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

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.

An experimental study of wall-injected flows in a rectangular cylinder

An experimental investigation of the flow inside a rectangular cylinder with air injected continuously along the wall is performed. This kind of flow is a two-dimensional approximation of what happens inside a solid rocket motor, where the lateral grain burns expelling exhaust gas or in processes with air filtration or devices to attain uniform flows. We propose a brief derivation of some analytical solutions and a comparison between these solutions and experimental data, which are obtained using the Particle Image Velocimetry (PIV) technique, in order to provide a global reconstruction of the flowfield. The flow, which enters orthogonal to the injecting wall, turns suddenly its direction being pushed towards the exit of the chamber. Under the incompressible and inviscid flow hypothesis, two analytical solutions are reported and compared. The first one, known as Hart-McClure solution, is irrotational and the injection velocity is non-perpendicular to the injecting wall. The other one, due to Taylor and Culick, has non-zero vorticity and constant, vertical injection velocity. The comparison with laminar solutions is useful to assess whether transition to turbulence is reached and how the disturbance thrown in by the porous injection influences and modifies those solutions

Two years of monitoring Supergiant Fast X-ray Transients with Swift

We present two years of intense Swift monitoring of three SFXTs, IGR J16479-4514, XTE J1739-302, and IGR J17544-2619 (since October 2007). Out-of-outburst intensity-based X-ray (0.3-10keV) spectroscopy yields absorbed power laws with by hard photon indices (G~1-2). Their outburst broad-band (0.3-150 keV) spectra can be fit well with models typically used to describe the X-ray emission from accreting NSs in HMXBs. We assess how long each source spends in each state using a systematic monitoring with a sensitive instrument. These sources spend 3-5% of the total in bright outbursts. The most probable flux is 1-2E-11 erg cm^{-2} s^{-1} (2-10 keV, unabsorbed), corresponding to luminosities in the order of a few 10^{33} to 10^{34} erg s^{-1} (two orders of magnitude lower than the bright outbursts). The duty-cycle of inactivity is 19, 39, 55%, for IGR J16479-4514, XTE J1739-302, and IGR J17544-2619, respectively. We present a complete list of BAT on-board detections further confirming the continued activity of these sources. This demonstrates that true quiescence is a rare state, and that these transients accrete matter throughout their life at different rates. X-ray variability is observed at all timescales and intensities we can probe. Superimposed on the day-to-day variability is intra-day flaring which involves variations up to one order of magnitude that can occur down to timescales as short as ~1ks, and whichcan be explained by the accretion of single clumps composing the donor wind with masses M_cl~0.3-2x10^{19} g. (Abridged)Comment: Accepted for publication in MNRAS. 17 pages, 11 figures, 8 table

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

The MESS of cosmological perturbations

We introduce two new effective quantities for the study of comoving curvature perturbations $\zeta$: the space dependent effective sound speed (SESS) and the momentum dependent effective sound speed (MESS) . We use the SESS and the MESS to derive a new set of equations which can be applied to any system described by an effective stress-energy-momentum tensor (EST), including multi-fields systems, supergravity and modified gravity theories. We show that this approach is completely equivalent to the standard one and it has the advantage of requiring to solve only one differential equation for $\zeta$ instead of a system, without the need of explicitly computing the evolution of entropy perturbations. The equations are valid for perturbations respect to any arbitrary flat spatially homogeneous background, including any inflationary and bounce model. As an application we derive the equation for $\zeta$ for multi-fields $KGB$ models and show that observed features of the primordial curvature perturbation spectrum are compatible with the effects of an appropriate local variation of the MESS in momentum space. The MESS is the natural quantity to parametrize in a model independent way the effects produced on curvature perturbations by multi-fields systems, particle production and modified gravity theories and could be conveniently used in the analysis of LSS observations, such as the ones from the upcoming EUCLID mission or CMB radiation measurements.Comment: We study the MESS of cosmological perturbations, version accepted in Physics Letters

The boson-fermion model: An exact diagonalization study

The main features of a generic boson-fermion scenario for electron pairing in a many-body correlated fermionic system are: i) a cross-over from a poor metal to an insulator and finally a superconductor as the temperature decreases, ii) the build-up of a finite amplitude of local electron pairing below a certain temperature $T^*$, followed by the onset of long-range phase correlations among electron pairs below a second characteristic temperature $T_{\phi}$, iii) the opening of a pseudogap in the DOS of the electrons below $T^*$, rendering these electrons poorer and poorer quasi-particles as the temperature decreases, with the electron transport becoming ensured by electron pairs rather than by individual electrons. A number of these features have been so far obtained on the basis of different many-body techniques, all of which have their built-in shortcomings in the intermediate coupling regime, which is of interest here. In order to substantiate these features, we investigate them on the basis of an exact diagonalization study on rings up to eight sites. Particular emphasis has been put on the possibility of having persistent currents in mesoscopic rings tracking the change-over from single- to two-particle transport as the temperature decreases and the superconducting state is approached.Comment: 7 pages, 8 figures; to be published in Phys. Rev.