Measuring the Generalized Friendship Paradox in Networks with Quality-dependent Connectivity

The friendship paradox is a sociological phenomenon stating that most people have fewer friends than their friends do. The generalized friendship paradox refers to the same observation for attributes other than degree, and it has been observed in Twitter and scientific collaboration networks. This paper takes an analytical approach to model this phenomenon. We consider a preferential attachment-like network growth mechanism governed by both node degrees and `qualities'. We introduce measures to quantify paradoxes, and contrast the results obtained in our model to those obtained for an uncorrelated network, where the degrees and qualities of adjacent nodes are uncorrelated. We shed light on the effect of the distribution of node qualities on the friendship paradox. We consider both the mean and the median to measure paradoxes, and compare the results obtained by using these two statistics

Direct Observation of Second Order Atom Tunnelling

Tunnelling of material particles through a classically impenetrable barrier constitutes one of the hallmark effects of quantum physics. When interactions between the particles compete with their mobility through a tunnel junction, intriguing novel dynamical behaviour can arise where particles do not tunnel independently. In single-electron or Bloch transistors, for example, the tunnelling of an electron or Cooper pair can be enabled or suppressed by the presence of a second charge carrier due to Coulomb blockade. Here we report on the first direct and time-resolved observation of correlated tunnelling of two interacting atoms through a barrier in a double well potential. We show that for weak interactions between the atoms and dominating tunnel coupling, individual atoms can tunnel independently, similar to the case in a normal Josephson junction. With strong repulsive interactions present, two atoms located on one side of the barrier cannot separate, but are observed to tunnel together as a pair in a second order co-tunnelling process. By recording both the atom position and phase coherence over time, we fully characterize the tunnelling process for a single atom as well as the correlated dynamics of a pair of atoms for weak and strong interactions. In addition, we identify a conditional tunnelling regime, where a single atom can only tunnel in the presence of a second particle, acting as a single atom switch. Our work constitutes the first direct observation of second order tunnelling events with ultracold atoms, which are the dominating dynamical effect in the strongly interacting regime. Similar second-order processes form the basis of superexchange interactions between atoms on neighbouring lattice sites of a periodic potential, a central component of quantum magnetism.Comment: 18 pages, 4 figures, accepted for publication in Natur

Driver-pressure-impact and response-recovery chains in European rivers: observed and predicted effects on BQEs

The report presented in the following is part of the outcome of WISER’s river Workpackage WP5.1 and as such part of the module on aquatic ecosystem management and restoration. The ultimate goal of WP5.1 is to provide guidance on best practice restoration and management to the practitioners in River Basin Management. Therefore, a series of analyses was undertaken, each of which used a part of the WP5.1 database in order to track two major pathways of biological response: 1) the response of riverine biota to environmental pressures (degradation) and 2) the response of biota to the reduction of these impacts (restoration). This report attempts to provide empirical evidence on the environment-biota relationships for both pathways

Efficient Immunization Strategies for Computer Networks and Populations

We present an effective immunization strategy for computer networks and populations with broad and, in particular, scale-free degree distributions. The proposed strategy, acquaintance immunization, calls for the immunization of random acquaintances of random nodes (individuals). The strategy requires no knowledge of the node degrees or any other global knowledge, as do targeted immunization strategies. We study analytically the critical threshold for complete immunization. We also study the strategy with respect to the susceptible-infected-removed epidemiological model. We show that the immunization threshold is dramatically reduced with the suggested strategy, for all studied cases.Comment: Revtex, 5 pages, 4 ps fig

A two-dimensional Fermi liquid with attractive interactions

We realize and study an attractively interacting two-dimensional Fermi liquid. Using momentum resolved photoemission spectroscopy, we measure the self-energy, determine the contact parameter of the short-range interaction potential, and find their dependence on the interaction strength. We successfully compare the measurements to a theoretical analysis, properly taking into account the finite temperature, harmonic trap, and the averaging over several two-dimensional gases with different peak densities

Time-resolved Observation and Control of Superexchange Interactions with Ultracold Atoms in Optical Lattices

Quantum mechanical superexchange interactions form the basis of quantum magnetism in strongly correlated electronic media. We report on the direct measurement of superexchange interactions with ultracold atoms in optical lattices. After preparing a spin-mixture of ultracold atoms in an antiferromagnetically ordered state, we measure a coherent superexchange-mediated spin dynamics with coupling energies from 5 Hz up to 1 kHz. By dynamically modifying the potential bias between neighboring lattice sites, the magnitude and sign of the superexchange interaction can be controlled, thus allowing the system to be switched between antiferromagnetic or ferromagnetic spin interactions. We compare our findings to predictions of a two-site Bose-Hubbard model and find very good agreement, but are also able to identify corrections which can be explained by the inclusion of direct nearest-neighbor interactions.Comment: 24 pages, 7 figure

From the internal market to a banking union: A proposal by the German Council of Economic Experts

The European sovereign debt crisis has revealed severe flaws in the design of the internal market. Both, private and public borrowers had incentives for excessive borrowing, which have been created by deficits in the regulatory structure of financial markets. Capital requirements for banks were too low and had procyclical effects (Favara and Ratnovski 2012). Supervision has been ineffective with regard to containing the build-up of risks in banks' balance sheets. Common monetary policy in the Euro Area has not been accompanied by the transfer of authority to supervise and restructure banks which has, in turn, created incentives to shift risks to the European level. Risks of banks and states have become dangerously intertwined. Proposals for a banking union aim at correcting these deficits. In principle, a banking union is a necessary complement to other elements of the internal market. In an integrated capital market, banking distress in one country can have negative externalities for the stability of financial systems in other countries. Such risks are even more pronounced if other countries are affected through a common monetary policy. The banking union currently being discussed has three elements (President of the European Council 2012): banking supervision at the European level, a European authority for bank restructuring and resolution financed by a bank resolution fund, and a European deposit insurance fund. So far, concrete proposals have been made for the establishment of a Single Supervisory Mechanism only. However, a banking union is a long-term project. It is not the key to a solution to the acute problems in Europe's banking sectors (Buch and Weigert 2012, GCEE 2012a).Die europäische Schuldenkrise hat zentrale Defizite des Binnenmarkts offengelegt. Regulatorische Defizite haben Anreize geschaffen, eine zu hohe Verschuldung einzugehen. Zwar wurde eine einheitliche Geldpolitik verfolgt, gleichzeitig aber die Kompetenzen für die Aufsicht und Restrukturierung von Banken auf nationaler Ebene belassen. So sind Anreize entstanden, Risiken auf die europäische Ebene zu verlagern. Eine Bankenunion und die Etablierung eines einheitlichen Aufsichtsmechanismus haben das Ziel, diese Defizite zu beheben. Grundsätzlich ist eine Bankenunion eine notwendige Ergänzung des Binnenmarkts. Sie ist aber vor allem ein langfristiges Projekt, das keinesfalls das aktuelle Problem des privaten und öffentlichen Schuldenüberhangs lösen kann (Buch und Weigert, 2012; SVR, 2012a)

Gravity a la Born-Infeld

A simple technique for the construction of gravity theories in Born-Infeld style is presented, and the properties of some of these novel theories are investigated. They regularize the positive energy Schwarzschild singularity, and a large class of models allows for the cancellation of ghosts. The possible correspondence to low energy string theory is discussed. By including curvature corrections to all orders in alpha', the new theories nicely illustrate a mechanism that string theory might use to regularize gravitational singularities.Comment: 21 pages, 2 figures, new appendix B with corrigendum: Class. Quantum Grav. 21 (2004) 529

Nuclear quantum optics with x-ray laser pulses

The direct interaction of nuclei with super-intense laser fields is studied. We show that present and upcoming high-frequency laser facilities, especially together with a moderate acceleration of the target nuclei, do allow for resonant laser-nucleus interaction. These direct interactions may be utilized for the optical measurement of nuclear properties such as the transition frequency and the dipole moment, thus opening the field of nuclear quantum optics. As ultimate goal, one may hope that direct laser-nucleus interactions could become a versatile tool to enhance preparation, control and detection in nuclear physics.Comment: 5 pages, 3 eps figures, revised versio