Non-Markovian continuous-time quantum walks on lattices with dynamical noise

We address the dynamics of continuous-time quantum walks on one-dimensional disordered lattices inducing dynamical noise in the system. Noise is described as time-dependent fluctuations of the tunneling amplitudes between adjacent sites, and attention is focused on non-Gaussian telegraph noise, going beyond the usual assumption of fast Gaussian noise. We observe the emergence of two different dynamical behaviors for the walker, corresponding to two opposite noise regimes: slow noise (i.e. strong coupling with the environment) confines the walker into few lattice nodes, while fast noise (weak coupling) induces a transition between quantum and classical diffusion over the lattice. A phase transition between the two dynamical regimes may be observed by tuning the ratio between the autocorrelation time of the noise and the coupling between the walker and the external environment generating the noise. We also address the non-Markovianity of the quantum map by assessing its memory effects, as well as evaluating the information backflow to the system. Our results suggest that the non-Markovian character of the evolution is linked to the dynamical behavior in the slow noise regime, and that fast noise induces a Markovian dynamics for the walker.Comment: 10 pages, 8 figure

Dynamics of quantum correlations in colored environments

We address the dynamics of entanglement and quantum discord for two non interacting qubits initially prepared in a maximally entangled state and then subjected to a classical colored noise, i.e. coupled with an external environment characterized by a noise spectrum of the form $1/f^{\alpha}$. More specifically, we address systems where the Gaussian approximation fails, i.e. the sole knowledge of the spectrum is not enough to determine the dynamics of quantum correlations. We thus investigate the dynamics for two different configurations of the environment: in the first case the noise spectrum is due to the interaction of each qubit with a single bistable fluctuator with an undetermined switching rate, whereas in the second case we consider a collection of classical fluctuators with fixed switching rates. In both cases we found analytical expressions for the time dependence of entanglement and quantum discord, which may be also extended to a collection of flcutuators with random switching rates. The environmental noise is introduced by means of stochastic time-dependent terms in the Hamiltonian and this allows us to describe the effects of both separate and common environments. We show that the non-Gaussian character of the noise may lead to significant effects, e.g. environments with the same power spectrum, but different configurations, give raise to opposite behavior for the quantum correlations. In particular, depending on the characteristics of the environmental noise considered, both entanglement and discord display either a monotonic decay or the phenomena of sudden death and revivals. Our results show that the microscopic structure of environment, besides its noise spectrum, is relevant for the dynamics of quantum correlations, and may be a valid starting point for the engineering of non-Gaussian colored environments.Comment: 8 pages, 3 figure

Probing the sign of on-site Hubbard interaction by two-particle quantum walks

We consider two identical bosons propagating on a one-dimensional lattice and address the prob- lem of discriminating whether their mutual on-site interaction is attractive or repulsive. We suggest a probing scheme based on the properties of the corresponding two-particle quantum walks, and show that the sign of the interaction introduces specific and detectable features in the dynamics of quantum correlations, thus permitting to discriminate between the two cases. We also discuss how these features are connected to the band-structure of the Hubbard Hamiltonian, and prove that discrimination may be obtained only when the two walkers are initially prepared in a superposition of localized states.Comment: 9 pages, 9 figure

Quantum teleportation of electrons in quantum wires with surface acoustic waves

We propose and numerically simulate a semiconductor device based on coupled quantum wires, suitable for deterministic quantum teleportation of electrons trapped in the minima of surface acoustic waves.We exploit a network of interacting semiconductor quantum wires able to provide the universal set of gates for quantum information processing, with the qubit defined by the localization of a single electron in one of two coupled channels.The numerical approach is based on a time-dependent solution of the three-particle Schr\"odinger equation. First, a maximally entangled pair of electrons is obtained via Coulomb interaction between carriers in different channels. Then, a complete Bell-state measurement involving one electron from this pair and a third electron is performed. Finally, the teleported state is reconstructed by means of local one-qubit operations. The large estimated fidelity explicitely suggests that an efficient teleportation process could be reached in an experimental setup.Comment: 7 pages,4 figures, 1 tabl

Intergenerational support among migrant families in Europe

Intergenerational support is important throughout the individual life course and a major mechanism of cultural continuity. In this study, we analyse support between older parents and their adult children among international migrant and non-migrant populations in North, Centre and Southern Europe. Data from the Survey of Health, Ageing and Retirement in Europe are used to compare upward and downward practical support, grandparenting, and frequency of contact among 62,213 parent–child dyads. Findings indicate limited differences in support between migrants and non-migrants as well as between migrants of various origins. However, persistent differences in intergenerational support across Europe along a north–south gradient are found irrespective of migrant status

Becoming a grandparent and its effect on well-being: the role of order of transitions, time, and gender

Objectives Although the majority of older people are grandparents, little is known on whether and how the transition into grandparenthood affects their well-being. Moreover, evidence on whether the order of the transition, the time since grandchild’s birth, and the socio-demographic characteristics of the offspring modify the grandparental well-being is scarce. Taking into account these factors, our study examines the association between becoming a grandparent and subsequent well-being. Methods Our study is based on grandparents aged 50 and over from waves 4-6 of the Survey of Health, Ageing and Retirement in Europe. Using longitudinal analyses, we investigate associations between becoming a grandparent and subsequent life satisfaction, positive affect, and depression controlling for demographic and socio-economic factors as well as health and well-being at baseline. Furthermore, we explore the role of modifying factors such as whether the grandchild was first-born, the time since transition, and socio-demographic characteristics of the offspring who became a parent. Results Becoming a grandparent has a positive effect on well-being only among women who became grandmothers for the first time and via their daughters. Moreover, this effect is particularly strong in the proximity of the birth of the grandchild. No effects were found among first-time grandfathers. Having an additional grandchild does not affect well-being of grandparents, regardless of the offspring’s characteristics

Noisy quantum walks of two indistinguishable interacting particles

We investigate the dynamics of continuous-time two-particle quantum walks on a one-dimensional noisy lattice. Depending on the initial condition, we show how the interplay between particle indistinguishability and interaction determines distinct propagation regimes. A realistic model for the environment is considered by introducing non-Gaussian noise as time-dependent fluctuations of the tunneling amplitudes between adjacent sites. We observe that the combined effect of particle interaction and fast noise (weak coupling with the environment) provides a faster propagation compared to the noiseless case. This effect can be understood in terms of the band structure of the Hubbard model, and a detailed analysis as a function of both noise and system parameters is presented.Comment: 9 pages, 8 figure

Bowling Together: Scientific Collaboration Network of Demographers at European Population Conferences.

Exploiting a unique database of metadata for papers presented at six European Population Conferences (EPC) for the years 2006-2016, this paper explores: 1) development of research in population studies; 2) trends and patterns of scientific collaboration networks among demographers; and 3) gender differences in demographic research. The data are organised in a panel format whereby each author, institution and country are linked across the six conferences. We find that collaboration among demographers has increased substantially over the past ten years. While there is no gender disparity in the likelihood of co-authoring a paper, men are significantly more likely than women to collaborate with authors from other institutions. Likewise, the fields of research vary considerably by gender where women are particularly over represented in the subfield ‘fertility and family’ whereas men dominate the subfield ‘data and methods’. Compared to other subfields, research on ‘data and methods’ is more likely to involve collaboration across multiple institutions. With respect to collaboration patterns at the institutional level, a chord diagram plot shows that scientific collaborations across institutions are more common between institutions sharing geographical proximity. Finally, using network centrality measures, we identify key demographic research institutes which play a role in driving demographic research in Europe

Universality of the fully connected vertex in Laplacian continuous-time quantum walk problems

A fully connected vertex w in a simple graph G of order N is a vertex connected to all the other N − 1 vertices. Upon denoting by L the Laplacian matrix of the graph, we prove that the continuous-time quantum walk (CTQW)—with Hamiltonian H = γL—of a walker initially localized at |w does not depend on the graph G. We also prove that for any Grover-like CTQW—with Hamiltonian H = γL + w λw|w w|—the probability amplitude at the fully connected marked vertices w does not depend on G. The result does not hold for CTQW with Hamiltonian H = γA (adjacency matrix). We apply our results to spatial search and quantum transport for single and multiple fully connected marked vertices, proving that CTQWs on any graph G inherit the properties already known for the complete graph of the same order, including the optimality of the spatial search. Our results provide a unified framework for several partial results already reported in literature for fully connected vertices, such as the equivalence of CTQWand of spatial search for the central vertex of the star and wheel graph, and any vertex of the complete graph

Transport efficiency of continuous-time quantum walks on graphs

Continuous-time quantum walk describes the propagation of a quantum particle (or an excitation) evolving continuously in time on a graph. As such, it provides a natural framework for modeling transport processes, e.g., in light-harvesting systems. In particular, the transport properties strongly depend on the initial state and specific features of the graph under investigation. In this paper, we address the role of graph topology, and investigate the transport properties of graphs with different regularity, symmetry, and connectivity. We neglect disorder and decoherence, and assume a single trap vertex that is accountable for the loss processes. In particular, for each graph, we analytically determine the subspace of states having maximum transport efficiency. Our results provide a set of benchmarks for environment-assisted quantum transport, and suggest that connectivity is a poor indicator for transport efficiency. Indeed, we observe some specific correlations between transport efficiency and connectivity for certain graphs, but, in general, they are uncorrelated