Publications (29)76.14 Total impact
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ABSTRACT: We study translationally invariant rings of qubits with a finite number of sites N, and determine the maximal nearestneighbor entanglement for a fixed zcomponent of the total spin. For small numbers of sites we present analytical results. We establish a relation between the maximal nearestneighbor concurrence and the ground state energy of an XXZ spin model. This connection allows us to calculate the concurrence numerically for N≤24. We point out some interesting finitesize effects. Finally, we generalize our results beyond nearest neighbors.  [Show abstract] [Hide abstract]
ABSTRACT: We show how spinspin correlations, detected in a nondestructive way via spatially resolved quantum polarization spectroscopy, strongly characterize various phases realized in trapped ultracold fermionic atoms. Polarization degrees of freedom of the light couple to spatially resolved components of the atomic spin. In this way quantum fluctuations of matter are faithfully mapped onto those of light. In particular we demonstrate that quantum spin polarization spectroscopy provides a direct method to detect the FuldeFerrellLarkinOvchinnikov phase realized in a onedimensional imbalanced Fermi system. Comment: 16 pages, 9 figures 
Article: Generation of entangled photon pairs in optical cavityQED: Operating in the bad cavity limit
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ABSTRACT: We propose an optical cavityQED scheme for the deterministic generation of polarization entangled photon pairs that operates with high fidelity even in the bad cavity limit. The scheme is based on the interaction of an excited fourlevel atom with two empty optical cavity modes via an adiabatic passage process. MonteCarlo wave function simulations are used to evaluate the fidelity of the cavityQED source and its entanglement capability in the presence of decoherence. In the bad cavity limit, fidelities close to one are predicted for stateoftheart experimental parameter values. Comment: 9 pages and 5 figures  [Show abstract] [Hide abstract]
ABSTRACT: We extend the recently developed threelevel atom optics (TLAO) techniques, e.g., the spatial analogue of Stimulated Raman Adiabatic Passage, to (i) the coherent control of defects, e.g., empty sites, in optical lattices and microtraps; and (ii) to the robust and efficient transport of atoms between optical waveguides.  [Show abstract] [Hide abstract]
ABSTRACT: We describe a cavityQED scheme to deterministically generate polarization entangled photon pairs by using a single atom successively coupled to two single longitudinal mode cavities presenting polarization degeneracy. The cavities are initially prepared either in the vacuum state or in a single photon Fock state for each orthogonal polarization. Sharing the same basic elements, the source can operate on different physical processes. For a Vtype threelevel atom initially prepared in the ground state two implementations of the source are possible using either: i) two truncated Rabi Oscillations, or ii) a counterintuitive Stimulated Raman Adiabatic Passage process. Although slower than the former implementation, this second one is very efficient and robust under fluctuations of the experimental parameters and, particularly interesting, almost insensitive to atomic decay. For a fourlevel atom in a diamond configuration initially prepared in the upper state, the source can produce entangled photon pairs even in the bad cavity limit via an adiabatic passage process. We have performed Monte Carlo wave function simulations to characterize these sources by means of: i) the success probability P of producing the desired entangled state, ii) the fidelity F in the reduced space of two emitted cavity photons, and iii) the S parameter of the ClauserHorneShimonyHolt (CHSH) inequality to quantify the entanglement capability.  [Show abstract] [Hide abstract]
ABSTRACT: Preparation, manipulation, and detection of strongly correlated states of quantum many body systems are among the most important goals and challenges of modern physics. Ultracold atoms offer an unprecedented playground for realization of these goals. Here we show how strongly correlated states of ultracold atoms can be detected in a quantum nondemolition scheme, that is, in the fundamentally least destructive way permitted by quantum mechanics. In our method, spatially resolved components of atomic spins couple to quantum polarization degrees of freedom of light. In this way quantum correlations of matter are faithfully mapped on those of light; the latter can then be efficiently measured using homodyne detection. We illustrate the power of such spatially resolved quantum noise limited polarization measurement by applying it to detect various standard and "exotic" types of antiferromagnetic order in lattice systems and by indicating the feasibility of detection of superfluid order in Fermi liquids.  [Show abstract] [Hide abstract]
ABSTRACT: We study entanglement generation via particle transport across a onedimensional system described by the BoseHubbard Hamiltonian. We analyze how the competition between interactions and tunneling affects transport properties and the creation of entanglement in the occupation number basis. Alternatively, we propose to use spatially delocalized quantum bits, where a quantum bit is defined by the presence of a particle either in a site or in the adjacent one. Our results can serve as a guidance for future experiments to characterize entanglement of ultracold gases in onedimensional optical lattices.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a method for the detection of ground state quantum phases of spinor gases through a series of two quantum nondemolition measurements performed by sending offresonant, polarized light pulses through the gas. Signatures of various meanfield as well as strongly correlated phases of F=1 and F=2 spinor gases obtained by detecting quantum fluctuations and mean values of polarization of transmitted light are identified.  [Show abstract] [Hide abstract]
ABSTRACT: We propose a method for quantum state transfer in spin chains using an adiabatic passage technique. Modifying even and odd nearestneighbour couplings in time allows to achieve transfer fidelities arbitrarily close to one, without the need for a precise control of coupling strengths and timing. We study in detail transfer by adiabatic passage in a spin1 chain governed by a generalized Heisenberg Hamiltonian. We consider optimization of the transfer process applying optimal control techniques. We discuss a realistic experimental implementation using cold atomic gases confined in deep optical lattices. 
Article: Cavityquantumelectrodynamics entangled photon source based on two truncated Rabi oscillations
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ABSTRACT: We discuss a cavityQED scheme to deterministically generate entangled photons pairs by using a threelevel atom successively coupled to two single longitudinal mode highQ cavities presenting polarization degeneracy. The first cavity is prepared in a welldefined Fock state with two photons with opposite circular polarizations while the second cavity remains in the vacuum state. Half of a resonant Rabi oscillation in each cavity transfers one photon from the first to the second cavity, leaving the photons entangled in their polarization degree of freedom. The feasibility of this implementation and some practical considerations are discussed for both microwave and optical regimes. In particular, Monte Carlo wavefunction simulations have been performed with stateoftheart parameter values to evaluate the success probability of the cavityQED source in producing entangled photon pairs as well as its entanglement capability.  [Show abstract] [Hide abstract]
ABSTRACT: Quantum key distribution (QKD) refers to specific quantum strategies which permit the secure distribution of a secret key between two parties that wish to communicate secretly. Quantum cryptography has proven unconditionally secure in ideal scenarios and has been successfully implemented using quantum states with finite (discrete) as well as infinite (continuous) degrees of freedom. Here, we analyze the efficiency of QKD protocols that use as a resource entangled gaussian states and gaussian operations only. In this framework, it has already been shown that QKD is possible (M. Navascu\'es et al. Phys. Rev. Lett. 94, 010502 (2005)) but the issue of its efficiency has not been considered. We propose a figure of merit (the efficiency $E$) to quantify the number of classical correlated bits that can be used to distill a key from a sample of $N$ entangled states. We relate the efficiency of the protocol to the entanglement and purity of the states shared between the parties.  [Show abstract] [Hide abstract]
ABSTRACT: We study the transfer of quantum information through a Heisenberg spin1 chain prepared in its ground state. We measure the efficiency of such a quantum channel {\em via} the fidelity of retrieving an arbitrarily prepared state and {\em via} the transfer of quantum entanglement. The Heisenberg spin1 chain has a very rich quantum phase diagram. We show that the phase boundaries are reflected in sharp variations of the transfer efficiency. In the vicinity of the border between the dimer and the ferromagnetic phase (in the conjectured spinnematic region), we find strong indications for a qualitative change of the excitation spectrum. Moreover, we identify two regions of the phase diagram which give rise to particularly high transfer efficiency; the channel might be nonclassical even for chains of arbitrary length, in contrast to spin1/2 chains.  [Show abstract] [Hide abstract]
ABSTRACT: An analogy is explored between a setup of three atomic traps coupled via tunneling and an internal atomic threelevel system interacting with two laser fields. Within this scenario we describe a STIRAP like process which allows to move an atom between the ground states of two trapping potentials and analyze its robustness. This analogy is extended to other robust and coherent transport schemes and to systems of more than a single atom. Finally it is applied to manipulate external degrees of freedom of atomic wave packets propagating in waveguides.  [Show abstract] [Hide abstract]
ABSTRACT: We discuss a cavityQED scheme to deterministically generate entangled photons pairs by using a threelevel atom successively coupled to two single longitudinal mode highQ cavities presenting polarization degeneracy. The first cavity is prepared in a well defined Fock state with two photons with opposite circular polarizations while the second cavity remains in the vacuum state. A halfofaresonant Rabi oscillation in each cavity transfers one photon from the first to the second cavity, leaving the photons entangled in their polarization degree of freedom. The feasibility of this implementation and some practical considerations are discussed for both, microwave and optical regimes. In particular, Monte Carlo wave function simulations have been performed with stateoftheart parameter values to evaluate the success probability of the cavityQED source in producing entangled photon pairs as well as its entanglement capability.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate magnetic properties of Mottinsulating phases of ultracold Bose and Fermi spinor gases in optical lattices. We consider in particular the F=2 Bose gas, and the F=3/2 and F=5/2 Fermi gases. We derive effective spin Hamiltonians for one and two atoms per site and discuss the possibilities of manipulating the magnetic properties of the system using optical Feshbach resonances. We discuss low temperature quantum phases of a 87Rb gas in the F=2 hyperfine state, as well as possible realizations of high spin Fermi gases with either 6Li or 132Cs atoms in the F=3/2 state, and with 173Yb atoms in the F=5/2 state. Comment: 15 pages, 5 figures; a completely new and substantially expanded version with several errors corrected  [Show abstract] [Hide abstract]
ABSTRACT: We present two cavity quantum electrodynamics proposals that, sharing the same basic elements, allow for the deterministic generation of entangled photons pairs by means of a threelevel atom successively coupled to two single longitudinal mode highQ optical resonators presenting polarization degeneracy. In the faster proposal, the threelevel atom yields a polarization entangled photon pair via two truncated Rabi oscillations, whereas in the adiabatic proposal a counterintuitive Stimulated Raman Adiabatic Passage process is considered. Although slower than the former process, this second method is very efficient and robust under fluctuations of the experimental parameters and, particularly interesting, almost completely insensitive to atomic decay. Comment: 5 pages, 5 figures  [Show abstract] [Hide abstract]
ABSTRACT: We present here an overview of our work concerning entanglement properties of composite quantum systems. The characterization of entanglement, i.e., the possibility to assert wether a given quantum state is entangled with others and how much entangled it is, remains one of the most fundamental open questions in quantum information theory. We discuss our recent results related to the problem of separability and distillability for distinguishable particles, employing the tool of witness operators. Finally, we also state our results concerning quantum correlations for indistinguishable particles.  [Show abstract] [Hide abstract]
ABSTRACT: We analyze methods to go beyond the standard quantum limit for a class of atomic interferometers, where the quantity of interest is the difference of phase shifts obtained by two independent atomic ensembles. An example is given by an atomic Sagnac interferometer, where for two ensembles propagating in opposite directions in the interferometer this phase difference encodes the angular velocity of the experimental setup. We discuss methods of squeezing separately or jointly observables of the two atomic ensembles, and compare in detail advantages and drawbacks of such schemes. In particular we show that the method of joint squeezing may improve the variance by up to a factor of 2. We take into account fluctuations of the number of atoms in both the preparation and the measurement stage, and obtain bounds on the difference of the numbers of atoms in the two ensembles, as well as on the detection efficiency, which have to be fulfilled in order to surpass the standard quantum limit. Under realistic conditions, the performance of both schemes can be improved significantly by reading out the phase difference via a quantum nondemolition (QND) measurement. Finally, we discuss a scheme using macroscopically entangled ensembles. Comment: 10 pages, 5 figures; eq. (3) corrected and other minor changes 
Conference Paper: CavityQEDbased quantum phase gates for a single longitudinal mode
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ABSTRACT: A three level atom and a longitudinal mode of a cavity can implement twoqubit quantum phase gates. The qubits are represented by the zeroand onephoton states of the two circular polarization states of the field. 
Article: Quantum Information Processing
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ABSTRACT: IntroductionSeparability of composite quantum systems The separability problemResults on the separability problemThe distillability problem Results on the distillability problemWitness operators for the detection of entanglement Definition and geometrical interpretation of witness operatorsResults on witness operatorsQuantum correlations in systems of fermionic and bosonic states What is different with indistinguishable particles?Results on quantum correlations for indistinguishable particlesImplementation of an entangling gate with bosonsSummaryReferences The separability problemResults on the separability problem Results on the distillability problem Definition and geometrical interpretation of witness operatorsResults on witness operators What is different with indistinguishable particles?Results on quantum correlations for indistinguishable particlesImplementation of an entangling gate with bosons
Publication Stats
810  Citations  
76.14  Total Impact Points  
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Institutions

20062009

Autonomous University of Barcelona
 Department of Physics
Cerdanyola del Vallès, Catalonia, Spain
