Bose-Einstein Condensation and strong-correlation behavior of phonons in ion traps

We show that the dynamics of phonons in a set of trapped ions interacting with lasers is described by a Bose-Hubbard model whose parameters can be externally adjusted. We investigate the possibility of observing several quantum many-body phenomena, including (quasi) Bose-Einstein condensation as well as a superfluid-Mott insulator quantum phase transition.Comment: 5 pages, 3 figure

Mesoscopic Spin-Boson Models of Trapped Ions

Trapped ions arranged in Coulomb crystals provide us with the elements to study the physics of a single spin coupled to a boson bath. In this work we show that optical forces allow us to realize a variety of spin-boson models, depending on the crystal geometry and the laser configuration. We study in detail the Ohmic case, which can be implemented by illuminating a single ion with a travelling wave. The mesoscopic character of the phonon bath in trapped ions induces new effects like the appearance of quantum revivals in the spin evolution.Comment: 4.4 pages, 5 figure

Collective generation of quantum states of light by entangled atoms

We present a theoretical framework to describe the collective emission of light by entangled atomic states. Our theory applies to the low excitation regime, where most of the atoms are initially in the ground state, and relies on a bosonic description of the atomic excitations. In this way, the problem of light emission by an ensemble of atoms can be solved exactly, including dipole-dipole interactions and multiple light scattering. Explicit expressions for the emitted photonic states are obtained in several situations, such as those of atoms in regular lattices and atomic vapors. We determine the directionality of the photonic beam, the purity of the photonic state, and the renormalization of the emission rates. We also show how to observe collective phenomena with ultracold atoms in optical lattices, and how to use these ideas to generate photonic states that are useful in the context of quantum information.Comment: 15 pages, 10 figure

Quantum phases of interacting phonons in ion traps

The vibrations of a chain of trapped ions can be considered, under suitable experimental conditions, as an ensemble of interacting phonons, whose quantum dynamics is governed by a Bose--Hubbard Hamiltonian. In this work we study the quantum phases which appear in this system, and show that thermodynamical properties, such as critical parameters and critical exponents, can be measured in experiments with a limited number of ions. Besides that, interacting phonons in trapped ions offer us the possibility to access regimes which are difficult to study with ultracold bosons in optical lattices, like models with attractive or site--dependent phonon-phonon interactions.Comment: 10 page

DMRG and periodic boundary conditions: a quantum information perspective

We introduce a picture to analyze the density matrix renormalization group (DMRG) numerical method from a quantum information perspective. This leads us to introduce some modifications for problems with periodic boundary conditions in which the results are dramatically improved. The picture also explains some features of the method in terms of entanglement and teleportation.Comment: 4 page

Zeno dynamics in wave-packet diffraction spreading

We analyze a simple and feasible practical scheme displaying Zeno, anti-Zeno, and inverse-Zeno effects in the observation of wave-packet spreading caused by free evolution. The scheme is valid both in spatial diffraction of classical optical waves and in time diffraction of a quantum wave packet. In the optical realization, diffraction spreading is observed by placing slits between a light source and a light-power detector. We show that the occurrence of Zeno or anti-Zeno effects depends just on the frequency of observations between the source and detector. These effects are seen to be related to the diffraction mode theory in Fabry-Perot resonators.Comment: 7 pages, 8 figure

Exploring the blazar zone in High Energy flares of FSRQs

The gamma-ray emission offers a powerful diagnostic tool to probe jets and their surroundings in flat spectrum radio quasars (FSRQ). In particular, sources emitting at high energies (>10 GeV) give us the strongest constraints. This motivates us to start a systematic study of flares with bright emission above 10 GeV, examining archival data of Fermi-LAT gamma-ray telescope. At the same time, we began to trigger Target of Opportunity observations to the Swift observatory at the occurrence of high-energy flares, obtaining a wide coverage of the spectral energy distributions for several FSRQs during flares. Among the others we investigate the SED of a peculiar flare of 3C 454.3, showing a remarkable hard gamma-ray spectrum, quite different from the brightest flares of this source, and a bright flare of CTA 102. We modeled the SED in the framework of the one--zone leptonic model, using also archival optical spectroscopic data to derive the luminosity of the broad lines and thus estimate the disk luminosity, from which the structural parameters of the FSRQ nucleus can be inferred. The model allowed us to evaluate the magnetic field intensity in the blazar zone, and to locate the emitting region of gamma rays in the particular case in which gamma-ray spectra show neither absorption from the BLR, nor the Klein-Nishina curvature expected in leptonic models assuming the BLR as source of seed photons for the External Compton. For FSRQs bright above 10 GeV, we where able to identify short periods lasting less than 1 day characterized by high rate of high energy gamma rays, and hard gamma-ray spectra. We discussed the observed spectra and variability timescales in terms of injection and cooling of energetic particles, arguing that these flares could be triggered by magnetic reconnections events or turbulence in the flow.Comment: 20 pages, 8 figures, 6 tables, accepted for publication in Ap

Renormalization algorithm for the calculation of spectra of interacting quantum systems

We present an algorithm for the calculation of eigenstates with definite linear momentum in quantum lattices. Our method is related to the Density Matrix Renormalization Group, and makes use of the distribution of multipartite entanglement to build variational wave--functions with translational symmetry. Its virtues are shown in the study of bilinear--biquadratic S=1 chains.Comment: Corrected version. We have added an appendix with an extended explanation of the implementation of our algorith

Simulating quantum-optical phenomena with cold atoms in optical lattices

We propose a scheme involving cold atoms trapped in optical lattices to observe different phenomena traditionally linked to quantum-optical systems. The basic idea consists of connecting the trapped atomic state to a non-trapped state through a Raman scheme. The coupling between these two types of atoms (trapped and free) turns out to be similar to that describing light-matter interaction within the rotating-wave approximation, the role of matter and photons being played by the trapped and free atoms, respectively. We explain in particular how to observe phenomena arising from the collective spontaneous emission of atomic and harmonic oscillator samples such as superradiance and directional emission. We also show how the same setup can simulate Bose-Hubbard Hamiltonians with extended hopping as well as Ising models with long-range interactions. We believe that this system can be realized with state of the art technology