Resonant purification of mixed states for closed and open quantum systems

Pure states are fundamental for the implementation of quantum technologies, and several methods for the purification of the state of a quantum system S have been developed in the past years. In this letter we present a new approach, based on the interaction of S with an auxiliary system P, having a wide range of applicability. Considering two-level systems S and P and assuming a particular interaction between them, we prove that complete purifications can be obtained under suitable conditions on the parameters characterizing P. Using analytical and numerical tools, we show that the purification process exhibits a resonant behavior in both the cases of system isolated from the external environment or not.Comment: 4 pages, LaTe

Effects of particle production during inflation

The impact of particle production during inflation on the primordial curvature perturbation spectrum is investigated both analytically and numerically. We obtain an oscillatory behavior on small scales, while on large scales the spectrum is unaffected. The amplitude of the oscillations is proportional to the number of coupled fields, their mass, and the square of the coupling constant. The oscillations are due a discontinuity in the second time derivative of the inflaton, arising from a temporary violation of the slow-roll conditions. A similar effect on the power spectrum should be produced also in other inflationary models where the slow-roll conditions are temporarily violated.Comment: 7 pages, 5 figure

Citizens and Institutions as Information Prosumers. The Case Study of Italian Municipalities on Twitter

The aim of this paper is to address changes in public communication following the advent of Internet social networking tools and the emerging web 2.0 technologies which are providing new ways of sharing information and knowledge. In particular public administrations are called upon to reinvent the governance of public affairs and to update the means for interacting with their communities. The paper develops an analysis of the distribution, diffusion and performance of the official profiles on Twitter adopted by the Italian municipalities (comuni) up to November 2013. It aims to identify the patterns of spatial distribution and the drivers of the diffusion of Twitter profiles; the performance of the profiles through an aggregated index, called the Twitter performance index (Twiperindex), which evaluates the profiles' activity with reference to the gravitational areas of the municipalities in order to enable comparisons of the activity of municipalities with different demographic sizes and functional roles. The results show that only a small portion of innovative municipalities have adopted Twitter to enhance e-participation and e-governance and that the drivers of the diffusion seem to be related either to past experiences and existing conditions (i.e. civic networks, digital infrastructures) developed over time or to strong local community awareness. The better performances are achieved mainly by small and medium-sized municipalities. Of course, the phenomenon is very new and fluid, therefore this analysis should be considered as a first step in ongoing research which aims to grasp the dynamics of these new means of public communication

The Chemical Evolution of the Milky Way: the Three Infall Model

We present a new chemical evolution model for the Galaxy that assumes three main infall episodes of primordial gas for the formation of halo, thick and thin disk, respectively. We compare our results with selected data taking into account NLTE effects. The most important parameters of the model are (i) the timescale for gas accretion, (ii) the efficiency of star formation and (iii) a threshold in the gas density for the star formation process, for each Galactic component. We find that, in order to best fit the features of the solar neighbourhood, the halo and thick disk must form on short timescales (~0.2 and ~1.25 Gyr, respectively), while a longer timescale is required for the thin-disk formation. The efficiency of star formation must be maximum (10 Gyr-1) during the thick-disk phase and minimum (1 Gyr-1) during the thin-disk formation. Also the threshold gas density for star formation is suggested to be different in the three Galactic components. Our main conclusion is that in the framework of our model an independent episode of accretion of extragalactic gas, which gives rise to a burst of star formation, is fundamental to explain the formation of the thick disk. We discuss our results in comparison to previous studies and in the framework of modern galaxy formation theories.Comment: 12 pages, 7 figures, accepted for publication in MNRA

Resonant pairing isotope effect in polaronic systems

The intermediate coupling regime in polaronic systems, situated between the adiabatic and the anti-adiabatic limit, is characterized by resonant pairing between quasi-free electrons which is induced by an exchange interaction with localized bipolarons. The onset of this resonant pairing takes place below a characteristic temperature T* and is manifest in the opening of a pseudogap in the density of states of the electrons. The variation of T* is examined here as a function of (i) the typical frequency \omega_0 of the local lattice modes, which determines the binding energy of the bipolarons, and (ii) the doping, which amounts to a relative change of the bipolaron concentration n_B to that of the free electrons n_F. We concentrate on a doping regime, where small changes in doping give rise to a large change in T*, which is the case when n_B is small (< 0.1 per site). For finite values of n_B we find negative and practically doping independent values of the isotope coefficient \alpha^* which characterizes the formation of resonating electron pairs. Upon decreasing the total particle density such that n_B becomes exponentially small, we find a rapid change in sign of \alpha^*. This is related to the fact that the system approaches a state which is more BCS-like, where electron pairing occurs via virtual excitations into bipolaronic states and where T* coincides with the onset of superconductivity.Comment: 7 pages, 6 figures, enlarged discussion on the limits of validity of the model, to be published in Phys. Rev.

Towards a classification of branes in theories with eight supercharges

We provide a classification of half-supersymmetric branes in quarter-maximal supergravity theories with scalars parametrising coset manifolds. Guided by the results previously obtained for the half-maximal theories, we are able to show that half-supersymmetric branes correspond to the real longest weights of the representations of the brane charges, where the reality properties of the weights are determined from the Tits-Satake diagrams associated to the global symmetry groups. We show that the resulting brane structure is universal for all theories that can be uplifted to six dimensions. We also show that when viewing these theories as low-energy theories for the suitably compactified heterotic string, the classification we obtain is in perfect agreement with the wrapping rules derived in previous works for the same theory compactified on tori. Finally, we relate the branes to the R-symmetry representations of the central charges and we show that in general the degeneracies of the BPS conditions are twice those of the half-maximal theories and four times those of the maximal ones.Comment: 47 pages, 8 figure