A kk-polycosymplectic Marsden-Weinstein reduction

This work reviews and slightly improves the known $k$-polysymplectic Marsden-Weinstein reduction theory by removing some technical conditions concerning $k$-polysymplectic momentum maps and the existence of manifold structures. This is mainly accomplished by developing a theory of affine Lie group actions for $k$-polysymplectic momentum maps, which removes the necessity of their co-adjoint equivariance. Then, we focus on the analysis of a particular case of $k$-polysymplectic manifolds, the so-called fibred ones, and we study their $k$-polysymplectic Marsden-Weinstein reductions. Previous results allow us to devise a k-polycosymplectic Marsden-Weinstein reduction theory, which represents one of our main results. Our findings are applied to study coupled vibrating strings and, more generally, $k$-polycosymplectic Hamiltonian systems with field symmetries. As a very relevant byproduct, we show that $k$-polycosymplectic geometry can be understood as a particular type of $k$-polysymplectic geometry.Comment: 49 pages. Revised version. Added a reduction procedure of the space-time coordinate

Probing quantum coherence in qubit arrays

We discuss how the observation of population localization effects in periodically driven systems can be used to quantify the presence of quantum coherence in interacting qubit arrays. Essential for our proposal is the fact that these localization effects persist beyond tight-binding Hamiltonian models. This result is of special practical relevance in those situations where direct system probing using tomographic schemes becomes infeasible beyond a very small number of qubits. As a proof of principle, we study analytically a Hamiltonian system consisting of a chain of superconducting flux qubits under the effect of a periodic driving. We provide extensive numerical support of our results in the simple case of a two-qubits chain. For this system we also study the robustness of the scheme against different types of noise and disorder. We show that localization effects underpinned by quantum coherent interactions should be observable within realistic parameter regimes in chains with a larger number o

To freeze or not to: Quantum correlations under local decoherence

We provide necessary and sufficient conditions for freezing of quantum correlations as measured by quantum discord and quantum work deficit in the case of bipartite as well as multipartite states subjected to local noisy channels. We recognize that inhomogeneity of the magnetizations of the shared quantum states plays an important role in the freezing phenomena. We show that the frozen value of the quantum correlation and the time interval for freezing follow a complementarity relation. For states which do not exhibit "exact" freezing, but can be frozen "effectively", by having a very slow decay rate with suitable tuning of the state parameters, we introduce an index -- the freezing index -- to quantify the goodness of freezing. We find that the freezing index can be used to detect quantum phase transitions and discuss the corresponding scaling behavior.Comment: 14 pages, 9 figures, close to published version, title changed by Phys. Rev. A. to 'Freezing of quantum correlations under local decoherence

Hyperbolic Scar Patterns in Phase Space

We develop a semiclassical approximation for the spectral Wigner and Husimi functions in the neighbourhood of a classically unstable periodic orbit of chaotic two dimensional maps. The prediction of hyperbolic fringes for the Wigner function, asymptotic to the stable and unstable manifolds, is verified computationally for a (linear) cat map, after the theory is adapted to a discrete phase space appropriate to a quantized torus. The characteristic fringe patterns can be distinguished even for quasi-energies where the fixed point is not Bohr-quantized. The corresponding Husimi function dampens these fringes with a Gaussian envelope centered on the periodic point. Even though the hyperbolic structure is then barely perceptible, more periodic points stand out due to the weakened interference.Comment: 12 pages, 10 figures, Submited to Phys. Rev.

RNA denaturation: excluded volume, pseudoknots and transition scenarios

A lattice model of RNA denaturation which fully accounts for the excluded volume effects among nucleotides is proposed. A numerical study shows that interactions forming pseudoknots must be included in order to get a sharp continuous transition. Otherwise a smooth crossover occurs from the swollen linear polymer behavior to highly ramified, almost compact conformations with secondary structures. In the latter scenario, which is appropriate when these structures are much more stable than pseudoknot links, probability distributions for the lengths of both loops and main branches obey scaling with nonclassical exponents.Comment: 4 pages 3 figure

Quantum Rod Emission Coupled to Plasmonic Lattice Resonances: A Collective Directional Source of Polarized Light

We demonstrate that an array of optical antennas may render a thin layer of randomly oriented semiconductor nanocrystals into an enhanced and highly directional source of polarized light. The array sustains collective plasmonic lattice resonances which are in spectral overlap with the emission of the nanocrystals over narrow angular regions. Consequently, different photon energies of visible light are enhanced and beamed into definite directions.Comment: 4 pages, 3 figure

Structural Stability of the PsbQ Protein of Higher Plant Photosystem II

We have characterized the stability and folding behavior of the isolated extrinsic PsbQ protein of photosystem II (PSII) from a higher plant, Spinacia oleracea, using intrinsic protein fluorescence emission and near- and far-UV circular dichroism (CD) spectroscopy in combination with differential scanning calorimetry (DSC). Experimental results reveal that both chemical denaturation using guanidine hydrochloride (GdnHCl) and thermal unfolding of PsbQ proceed as a two-state reversible process. The denaturation free-energy changes (GD) at 20 C extrapolated from GdnHCl (4.0 ± 0.6 kcal mol-1) or thermal unfolding (4.4 ± 0.8 kcal mol-1) are very close. Moreover, the far-UV CD spectra of the denatured PsbQ registered at 90 C in the absence and presence of 6.0 M GdnHCl superimpose, leading us to conclude that both denatured states of PsbQ are structurally and energetically similar. The thermal unfolding of PsbQ has been also characterized by CD and DSC over a wide pH range. The stability of PsbQ is at its maximum at pH comprised between 5 and 8, being wider than the optimal pH for oxygen evolution in the lumen of thylakoid membranes. In addition, no significant structural changes were detected in PsbQ between 50 and 55 C in the pH range of 3-8, suggesting that PsbQ behaves as a soluble and stable particle in the lumen when it detaches from PSII under physiological stress conditions such as high temperature (45-50 C) or low pH (<5.0). Sedimentation experiments showed that, in solution at 20 C, the PsbQ protein is a monomer with an elongated shape.Spanish Minitry of Science and Technology (PB1998-0480 and AGL2004-00045)This work was funded by the Spanish Ministry of Science and Technology (project references PB1998-0480 and AGL2003-0045). M.B. holds a fellowship from the Spanish Ministry of Science and Technology.Peer reviewe

Quantum dynamics in photonic crystals

Employing a recently developed method that is numerically accurate within a model space simulating the real-time dynamics of few-body systems interacting with macroscopic environmental quantum fields, we analyze the full dynamics of an atomic system coupled to a continuum light-field with a gapped spectral density. This is a situation encountered, for example, in the radiation field in a photonic crystal, whose analysis has been so far been confined to limiting cases due to the lack of suitable numerical techniques. We show that both atomic population and coherence dynamics can drastically deviate from the results predicted when using the rotating wave approximation, particularly in the strong coupling regime. Experimental conditions required to observe these corrections are also discussed.Comment: 5 pages, 2 figures Updated with published versio