Optimal correlations in many-body quantum systems

Information and correlations in a quantum system are closely related through the process of measurement. We explore such relation in a many-body quantum setting, effectively bridging between quantum metrology and condensed matter physics. To this aim we adopt the information-theory view of correlations, and study the amount of correlations after certain classes of Positive-Operator-Valued Measurements are locally performed. As many-body system we consider a one-dimensional array of interacting two-level systems (a spin chain) at zero temperature, where quantum effects are most pronounced. We demonstrate how the optimal strategy to extract the correlations depends on the quantum phase through a subtle interplay between local interactions and coherence.Comment: 5 pages, 5 figures + supplementary material. To be published in PR

Alternative Food Networks (AFNs) in Calabria : a sociological exploration of interaction dynamics

This thesis aims to advance the understanding of identities and roles of Alternative Food Networks (AFNs). It focusses on AFNs which operate in contexts where traditional aspects are still dominant within the local agrifood systems, and which act on both food provisioning and raising awareness about civic issues. This research examines the identities and roles of AFNs by shedding light on the dynamics of the organisation and implementation of their activities. For this, the thesis firstly adapts a theoretical and methodological framework – Interaction Ritual (IR) theory - for the investigation of social phenomena, starting by unveiling the functioning of their internal dynamics; secondly, it operationalises and applies the selected theoretical and methodological framework in order to analyse GAS M which is a case of an AFN. Namely it is a case of ‘Gruppo di Acquisto Solidale’ (Solidarity Purchasing Group - GAS). Below the main contributions and recommendations of this research are synthesised. 1) This research contributes to the body of knowledge on AFNs by generating insight into the identity of an AFN which operates in a context of a traditional agrifood system, attributes to both consumers and consumer-based civil society organisations leading roles, and works on both food provisioning and activities of civic relevance. The research demonstrates that the identity of GAS M is not clearly defined. Instead, it is continuously re-shaped in response to the varying combination of material and civic activities and interests. The mentioned activities and interests attract different groups of people. It is especially the small group that manages GAS M that is interested in civic activities. Often people in this group choose the organisational arrangements that are in line with their interests so their interests override other people’s interests. GAS M is undergoing a process of ideological and material selection which, together with the pre-eminence of civic interests, has three main implications: a) the equity of the food provisioning system recreated by GAS M is reduced; b) its role as a creator of occasions where producers and consumers can satisfy their needs of buying and selling products with specific physical features is reduced. Furthermore, while its role in building community and creating spaces for socialising and learning is supported, it only reaches people sharing certain interests; c) the durability of GAS M and its likelihood of expansion are limited. 2) The second result and contribution of this research is theoretical and methodological. First of all, the study contributes to refining IR theory. It represents a first attempt to apply the IR model in a study of AFN activities. The research reinterprets IR theory in an explorative and strictly dynamic-centred way. It operationalises the guidelines of IR theory for the collection and analysis of data that result naturally from normal everyday AFN interactions. Furthermore, this approach provides analytical tools to ascertain the intensity of AFNs’ activities, what attracts attention in these activities, which sort of attention, and what its significance is.  Secondly, this research has contributed towards advancing the existing body of research into AFN dynamics. Previous studies have focussed on processes of negotiating meaning in order to understand AFN identities and roles. Following IR theory, this research goes beyond the negotiation of meaning by analysing not only the content of interaction but also its intensity. In doing so the study shows not only which issues influence AFN identities and roles, but also which activities play an important role. In addition, the study provides insight into the dynamics of power and inclusivity in AFN interaction that is, among others, reflected in decision making about activities. Unlike previous studies which have looked into these questions by interviewing actors and relying on their rationalisations, this study explores these questions through the analysis of normal every day activities and interactions, and what is also being indicated as ‘natural data’. In this way, this research contributes to overcoming actors’ personal preferences and ideas, which may bias their rationalisations about these matters. 3) Finally, this research gives some recommendations for further research. It advises to study more cases of AFN in order to check to what extent the results of this case-study may be generalizable.  Furthermore, it offers some inputs on how the approach can be used so as to consider a broader variety of cases in the analysis. Secondly, this research invites future researchers look into what influences the intensity of AFNs’ activities and contributes to their stabilisation, and to explore the causal relationship between civic and material activities and interests. &nbsp

Entanglement crossover close to a quantum critical point

We discuss the thermal entanglement close to a quantum phase transition by analyzing the concurrence for one dimensional models in the quantum Ising universality class. We demonstrate that the entanglement sensitivity to thermal and to quantum fluctuations obeys universal $T\neq 0$--scaling behaviour. We show that the entanglement, together with its criticality, exhibits a peculiar universal crossover behaviour.Comment: 12 pages; 5 figures (eps). References added; to be published in Europhysics Letter

Conserved Ising Model on the Human Connectome

Dynamical models implemented on the large scale architecture of the human brain may shed light on how function arises from the underlying structure. This is the case notably for simple abstract models, such as the Ising model. We compare the spin correlations of the Ising model and the empirical functional brain correlations, both at the single link level and at the modular level, and show that their match increases at the modular level in anesthesia, in line with recent results and theories. Moreover, we show that at the peak of the specific heat (the \it{critical state}) the spin correlations are minimally shaped by the underlying structural network, explaining how the best match between structure and function is obtained at the onset of criticality, as previously observed. These findings confirm that brain dynamics under anesthesia shows a departure from criticality and could open the way to novel perspectives when the conserved magnetization is interpreted in terms of an homeostatic principle imposed to neural activity

Hidden order in bosonic gases confined in one dimensional optical lattices

We analyze the effective Hamiltonian arising from a suitable power series expansion of the overlap integrals of Wannier functions for confined bosonic atoms in a 1d optical lattice. For certain constraints between the coupling constants, we construct an explicit relation between such an effective bosonic Hamiltonian and the integrable spin-$S$ anisotropic Heisenberg model. Therefore the former results to be integrable by construction. The field theory is governed by an anisotropic non linear $\sigma$-model with singlet and triplet massive excitations; such a result holds also in the generic non-integrable cases. The criticality of the bosonic system is investigated. The schematic phase diagram is drawn. Our study is shedding light on the hidden symmetry of the Haldane type for one dimensional bosons.Comment: 5 pages; 1 eps figure. Revised version, to be published in New. J. Phy

Bose-Einstein condensation and entanglement in magnetic systems

We present a study of magnetic field induced quantum phase transitions in insulating systems. A generalized scaling theory is used to obtain the temperature dependence of several physical quantities along the quantum critical trajectory ($H=H_{C}$, $T\to0$) where $H$ is a longitudinal external magnetic field and $H_{C}$ the critical value at which the transition occurs. We consider transitions from a spin liquid at a critical field $H_{C1}$ and from a fully polarized paramagnet, at $H_{C2}$, into phases with long range order in the transverse components. The transitions at $H_{C1}$ and $H_{C2}$ can be viewed as Bose-Einstein condensations of magnons which however belong to different universality classes since they have different values of the dynamic critical exponent $z$. Finally, we use that the magnetic susceptibility is an entanglement witness to discuss how this type of correlation sets in as the system approaches the quantum critical point along the critical trajectory, $H=H_{C2}$, $T\to0$.Comment: 7 pages, 1 Table; accepted version; changes in text and new reference

Self-trapping mechanisms in the dynamics of three coupled Bose-Einstein condensates

We formulate the dynamics of three coupled Bose-Einstein condensates within a semiclassical scenario based on the standard boson coherent states. We compare such a picture with that of Ref. 1 and show how our approach entails a simple formulation of the dimeric regime therein studied. This allows to recognize the parameters that govern the bifurcation mechanism causing self-trapping, and paves the way to the construction of analytic solutions. We present the results of a numerical simulation showing how the three-well dynamics has, in general, a cahotic behavior.Comment: 4 pages, 5 figure

Asymmetry of bipartite quantum discord

It is known from the analysis of the density matrix for bipartite systems that the quantum discord (as a measure of quantum correlations) depends on the particular subsystem chosen for the projective measurements. We study asymmetry of the discord in a simple physical model of two spin-1/2 particles with the dipole-dipole interaction governed by the XY Hamiltonian in the inhomogeneous magnetic field. The dependence of the above discord asymmetry on the Larmour frequencies at both T=0 (the ground state) and $T>0$ has been investigated. It is demonstrated, in particular, that the asymmetry is negligible for high temperatures but it may become significant with the decrease in temperature.Comment: 5 pages 3 figure

Sparse aperture masking at the VLT I. Faint companion detection limits for the two debris disk stars HD 92945 and HD 141569

Observational data on companion statistics around young stellar systems is needed to flesh out the formation pathways for extrasolar planets and brown dwarfs. Aperture masking is a new technique that is able to address an important part of this discovery space. We observed the two debris disk systems HD 92945 and HD 141569 with sparse aperture masking (SAM), a new mode offered on the NaCo instrument at the VLT. A search for faint companions was performed using a detection strategy based on the analysis of closure phases recovered from interferograms recorded on the Conica camera. Our results demonstrate that SAM is a very competitive mode in the field of companion detection. We obtained 5 sigma high-contrast detection limits at lambda/D of 2.5x10^{-3} (\Delta L' = 6.5) for HD 92945 and 4.6x10^{-3} (\Delta L' = 5.8) for HD 141569. According to brown dwarf evolutionary models, our data impose an upper mass boundary for any companion for the two stars to, respectively, 18 and 22 Jupiter masses at minimum separations of 1.5 and 7 AU. The detection limits is mostly independent of angular separation, until reaching the diffraction limit of the telescope. We have placed upper limits on the existence of companions to our target systems that fall close to the planetary mass regime. This demonstrates the potential for SAM mode to contribute to studies of faint companions. We furthermore show that the final dynamic range obtained is directly proportional to the error on the closure phase measurement. At the present performance levels of 0.28 degree closure phase error, SAM is among the most competitive techniques for recovering companions at scales of one to several times the diffraction limit of the telescope. Further improvements to the detection threshold can be expected with more accurate phase calibration.Comment: accepted in A&A, 9 page