Degradability of Bosonic Gaussian channels

The notion of weak-degradability of quantum channels is introduced by generalizing the degradability definition given by Devetak and Shor. Exploiting the unitary equivalence with beam-splitter/amplifier channels we then prove that a large class of one-mode Bosonic Gaussian channels are either weakly degradable or anti-degradable. In the latter case this implies that their quantum capacity Q is null. In the former case instead, this allows us to establish the additivity of the coherent information for those maps which admit unitary representation with single-mode pure environment.Comment: 7 pages, 1 figure, 1 table (minor editing

Algorithms for Graph-Constrained Coalition Formation in the Real World

Coalition formation typically involves the coming together of multiple, heterogeneous, agents to achieve both their individual and collective goals. In this paper, we focus on a special case of coalition formation known as Graph-Constrained Coalition Formation (GCCF) whereby a network connecting the agents constrains the formation of coalitions. We focus on this type of problem given that in many real-world applications, agents may be connected by a communication network or only trust certain peers in their social network. We propose a novel representation of this problem based on the concept of edge contraction, which allows us to model the search space induced by the GCCF problem as a rooted tree. Then, we propose an anytime solution algorithm (CFSS), which is particularly efficient when applied to a general class of characteristic functions called $m+a$ functions. Moreover, we show how CFSS can be efficiently parallelised to solve GCCF using a non-redundant partition of the search space. We benchmark CFSS on both synthetic and realistic scenarios, using a real-world dataset consisting of the energy consumption of a large number of households in the UK. Our results show that, in the best case, the serial version of CFSS is 4 orders of magnitude faster than the state of the art, while the parallel version is 9.44 times faster than the serial version on a 12-core machine. Moreover, CFSS is the first approach to provide anytime approximate solutions with quality guarantees for very large systems of agents (i.e., with more than 2700 agents).Comment: Accepted for publication, cite as "in press

The hard X-ray tails in neutron star low mass X-ray binaries: BeppoSAX observations and possible theoretical explanation of the GX 17+2 case

We report results of a new spectral analysis of two BeppoSAX observations of the Z source GX 17+2. In one of the two observations the source exhibits a powerlaw-like hard (> 30 keV) X-ray tail which was described in a previous work by a hybrid Comptonization model. Recent high-energy observations with INTEGRAL of a sample of Low Mass X-Ray Binaries including both Z and atoll classes have shown that bulk (dynamical) Comptonization of soft photons can be a possible alternative mechanism for producing hard X-ray tails in such systems. We start from the INTEGRAL results and we exploit the broad-band capability of BeppoSAX to better investigate the physical processes at work. We use GX 17+2 as a representative case. Moreover, we suggest that weakening (or disappearance) of the hard X-ray tail can be explained by increasing radiation pressure originated at the surface of the neutron star (NS). As a result the high radiation pressure stops the bulk inflow and consequently this radiation feedback of the NS surface leads to quenching the bulk Comptonization.Comment: 6 pages, 3 figures, Accepted for publication in Ap

Advances in diagnostic and interventional radiology

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Qubit quantum channels: A characteristic function approach

A characterization of qubit quantum channels is introduced. In analogy to what happens in the context of Bosonic channels we exploit the possibility of representing the states of the system in terms of characteristic function. The latter are functions of non-commuting variables (Grassmann variables) and are defined in terms of generalized displacement operators. In this context we introduce the set of Gaussian channels and show that they share similar properties with the corresponding Bosonic counterpart.Comment: 10 pages (minor editing

Uncovering the host galaxy of the γ\gamma-ray-emitting narrow-line Seyfert 1 galaxy FBQS J1644+2619

The discovery of $\gamma$-ray emission from radio-loud narrow-line Seyfert 1 (NLSy1) galaxies has questioned the need for large black hole masses (> 10$^8$ M$_{\odot}$) to launch relativistic jets. We present near-infrared data of the $\gamma$-ray-emitting NLSy1 FBQS J1644+2619 that were collected using the camera CIRCE (Canarias InfraRed Camera Experiment) at the 10.4-m Gran Telescopio Canarias to investigate the structural properties of its host galaxy and to infer the black hole mass. The 2D surface brightness profile is modelled by the combination of a nuclear and a bulge component with a S\'ersic profile with index $n$ = 3.7, indicative of an elliptical galaxy. The structural parameters of the host are consistent with the correlations of effective radius and surface brightness against absolute magnitude measured for elliptical galaxies. From the bulge luminosity, we estimated a black hole mass of (2.1$\pm$0.2) $\times$10$^8$ M$_{\odot}$, consistent with the values characterizing radio-loud active galactic nuclei.Comment: 5 pages, 3 figures, 1 table. Monthly Notices of the Royal Astronomical Society Letter, Vol. 469, L11-L1

Measurement of the Blackbody Radiation Shift of the 133Cs Hyperfine Transition in an Atomic Fountain

We used a Cs atomic fountain frequency standard to measure the Stark shift on the ground state hyperfine transiton frequency in cesium (9.2 GHz) due to the electric field generated by the blackbody radiation. The measures relative shift at 300 K is -1.43(11)e-14 and agrees with our theoretical evaluation -1.49(07)e-14. This value differs from the currently accepted one -1.69(04)e-14. The difference has a significant implication on the accuracy of frequency standards, in clocks comparison, and in a variety of high precision physics tests such as the time stability of fundamental constants.Comment: 4 pages, 2 figures, 2 table

Spin electric effects in molecular antiferromagnets

Molecular nanomagnets show clear signatures of coherent behavior and have a wide variety of effective low-energy spin Hamiltonians suitable for encoding qubits and implementing spin-based quantum information processing. At the nanoscale, the preferred mechanism for control of quantum systems is through application of electric fields, which are strong, can be locally applied, and rapidly switched. In this work, we provide the theoretical tools for the search for single molecule magnets suitable for electric control. By group-theoretical symmetry analysis we find that the spin-electric coupling in triangular molecules is governed by the modification of the exchange interaction, and is possible even in the absence of spin-orbit coupling. In pentagonal molecules the spin-electric coupling can exist only in the presence of spin-orbit interaction. This kind of coupling is allowed for both $s=1/2$ and $s=3/2$ spins at the magnetic centers. Within the Hubbard model, we find a relation between the spin-electric coupling and the properties of the chemical bonds in a molecule, suggesting that the best candidates for strong spin-electric coupling are molecules with nearly degenerate bond orbitals. We also investigate the possible experimental signatures of spin-electric coupling in nuclear magnetic resonance and electron spin resonance spectroscopy, as well as in the thermodynamic measurements of magnetization, electric polarization, and specific heat of the molecules.Comment: 31 pages, 24 figure

Statistical significance of communities in networks

Nodes in real-world networks are usually organized in local modules. These groups, called communities, are intuitively defined as sub-graphs with a larger density of internal connections than of external links. In this work, we introduce a new measure aimed at quantifying the statistical significance of single communities. Extreme and Order Statistics are used to predict the statistics associated with individual clusters in random graphs. These distributions allows us to define one community significance as the probability that a generic clustering algorithm finds such a group in a random graph. The method is successfully applied in the case of real-world networks for the evaluation of the significance of their communities.Comment: 9 pages, 8 figures, 2 tables. The software to calculate the C-score can be found at http://filrad.homelinux.org/cscor

UBQLN4 Represses Homologous Recombination and Is Overexpressed in Aggressive Tumors

Genomic instability can be a hallmark of both human genetic disease and cancer. We identify a deleterious UBQLN4 mutation in families with an autosomal recessive syndrome reminiscent of genome instability disorders. UBQLN4 deficiency leads to increased sensitivity to genotoxic stress and delayed DNA double-strand break (DSB) repair. The proteasomal shuttle factor UBQLN4 is phosphorylated by ATM and interacts with ubiquitylated MRE11 to mediate early steps of homologous recombination-mediated DSB repair (HRR). Loss of UBQLN4 leads to chromatin retention of MRE11, promoting non-physiological HRR activity in vitro and in vivo. Conversely, UBQLN4 overexpression represses HRR and favors non-homologous end joining. Moreover, we find UBQLN4 overexpressed in aggressive tumors. In line with an HRR defect in these tumors, UBQLN4 overexpression is associated with PARP1 inhibitor sensitivity. UBQLN4 therefore curtails HRR activity through removal of MRE11 from damaged chromatin and thus offers a therapeutic window for PARP1 inhibitor treatment in UBQLN4-overexpressing tumors