Classicality witness for two-qubit states

In the last few years one realized that if the state of a bipartite system can be written as $\sum_{i,j}p_{ij}|a_{i}> < b_{j}|$, where $\{|a_{i}> \}$ and $\{|b_{j}> \}$ form orthonormal basis for the subsystems and $\{p_{ij}\}$ is a probability distribution, then it possesses at most classical correlations. In this article we introduce a nonlinear witness providing a sufficient condition for classicality of correlations (absence of quantum discord) in a broad class of two-qubit systems. Such witness turns out to be necessary and sufficient condition in the case of Bell-diagonal states. We show that the witness introduced here can be readily experimentally implemented in nuclear magnetic resonance setups.Comment: To appear in IJQ

Role of quantum coherence in the thermodynamics of energy transfer

Recent research on the thermodynamic arrow of time, at the microscopic scale, has questioned the universality of its direction. Theoretical studies showed that quantum correlations can be used to revert the natural heat flow (from the hot body to the cold one), posing an apparent challenge to the second law of thermodynamics. Such an "anomalous" heat current was observed in a recent experiment (arXiv:1711.03323), by employing two spin systems initially quantum correlated. Nevertheless, the precise relationship between this intriguing phenomenon and the initial conditions that allow it is not fully evident. Here, we address energy transfer in a wider perspective, identifying a nonclassical contribution that applies to the reversion of the heat flow as well as to more general forms of energy exchange. We derive three theorems that describe the energy transfer between two microscopic systems, for arbitrary initial bipartite states. Using these theorems, we obtain an analytical bound showing that certain type of quantum coherence can optimize such a process, outperforming incoherent states. This genuine quantum advantage is corroborated through a characterization of the energy transfer between two qubits. For this system, it is shown that a large enough amount of coherence is necessary and sufficient to revert the thermodynamic arrow of time. As a second crucial consequence of the presented theorems, we introduce a class of nonequilibrium states that only allow unidirectional energy flow. In this way, we broaden the set where the standard Clausius statement of the second law applies

Peer Effects in Risk Taking

This paper examines the effect of peers on individual risk taking. In the absence of informational motives, we investigate why social utility concerns may drive peer effects. We test for two main channels: utility from payoff differences and from conforming to the peer. We show experimentally that social utility generates substantial peer effects in risk taking. These are mainly explained by utility from payoff differences, in line with outcomebased social preferences. Contrary to standard assumptions, we show that estimated social preference parameters change significantly when peers make active choices, compared to when lotteries are randomly assigned to them

Relational Contracting Under the Threat of Expropriation – Experimental Evidence

We examine how relational contracting in credit and investment relationships is affected by the potential expropriation of funds. We implement credit relationships in which repayment is not third-party enforceable, i.e. borrowers can default on their loans. In our main treatment the borrower can expropriate the lender’s funds: a defaulting borrower can reinvest the loaned funds in future periods. In a control treatment borrowers cannot expropriate borrowed funds, i.e. if they default they cannot reinvest these funds in future periods. We find that potential expropriation decreases the overall volume of credit as lenders offer smaller loans in initial periods. Borrowers are more likely to default in earlier periods of the relationship when expropriation is possible, especially when they receive large loans. Together these results suggest that relational contracts may be particularly difficult to establish in markets where the expropriation of funds is feasible. This finding is relevant to credit markets in which lenders’ rights are weak, but also to sovereign lending, as well as to foreign direct investment in countries with weak investor protection.Relational contracts;Investor protection;Banking;Sovereign debt;Foreign direct investment.

Multichannel effects in Rashba quantum wires

We investigate intersubband mixing effects in multichannel quantum wires in the presence of Rashba spin-orbit coupling and attached to two terminals. When the contacts are ferromagnetic and their magnetization direction is perpendicular to the Rashba field, the spin-transistor current is expected to depend in a oscillatory way on the Rashba coupling strength due to spin coherent oscillations of the travelling electrons. Nevertheless, we find that the presence of many propagating modes strongly influences the spin precession effect, leading to (i) a quenching of the oscillations and (ii) strongly irregular curves for high values of the Rashba coupling. We also observe that in the case of leads' magnetization parallel to the Rashba field, the conductance departs from a uniform value as the Rashba strength increases. We also discuss the Rashba interaction induced current polarization effects when the contacts are not magnetic and investigate how this mechanism is affected by the presence of several propagating channels.Comment: 8 pages, 7 figure

Tourism Demand in Portugal: Market Perspectives

Tourism has experienced different levels of development in the different regions of Portugal. To frame this development, several panel data models were estimated. The main objective is to explain the evolution of overnight stays by nationality in each region. Secondary data from 2000 to 2010 was used. The analysis includes the main tourism markets, such as the United Kingdom, Germany, the Netherlands, Ireland, France and Spain. Tourism literature suggests that, among others, the main determinants of tourism demand are Income (GDP), population, tourist´s income by place of residence, households’ consumption, unemployment rate, inflation rate, compensation of employees, comparative prices and households’ investment rate. It is observed that, although significant, the explanatory power of these variables varies according to the origin and the destination region considered