Competing many-body interactions in systems of trapped ions

We propose and theoretically analyse an experimental configuration in which lasers induce 3-spin interactions between trapped ions.By properly choosing the intensities and frequencies of the lasers, 3-spin couplings may be dominant or comparable to 2-spin terms and magnetic fields. In this way, trapped ions can be used to study exotic quantum phases which do not have a counterpart in nature. We study the conditions for the validity of the effective 3-spin Hamiltonian, and predict qualitatively the quantum phase diagram of the system.Comment: RevTex4 file, color figure

Critical Lines and Massive Phases in Quantum Spin Ladders with Dimerization

We determine the existence of critical lines in dimerized quantum spin ladders in their phase diagram of coupling constants using the finite-size DMRG algorithm. We consider both staggered and columnar dimerization patterns, and antiferromagnetic and ferromagnetic inter-leg couplings. The existence of critical phases depends on the precise combination of these patterns. The nature of the massive phases separating the critical lines are characterized with generalized string order parameters that determine their valence bond solid (VBS) content.Comment: 9 pages 10 figure

Topology induced anomalous defect production by crossing a quantum critical point

We study the influence of topology on the quench dynamics of a system driven across a quantum critical point. We show how the appearance of certain edge states, which fully characterise the topology of the system, dramatically modifies the process of defect production during the crossing of the critical point. Interestingly enough, the density of defects is no longer described by the Kibble-Zurek scaling, but determined instead by the non-universal topological features of the system. Edge states are shown to be robust against defect production, which highlights their topological nature.Comment: Phys. Rev. Lett. (to be published

Quantum Chinos Game: winning strategies through quantum fluctuations

We apply several quantization schemes to simple versions of the Chinos game. Classically, for two players with one coin each, there is a symmetric stable strategy that allows each player to win half of the times on average. A partial quantization of the game (semiclassical) allows us to find a winning strategy for the second player, but it is unstable w.r.t. the classical strategy. However, in a fully quantum version of the game we find a winning strategy for the first player that is optimal: the symmetric classical situation is broken at the quantum level.Comment: REVTEX4.b4 file, 3 table

The luminosity function of Palomar 5 and its tidal tails

We present the main sequence luminosity function of the tidally disrupted globular cluster Palomar 5 and its tidal tails. For this work we analyzed imaging data obtained with the Wide Field Camera at the INT (La Palma) and data from the Wide Field Imager at the MPG/ESO 2.2 m telescope at La Silla down to a limiting magnitude of approximately 24.5 mag in B. Our results indicate that preferentially fainter stars were removed from the cluster so that the LF of the cluster's main body exhibits a significant degree of flattening compared to other GCs. This is attributed to its advanced dynamical evolution. The LF of the tails is, in turn, enhanced with faint, low-mass stars, which we interpret as a consequence of mass segregation in the cluster.Comment: 4 pages, 3 figures, to be published in the proceedings of the conference "Satellites and tidal streams" held at La Palma, Canary Islands, May 26 - 30, 200

Quantum Google in a Complex Network

We investigate the behavior of the recently proposed quantum Google algorithm, or quantum PageRank, in large complex networks. Applying the quantum algorithm to a part of the real World Wide Web, we find that the algorithm is able to univocally reveal the underlying scale-free topology of the network and to clearly identify and order the most relevant nodes (hubs) of the graph according to their importance in the network structure. Moreover, our results show that the quantum PageRank algorithm generically leads to changes in the hierarchy of nodes. In addition, as compared to its classical counterpart, the quantum algorithm is capable to clearly highlight the structure of secondary hubs of the network, and to partially resolve the degeneracy in importance of the low lying part of the list of rankings, which represents a typical shortcoming of the classical PageRank algorithm. Complementary to this study, our analysis shows that the algorithm is able to clearly distinguish scale-free networks from other widespread and important classes of complex networks, such as Erd\H{o}s-R\'enyi networks and hierarchical graphs. We show that the ranking capabilities of the quantum PageRank algorithm are related to an increased stability with respect to a variation of the damping parameter $\alpha$ that appears in the Google algorithm, and to a more clearly pronounced power-law behavior in the distribution of importance among the nodes, as compared to the classical algorithm. Finally, we study to which extent the increased sensitivity of the quantum algorithm persists under coordinated attacks of the most important nodes in scale-free and Erd\H{o}s-R\'enyi random graphs

Electromagnetic multipole moments of elementary spin-1/2, 1, and 3/2 particles

We study multipole decompositions of the electromagnetic currents of spin-1/2, 1, and 3/2 particles described in terms of Lagrangians designed to reproduce representation specific wave equations which are second order in the momenta and which emerge within the recently elaborated Poincar\'e covariant projector method. We calculate the electric multipoles of the above spins for the spinor, the four-vector, and the four-vector--spinor representations, attend to the most general non-Lagrangian spin-3/2 currents which are allowed by Lorentz invariance to be of third order in the momenta and construct the linear current equivalent of identical multipole moments of one of them. We conclude that such non-Lagrangian currents are not necessarily more general than the two-term currents emerging within the covariant projector method. We compare our results with those of the conventional Proca-, and Rarita-Schwinger frameworks. Finally, we test the representation dependence of the multipoles by placing spin-1 and spin-3/2 in the respective (1,0)$\oplus$(0,1), and (3/2,0)$\oplus$(0,3/2) single-spin representations. We observe representation independence of the charge monopoles and the magnetic dipoles, in contrast to the higher multipoles, which turn out to be representation dependent. In particular, we find the bi-vector $(1,0)\oplus (0,1)$ to be characterized by an electric quadrupole moment of opposite sign to the one found in $(1/2,1/2)$, and consequently, to the $W$ boson. Our finding points toward the possibility that the $\rho$ meson could transform as part of an antisymmetric tensor with an $a_{1}$ meson-like state as its representation companion.Comment: 27 pages, 2 figure