We investigate the stability and dynamics of the orbital angular momentum modes of a repulsive Bose-Einstein condensate trapped in two tunnel-coupled rings in a stack configuration. Within mean-field theory, we derive a two-state model for the system in the case in which we populate both rings equally with a single orbital angular momentum mode and include small perturbations in other modes. Analyzing the fixed-point solutions of the model and the associated classical Hamiltonian, we characterize the destabilization of the stationary states and the subsequent dynamics. By populating a single orbital angular momentum mode with an arbitrary population imbalance between the rings, we derive analytically the boundary between the regimes of Josephson oscillations and macroscopic quantum self-trapping and study numerically the stability of these solutions.

}, doi = {10.1103/PhysRevA.102.023331}, url = {https://link.aps.org/doi/10.1103/PhysRevA.102.023331}, author = {N. Eul{\`a}lia and J. Mompart and B. Juli{\'a}-D{\'\i}az and V. Ahufinger} } @booklet {doi:10.1117/12.2184788, title = {Snapshot polarimeter based on the conical refraction phenomenon}, howpublished = {Proc. SPIE}, volume = {9526}, year = {2015}, pages = {952616-952616-8}, abstract = {A complete and punctual Stokes polarimeter based on the conical refraction (CR) phenomenon is presented. The CR phenomenon occurs when light travels along one of the optical axes of a biaxial crystal (BC), leading to a bright ring of light at the focal plane of the system. We propose using the connection between the intensity pattern of the CR ring and the state of polarization (SOP) of the incident beam as a new tool for polarization metrology. In order to implement a complete polarimeter, the instrument is designed with a beam splitter and two BCs, one BC for each sub-beam. In the second sub-beam, a retarder is introduced before the BC, allowing us to measure the ellipticity content of the input SOP. The CR-based polarimeter presents several appealing features compared to standard polarimeters. To name some of them, CR polarimeters retrieve the SOP of an input beam with a single snapshot measurement, allow for substantially enhancing the data redundancy without increasing measuring time, and avoid instrumental errors related to rotating elements or active polarization devices. This work shows the instrument design, in particular the parameters of the set-up have been optimized in order to reduce the amplification of noise. Then, the experimental implementation of the instrument is detailed, including the experimental calibration of the system. Finally, the implemented polarimeter is experimentally tested by measuring different SOPs, including fully and partially polarized light.

}, doi = {10.1117/12.2184788}, url = {http://dx.doi.org/10.1117/12.2184788}, author = {A. Peinado and A. Lizana and A. Turpin and I. Est{\'e}vez and C. Iemmi and T. K. Kalkandjiev and J. Mompart and J. Campos} } @article {Turpin:15, title = {Transformation of vector beams with radial and azimuthal polarizations in biaxial crystals}, journal = {J. Opt. Soc. Am. A}, volume = {32}, number = {5}, year = {2015}, month = {May}, pages = {1012{\textendash}1016}, publisher = {OSA}, abstract = {We present both experimentally and theoretically the transformation of radially and azimuthally polarized vector beams when they propagate through a biaxial crystal and are transformed by the conical refraction phenomenon. We show that, at the focal plane, the transverse pattern is formed by a ring-like light structure with an azimuthal node, this node being found at diametrically opposite points of the ring for radial/azimuthal polarizations. We also prove that the state of polarization of the transformed beams is conical refraction-like, i.e., that every two diametrically opposite points of the light ring are linearly orthogonally polarized.

}, keywords = {Birefringence, Crystal optics, Polarization}, doi = {10.1364/JOSAA.32.001012}, url = {http://josaa.osa.org/abstract.cfm?URI=josaa-32-5-1012}, author = {A. Turpin and A. Vargas and A. Lizana and F. A. Torres-Ruiz and I. Est{\'e}vez and I. Moreno and J. Campos and J. Mompart} } @article {0953-4075-41-4-045505, title = {Generation of entangled photon pairs in optical cavity-QED: operating in the bad cavity limit}, journal = {Journal of Physics B: Atomic, Molecular and Optical Physics}, volume = {41}, number = {4}, year = {2008}, pages = {045505}, abstract = {We propose an optical cavity-QED 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 four-level atom with two empty optical cavity modes via an adiabatic passage process. Monte Carlo wavefunction simulations are used to evaluate the fidelity of the cavity-QED source and its entanglement capability in the presence of decoherence. In the bad cavity limit, fidelities close to one are predicted for state-of-the-art experimental parameter values.

}, url = {http://stacks.iop.org/0953-4075/41/i=4/a=045505}, author = {R. Garc{\'\i}a-Maraver and K. Eckert and R. Corbal{\'a}n and J. Mompart} } @article {1742-6596-84-1-012008, title = {Cavity-QED-based entangled photon pair gun}, journal = {Journal of Physics: Conference Series}, volume = {84}, number = {1}, year = {2007}, pages = {012008}, abstract = {We describe a cavity-QED 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 V -type three-level 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 four-level 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 Clauser-Horne-Shimony-Holt (CHSH) inequality to quantify the entanglement capability.

}, url = {http://stacks.iop.org/1742-6596/84/i=1/a=012008}, author = {R. Garc{\'\i}a-Maraver and K. Eckert and R. Corbal{\'a}n and J. Mompart} } @article {Garcia-Maraver:07, title = {Cavity-quantum-electrodynamics entangled photon source based on two truncated Rabi oscillations}, journal = {J. Opt. Soc. Am. B}, volume = {24}, number = {2}, year = {2007}, month = {Feb}, pages = {257{\textendash}265}, publisher = {OSA}, abstract = {We discuss a cavity-QED scheme to deterministically generate entangled photons pairs by using a three-level atom successively coupled to two single longitudinal mode high-Q 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. 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 wave-function simulations have been performed with state-of-the-art parameter values to evaluate the success probability of the cavity-QED source in producing entangled photon pairs as well as its entanglement capability.

}, keywords = {Photon statistics, Quantum electrodynamics}, doi = {10.1364/JOSAB.24.000257}, url = {http://josab.osa.org/abstract.cfm?URI=josab-24-2-257}, author = {R. Garc{\'\i}a-Maraver and K. Eckert and R. Corbal{\'a}n and J. Mompart} } @article {PhysRevA.74.031801, title = {Deterministic cavity-QED source of polarization-entangled photon pairs}, journal = {Phys. Rev. A}, volume = {74}, year = {2006}, month = {Sep}, pages = {031801}, publisher = {American Physical Society}, abstract = {We present two cavity quantum electrodynamics proposals that, sharing the same basic elements, allow for the deterministic generation of entangled photon pairs by means of a three-level atom successively coupled to two single longitudinal mode high-Q optical resonators presenting polarization degeneracy. In the faster proposal, the three-level 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.

}, doi = {10.1103/PhysRevA.74.031801}, url = {http://link.aps.org/doi/10.1103/PhysRevA.74.031801}, author = {R. Garc{\'\i}a-Maraver and K. Eckert and R. Corbal{\'a}n and J. Mompart} } @article {Eckert2006264, title = {Three level atom optics in dipole traps and waveguides}, journal = {Optics Communications}, volume = {264}, number = {2}, year = {2006}, note = {Quantum Control of Light and Matter In honor of the 70th birthday of Bruce Shore}, pages = {264 - 270}, abstract = {An analogy is explored between a setup of three atomic traps coupled via tunneling and an internal atomic three-level 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.}, issn = {0030-4018}, doi = {http://dx.doi.org/10.1016/j.optcom.2006.02.056}, url = {http://www.sciencedirect.com/science/article/pii/S003040180600486X}, author = {K. Eckert and J. Mompart and R. Corbal{\'a}n and M. Lewenstein and G. Birkl} } @inbook { ISI:000304603600009, title = {Entanglement Properties of Composite Quantum Systems}, booktitle = {Quantum Information Processing}, year = {2005}, pages = {83-99}, publisher = {BLACKWELL SCIENCE PUBL}, organization = {BLACKWELL SCIENCE PUBL}, type = {{Article; Book Chapter}}, address = {OSNEY MEAD, OXFORD OX2 0EL, ENGLAND}, 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 if 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.}, isbn = {978-3-52760-600-9}, doi = {10.1002/3527606009.ch7}, author = {K. Eckert and O. Guehne and F. Hulpke and P. Hyllus and J. Korbicz and J. Mompart and D. Bruss and M. Lewenstein and A. Sanpera}, editor = {T. Beth and G. Leuchs} } @article {PhysRevA.72.012327, title = {One- and two-dimensional quantum walks in arrays of optical traps}, journal = {Phys. Rev. A}, volume = {72}, year = {2005}, month = {Jul}, pages = {012327}, publisher = {American Physical Society}, abstract = {We propose a different implementation of discrete-time quantum walks for a neutral atom in an array of optical microtraps or an optical lattice. We analyze a one-dimensional walk in position space, with the coin, the additional qubit degree of freedom that controls the displacement of the quantum walker, implemented as a spatially delocalized qubit, i.e., the coin is also encoded in position space. We analyze the dependence of the quantum walk on temperature and experimental imperfections such as shaking in the trap positions. Finally, combining a spatially delocalized qubit and a hyperfine qubit, we also give a scheme to realize a quantum walk on a two-dimensional square lattice with the possibility of implementing different coin operators.}, doi = {10.1103/PhysRevA.72.012327}, url = {http://link.aps.org/doi/10.1103/PhysRevA.72.012327}, author = {K. Eckert and J. Mompart and G. Birkl and M. Lewenstein} } @article {PhysRevA.70.062324, title = {Cavity QED quantum phase gates for a single longitudinal mode of the intracavity field}, journal = {Phys. Rev. A}, volume = {70}, year = {2004}, month = {Dec}, pages = {062324}, publisher = {American Physical Society}, abstract = {A single three-level atom driven by a longitudinal mode of a high-Q cavity is used to implement two-qubit quantum phase gates for the intracavity field. The two qubits are associated with the zero- and one-photon Fock states of each of the two opposite circular polarization states of the field. The three-level atom mediates the conditional phase gate provided the two polarization states and the atom interact in a V-type configuration and the two-photon resonance condition is satisfied. Microwave and optical implementations are discussed with gate fidelities being evaluated against several decoherence mechanisms such as atomic velocity fluctuations or the presence of a weak magnetic field. The use of coherent states for both polarization states is investigated to assess the entanglement capability of the proposed quantum gates.

}, doi = {10.1103/PhysRevA.70.062324}, url = {http://link.aps.org/doi/10.1103/PhysRevA.70.062324}, author = {R. Garc{\'\i}a-Maraver and R. Corbal{\'a}n and K. Eckert and S. Rebic and M. Artoni and J. Mompart} } @article {PhysRevA.70.023606, title = {Three-level atom optics via the tunneling interaction}, journal = {Phys. Rev. A}, volume = {70}, year = {2004}, month = {Aug}, pages = {023606}, publisher = {American Physical Society}, abstract = {Three-level atom optics is introduced as a simple, efficient, and robust method to coherently manipulate and transport neutral atoms. The tunneling interaction among three trapped states allows us to realize the spatial analog of the stimulated Raman adiabatic passage, coherent population trapping, and electromagnetically induced transparency techniques and offers a wide range of possible applications. We investigate an implementation in optical microtrap arrays and show that under realistic parameters the coherent manipulation and transfer of neutral atoms among dipole traps could be realized in the millisecond range.}, doi = {10.1103/PhysRevA.70.023606}, url = {http://link.aps.org/doi/10.1103/PhysRevA.70.023606}, author = {K. Eckert and M. Lewenstein and R. Corbal{\'a}n and G. Birkl and W. Ertmer and J. Mompart} } @article {PhysRevLett.90.147901, title = {Quantum Computing with Spatially Delocalized Qubits}, journal = {Phys. Rev. Lett.}, volume = {90}, year = {2003}, month = {Apr}, pages = {147901}, publisher = {American Physical Society}, abstract = {We analyze the operation of quantum gates for neutral atoms with qubits that are delocalized in space, i.e., the computational basis states are defined by the presence of a neutral atom in the ground state of one out of two trapping potentials. The implementation of single-qubit gates as well as a controlled phase gate between two qubits is discussed and explicit calculations are presented for rubidium atoms in optical microtraps. Furthermore, we show how multiqubit highly entangled states can be created in this scheme.}, doi = {10.1103/PhysRevLett.90.147901}, url = {http://link.aps.org/doi/10.1103/PhysRevLett.90.147901}, author = {J. Mompart and K. Eckert and W. Ertmer and G. Birkl and M. Lewenstein} } @article {PhysRevA.66.042317, title = {Quantum computing in optical microtraps based on the motional states of neutral atoms}, journal = {Phys. Rev. A}, volume = {66}, year = {2002}, month = {Oct}, pages = {042317}, publisher = {American Physical Society}, abstract = {We investigate quantum computation with neutral atoms in optical microtraps where the qubit is implemented in the motional states of the atoms, i.e., in the two lowest vibrational states of each trap. The quantum gate operation is performed by adiabatically approaching two traps and allowing tunneling and cold collisions to take place. We demonstrate the capability of this scheme to realize a square root of swap gate, and address the problem of double occupation and excitation to other unwanted states. We expand the two-particle wave function in an orthonormal basis and analyze quantum correlations throughout the whole gate process. Fidelity of the gate operation is evaluated as a function of the degree of adiabaticity in moving the traps. Simulations are based on rubidium atoms in state-of-the-art optical microtraps with quantum gate realizations in the few tens of milliseconds duration range.}, doi = {10.1103/PhysRevA.66.042317}, url = {http://link.aps.org/doi/10.1103/PhysRevA.66.042317}, author = {K. Eckert and J. Mompart and X. X. Yi and J. Schliemann and D. Bruss and G. Birkl and M. Lewenstein} }