### Browsing by Author "Porras Torres, Diego"

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Publication Adiabatic quantum metrology with strongly correlated quantum optical systems(American Physical Society, 2013-08-02) Ivanov, P. A.; Porras Torres, DiegoWe show that the quasiadiabatic evolution of a system governed by the Dicke Hamiltonian can be described in terms of a self-induced quantum many-body metrological protocol. This effect relies on the sensitivity of the ground state to a small symmetry-breaking perturbation at the quantum phase transition, which leads to the collapse of the wave function into one of two possible ground states. The scaling of the final-state properties with the number of atoms and with the intensity of the symmetry-breaking field can be interpreted in terms of the precession time of an effective quantum metrological protocol. We show that our ideas can be tested with spin-phonon interactions in trapped ion setups. Our work points to a classification of quantum phase transitions in terms of the capability of many-body quantum systems for parameter estimation.Publication Circuit QED bright source for chiral entangled light based on dissipation(American Physical Society, 2013-08-15) Quijandría, Fernando; Porras Torres, Diego; García Ripoll, Juan José; Zueco, DavidWe present a scalable and tunable framework for the quantum simulation of critical dissipative models based on a circuit QED cavity array interacting with driven superconducting qubits. We will show that the strongly correlated many-body state of the cavities can be mapped into the state of propagating photons in a transmission line. This allows not only for an efficient way of accessing the correlations in the many-body system, but also provides a bright source of chiral entangled light where directionality and entanglement are assisted by collective phenomena and breaking of reflection symmetry.Publication Collective generation of quantum states of light by entangled atoms(American Physical Society, 2008-11) Porras Torres, Diego; Cirac, J. I.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.Publication Competing many-body interactions in systems of trapped ions(American Physical Society, 2009-06) Bermúdez, A.; Porras Torres, Diego; Martin-Delgado Alcántara, Miguel ÁngelWe propose and theoretically analyze an experimental configuration in which lasers induce three-spin interactions between trapped ions. By properly choosing the intensities and frequencies of the lasers, three-spin couplings may be dominant or comparable to two-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 three-spin Hamiltonian and predict qualitatively the quantum phase diagram of the system.Publication Detection of spin correlations in optical lattices by light scattering(American Physical Society, 2008-05) Vega, Inés de; Cirac, J. Ignacio; Porras Torres, DiegoWe show that spin correlations of atoms in an optical lattice can be reconstructed by coupling the system to the light, and by measuring correlations between the emitted photons. This principle is the basis for a method to characterize states in quantum computation and simulation with optical lattices. As examples, we study the detection of spin correlations in a quantum magnetic phase, and the characterization of cluster states.Publication Effective spin quantum phases in systems of trapped ions(American Physical Society, 2005-12) Deng, X.-L.; Porras Torres, Diego; Cirac, J. IgnacioA system of trapped ions under the action of off-resonant standing waves can be used to simulate a variety of quantum spin models. In this work, we describe theoretically quantum phases that can be observed in the simplest realization of this idea: quantum Ising and XY models. Our numerical calculations with the density matrix renormalization group method show that experiments with ion traps should allow one to access general properties of quantum critical systems. On the other hand, ion trap quantum spin models show a few unusual features due to the peculiarities of induced effective spin-spin interactions which lead to interesting effects like long-range quantum correlations and the coexistence of different spin phases.Publication Matter-wave emission in optical lattices: single particle and collective effects(American Physical Society, 2008-12-31) de Vega, Inés; Porras Torres, Diego; Cirac, J. IgnacioWe introduce a simple setup corresponding to the matter-wave analogue of impurity atoms embedded in a photonic crystal and interacting with the radiation field. Atoms in a given internal level are trapped in an optical lattice, and play the role of the impurities. Atoms in an untrapped level play the role of the radiation field. The interaction is mediated by means of lasers that couple those levels. By tuning the lasers parameters, it is possible to drive the system through different regimes, and observe phenomena such as matter-wave superradiance, non-Markovian atom emission, and the appearance of bound atomic states.Publication Mesoscopic entanglement induced by spontaneous emission in solid-state quantum optics(American Physical Society, 2013-02-20) González Tudela, Alejandro; Porras Torres, DiegoImplementations of solid-state quantum optics provide us with devices where qubits are placed at fixed positions in photonic or plasmonic one-dimensional waveguides. We show that solely by controlling the position of the qubits and with the help of a coherent driving, collective spontaneous decay maybe engineered to yield an entangled mesoscopic steady state. Our scheme relies on the realization of pure superradiant Dicke models by a destructive interference that cancels dipole-dipole interactions in one dimension.Publication Mesoscopic mean-field theory for spin-boson chains in quantum optical systems(Springer Heidelberg, 2013-02) Nevado Serrano, Pedro; Porras Torres, DiegoWe present a theoretical description of a system of many spins strongly coupled to a bosonic chain. We rely on the use of a spin-wave theory describing the Gaussian fluctuations around the mean-field solution, and focus on spin-boson chains arising as a generalization of the Dicke Hamiltonian. Our model is motivated by experimental setups such as trapped ions, or atoms/qubits coupled to cavity arrays. This situation corresponds to the cooperative (E circle times beta) Jahn-Teller distortion studied in solid-state physics. However, the ability to tune the parameters of the model in quantum optical setups opens up a variety of novel intriguing situations. The main focus of this paper is to review the spin-wave theoretical description of this problem as well as to test the validity of mean-field theory. Our main result is that deviations from mean-field effects are determined by the interplay between magnetic order and mesoscopic cooperativity effects, being the latter strongly size-dependent.Publication Mesoscopic spin-boson models of trapped ions(American Physical Society, 2008-07) Porras Torres, Diego; Marquard, F.; Delft, J. von; Cirac, J. ITrapped 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 traveling wave. The mesoscopic character of the phonon bath in trapped ions induces effects such as the appearance of quantum revivals in the spin evolution.Publication Nonequilibrium and nonperturbative dynamics of ultrastrong coupling in open lines(Amer Physical Soc, 2013-12-09) Peropadre, B.; Zueco, D.; Porras Torres, Diego; Garcı´a-Ripoll, J. J.The time and space resolved dynamics of a qubit with an Ohmic coupling to propagating 1D photons is studied, from weak coupling to the ultrastrong coupling regime. A nonperturbative study based on matrix product states shows the following results, (i) The ground state of the combined systems contains excitations of both the qubit and the surrounding bosonic field. (ii) An initially excited qubit equilibrates through spontaneous emission to a state, which under certain conditions is locally close to that ground state, both in the qubit and the field. (iii) The resonances of the combined qubit-photon system match those of the spontaneous emission process and also the predictions of the adiabatic renormalization [A. J. Leggett et al., Rev. Mod. Phys. 59, 1 (1987)]. Finally, nonperturbative ab initio calculations show that this physics can be studied using a flux qubit galvanically coupled to a superconducting transmission line.Publication Photon-assisted-tunneling toolbox for quantum simulations in ion traps(IOP Publishing, 2012-05-31) Bermudez, Alejandro; Schaetz, Tobias; Porras Torres, DiegoWe describe a versatile toolbox for the quantum simulation of many-body lattice models, capable of exploring the combined effects of background Abelian and non-Abelian gauge fields, bond and site disorder and strong on-site interactions. We show how to control the quantum dynamics of particles trapped in lattice potentials by the photon-assisted tunneling induced by periodic drivings. This scheme is general enough to be applied to either bosons or fermions with the additional advantage of being non-perturbative. It finds an ideal application in microfabricated ion trap arrays, where the quantized vibrational modes of the ions can be described by a quantum lattice model. We present a detailed theoretical proposal for a quantum simulator in that experimental setup, and show that it is possible to explore phases of matter that range from the fractional quantum Hall effect, to exotic strongly correlated glasses or flux-lattice models decorated with arbitrary patterns of localized defects.Publication Quantum manipulation of trapped ions in two dimensional Coulomb crystals(American Physical Society, 2006-06-30) Porras Torres, Diego; Cirac, J. IgnacioWe show that a large number of ions forming a 2D Coulomb crystal provides an almost ideal system for scalable quantum computation and quantum simulation. In particular, the coupling of the internal states to the motion of the ions transverse to the crystal plane allows one to implement two-qubit quantum gates. We analyze in detail the decoherence induced by anharmonic couplings, and show that very high gate fidelities can be achieved with current experimental setups.Publication Quantum phases of interacting phonons in ion traps(American Physical Society, 2008-03) Deng, X.-L. D; Porras Torres, Diego; Cirac, J. IgnacioThe 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. In addition to that, interacting phonons in trapped ions offer us the possibility to access regimes which are difficult to study with ultracold bosons in optical lattices, such as models with attractive or site-dependent phonon-phonon interactions.Publication Quantum phases of trapped ions in an optical lattice(IOP Publishing, 2008-04-30) Schmied, R.; Roscilde, T.; Murg, V.; Porras Torres, Diego; Cirac, J. L.We propose loading trapped ions into microtraps formed by an optical lattice. For harmonic microtraps, the Coulomb coupling of the spatial motions of neighboring ions can be used to construct a broad class of effective short-range Hamiltonians acting on an internal degree of freedom of the ions. For large anharmonicities, on the other hand, the spatial motion of the ions itself represents a spin-1/2 model with frustrated dipolar XY interactions. We illustrate the latter setup with three systems: the linear chain, the zigzag ladder and the triangular lattice. In the frustrated zigzag ladder with dipolar interactions we find chiral ordering beyond what was predicted previously for a next-nearest-neighbor model. In the frustrated anisotropic triangular lattice with nearest-neighbor interactions we find that the transition from the one-dimensional (1D) gapless spin-liquid phase to the 2D spiraling ordered phase passes through a gapped spin-liquid phase, similar to what has been predicted for the same model with Heisenberg interactions. Further, a second gapped spin-liquid phase marks the transition to the 2D Neel-ordered phase.Publication Quantum processing photonic states in optical lattices(American Physical Society, 2008-02-15) Muschik, Christine A.; Vega, Inés de; Porras Torres, Diego; Cirac, J. IgnacioThe mapping of photonic states to collective excitations of atomic ensembles is a powerful tool which finds a useful application in the realization of quantum memories and quantum repeaters. In this work we show that cold atoms in optical lattices can be used to perform an entangling unitary operation on the transferred atomic excitations. After the release of the quantum atomic state, our protocol results in a deterministic two qubit gate for photons. The proposed scheme is feasible with current experimental techniques and robust against the dominant sources of noise.Publication Quantum simulation of the cooperative Jahn-Teller transition in 1D Ion crystals(American Physical Society, 2012-06-05) Porras Torres, Diego; Ivanov, Peter A.; Schmidt-Kale, FerdinandThe Jahn-Teller effect explains distortions and nondegenerate energy levels in molecular and solid-state physics via a coupling of effective spins to collective bosons. Here we propose and theoretically analyze the quantum simulation of a many-body Jahn-Teller model with linear ion crystals subjected to magnetic field gradients. We show that the system undergoes a quantum magnetic structural phase transition which leads to a reordering of particle positions and the formation of a spin-phonon quasicondensate in mesoscopic ion chains.Publication Renormalization algorithm for the calculation of spectra of interacting quantum systems(American Physical Society, 2006-01) Porras Torres, Diego; Verstraete, Frank; Cirac, J. IgnacioWe 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. The algorithm is applied to the study of bilinear-biquadratic S=1 chains, in particular to the region of phase space between the dimerized and ferromagnetic phases.Publication Simulating accelerated atoms coupled to a quantum field(American Physical Society, 2012-02-25) Rey, Marco del; Porras Torres, Diego; Martin Martinez, EduardoWe show an analogy between static quantum emitters coupled to a single mode of a quantum field and accelerated Unruh-DeWitt detectors. We envision a way to simulate a variety of relativistic quantum field settings beyond the reach of current computational power, such as a high number of qubits coupled to a quantum field following arbitrary noninertial trajectories. Our scheme may be implemented with trapped ions and circuit QED setups.Publication Simulating quantum-optical phenomena with cold atoms in optical lattices(IOP Publishing, 2011-02-10) Navarrete Benlloch, Carlos; de Vega, Inés; Porras Torres, Diego; Cirac, J IgnacioWe 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.