springer