In the last 30 days

• Open/close Probing the unitarity limit at low laser intensities

  Philippe Pellegrini and Robin Côté 2009 New J. Phys. 11 055047 Tag this article

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  We show how photoassociation (PA) near a magnetically tuned Feshbach resonance can probe the unitarity limit at low laser intensities. This limit occurs when the scattering matrix element in the PA process reaches its quantum limit of one, and is usually difficult to access, requiring large laser intensities. We show how giant enhancements of the PA rate in the vicinity of a Feshbach resonance can reach it, and we predict the appearance of new features with varying magnetic fields and laser intensities. We investigate these effects for a Bose gas of ⁷Li atoms at temperatures near quantum degeneracy and give a simple analytical model. We discuss potential applications of our results, obtained without adjustable parameters, to high-precision spectroscopy.

• Open/close Vortex knots in light

  J Leach et al 2005 New J. Phys. 7 55 Tag this article

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  Optical vortices generically arise when optical beams are combined. Recently, we reported how several laser beams containing optical vortices could be combined to form optical vortex loops, links and knots embedded in a light beam (Leach et al 2004 Nature 432 165). Here, we describe in detail the experiments in which vortex loops form these structures. The experimental construction follows a theoretical model originally proposed by Berry and Dennis, and the beams are synthesized using a programmable spatial light modulator and imaged using a CCD camera.

• Open/close Effect of 3d transition metal doping on the superconductivity in quaternary fluoroarsenide CaFeAsF

  Satoru Matsuishi et al 2009 New J. Phys. 11 025012 Tag this article

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  We examined the doping effect of 3d transition metal (TM) elements (Cr, Mn, Co, Ni and Cu) at the Fe site of a quaternary fluoroarsenide CaFeAsF, an analogue of 1111-type parent compound LaFeAsO. The anomaly at ~120 K observed in resistivity (ρ) versus temperature ( T) plot for the parent compound is suppressed by the doping of each TM element. Furthermore, Co and Ni doping (CaFe _{1− x}TM _x AsF, TM = Co,Ni) induces superconductivity with a transition temperature maximized at the nominal x = 0.10 for Co (22 K) and at x= 0.05 for Ni (12 K). These optimal doping levels may be understood by considering that Ni ²⁺(3d ⁸) adds double electrons to the FeAs layers compared with Co ²⁺ (3d ⁷). Increased x for Co or Ni breaks the superconductivity, while metallic nature dρ/d T > 0 is still kept. These observations indicate that Co and Ni serve as electron donors. In contrast, Cr, Mn and Cu doping does not induce superconductivity, yielding dρ/d T < 0 below the ρ– T anomaly temperature, indicating that these TM ions act as scattering centers. The two different types of behavior of TM replacing the Fe site are discussed in relation to the changes in the lattice constants with doping.

• Open/close Topological insulators and superconductors: tenfold way and dimensional hierarchy

  Shinsei Ryu et al 2010 New J. Phys. 12 065010 Tag this article

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  It has recently been shown that in every spatial dimension there exist precisely five distinct classes of topological insulators or superconductors. Within a given class, the different topological sectors can be distinguished, depending on the case, by a or a topological invariant. This is an exhaustive classification. Here we construct representatives of topological insulators and superconductors for all five classes and in arbitrary spatial dimension d, in terms of Dirac Hamiltonians. Using these representatives we demonstrate how topological insulators (superconductors) in different dimensions and different classes can be related via 'dimensional reduction' by compactifying one or more spatial dimensions (in 'Kaluza–Klein'-like fashion). For -topological insulators (superconductors) this proceeds by descending by one dimension at a time into a different class. The -topological insulators (superconductors), on the other hand, are shown to be lower-dimensional descendants of parent -topological insulators in the same class, from which they inherit their topological properties. The eightfold periodicity in dimension d that exists for topological insulators (superconductors) with Hamiltonians satisfying at least one reality condition (arising from time-reversal or charge-conjugation/particle–hole symmetries) is a reflection of the eightfold periodicity of the spinor representations of the orthogonal groups SO( N) (a form of Bott periodicity). Furthermore, we derive for general spatial dimensions a relation between the topological invariant that characterizes topological insulators and superconductors with chiral symmetry (i.e., the winding number) and the Chern–Simons invariant. For lower-dimensional cases, this formula relates the winding number to the electric polarization ( d=1 spatial dimensions) or to the magnetoelectric polarizability ( d=3 spatial dimensions). Finally, we also discuss topological field theories describing the spacetime theory of linear responses in topological insulators (superconductors) and study how the presence of inversion symmetry modifies the classification of topological insulators (superconductors).

• Open/close High rate, long-distance quantum key distribution over 250 km of ultra low loss fibres

  D Stucki et al 2009 New J. Phys. 11 075003 Tag this article

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  We present a fully automated quantum key distribution prototype running at 625 MHz clock rate. Taking advantage of ultra low loss (ULL) fibres and low-noise superconducting detectors, we can distribute 6000 secret bits s ⁻¹ over 100 km and 15 bits s ⁻¹ over 250 km.

• Open/close The resource theory of quantum reference frames: manipulations and monotones

  Gilad Gour and Robert W Spekkens 2008 New J. Phys. 10 033023 Tag this article

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  Every restriction on quantum operations defines a resource theory, determining how quantum states that cannot be prepared under the restriction may be manipulated and used to circumvent the restriction. A superselection rule (SSR) is a restriction that arises through the lack of a classical reference frame and the states that circumvent it (the resource) are quantum reference frames. We consider the resource theories that arise from three types of SSRs, associated respectively with lacking: (i) a phase reference, (ii) a frame for chirality, and (iii) a frame for spatial orientation. Focusing on pure unipartite quantum states (and in some cases restricting our attention even further to subsets of these), we explore single-copy and asymptotic manipulations. In particular, we identify the necessary and sufficient conditions for a deterministic transformation between two resource states to be possible and, when these conditions are not met, the maximum probability with which the transformation can be achieved. We also determine when a particular transformation can be achieved reversibly in the limit of arbitrarily many copies and find the maximum rate of conversion. A comparison of the three resource theories demonstrates that the extent to which resources can be interconverted decreases as the strength of the restriction increases. Along the way, we introduce several measures of frameness and prove that these are monotonically non-increasing under various classes of operations that are permitted by the SSR.

• Open/close Flexible metamaterials at visible wavelengths

  Andrea Di Falco et al 2010 New J. Phys. 12 113006 Tag this article

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  We report on the fabrication and characterization of plasmonic structures on flexible substrates (Metaflex) and demonstrate the optical properties of a single layer of Metaflex. The layer exhibits a plasmonic resonance in the visible region around 620 nm. We show experimental and numerical results for both nano-antennas and fishnet geometries. We anticipate the use of Metaflex as a building block for flexible metamaterials in the visible range.

• Open/close Characteristics of integrated magneto-optical traps for atom chips

  S Pollock et al 2011 New J. Phys. 13 043029 Tag this article

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  We investigate the operation of pyramidal magneto-optical traps (MOTs) microfabricated in silicon. Measurements of the loading and loss rates give insights into the role of the nearby surface in the MOT dynamics. Studies of the fluorescence versus laser frequency and intensity allow us to develop a simple theory of operation. The number of ⁸⁵Rb atoms trapped in the pyramid is approximately L ⁶, where L 6 is the size of the pyramid opening in mm. This follows quite naturally from the relation between capture velocity and size and differs from the L ^3.6 often used for describing larger MOTs. Our results represent substantial progress towards fully integrated atomic physics experiments and devices.

• Open/close Zipf's law unzipped

  Seung Ki Baek et al 2011 New J. Phys. 13 043004 Tag this article

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  Why does Zipf's law give a good description of data from seemingly completely unrelated phenomena? Here it is argued that the reason is that they can all be described as outcomes of a ubiquitous random group division: the elements can be citizens of a country and the groups family names, or the elements can be all the words making up a novel and the groups the unique words, or the elements could be inhabitants and the groups the cities in a country and so on. A random group formation (RGF) is presented from which a Bayesian estimate is obtained based on minimal information: it provides the best prediction for the number of groups with k elements, given the total number of elements, groups and the number of elements in the largest group. For each specification of these three values, the RGF predicts a unique group distribution N( k)∝exp(− bk)/ k ^γ, where the power-law index γ is a unique function of the same three values. The universality of the result is made possible by the fact that no system-specific assumptions are made about the mechanism responsible for the group division. The direct relation between γ and the total number of elements, groups and the number of elements in the largest group is calculated. The predictive power of the RGF model is demonstrated by direct comparison with data from a variety of systems. It is shown that γ usually takes values in the interval 1≤γ≤2 and that the value for a given phenomenon depends in a systematic way on the total size of the dataset. The results are put in the context of earlier discussions on Zipf's and Gibrat's laws, N( k)∝ k ⁻² and the connection between growth models and RGF is elucidated.

• Open/close Can intrinsic noise induce various resonant peaks?

  J J Torres et al 2011 New J. Phys. 13 053014 Tag this article

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  We theoretically describe how weak signals may be efficiently transmitted throughout more than one frequency range in noisy excitable media by a sort of stochastic multiresonance. This helps us to reinterpret recent experiments in neuroscience and to suggest that many other systems in nature might be able to exhibit several resonances. In fact, the observed behavior happens in our model as a result of competition between (i) changes in the transmitted signals as if the units were varying their activation threshold, and (ii) adaptive noise realized in the model as rapid activity-dependent fluctuations of the connection intensities. These two conditions are indeed known to characterize heterogeneously networked systems of excitable units, e.g. sets of neurons and synapses in the brain. Our results may find application also in the design of detector devices.