Bose-Einstein Condensation on the Surface of a Sphere

Motivated by the recent achievement of space-based Bose-Einstein condensates (BEC) with ultracold alkali-metal atoms under microgravity and by the proposal of bubble traps which confine atoms on a thin shell, we investigate the BEC thermodynamics on the surface of a sphere. We determine analytically the critical temperature and the condensate fraction of a noninteracting Bose gas. Then we consider the inclusion of a zero-range interatomic potential, extending the noninteracting results at zero and finite temperature. Both in the noninteracting and interacting cases the crucial role of the finite radius of the sphere is emphasized, showing that in the limit of infinite radius one recovers the familiar two-dimensional results. We also investigate the Berezinski-Kosterlitz-Thouless transition driven by vortical configurations on the surface of the sphere, analyzing the interplay of condensation and superfluidity in this finite-size system.Comment: 6 pages, 2 figure

Quantum solitons in spin-orbit-coupled Bose-Bose mixtures

Recent experimental and theoretical results show that weakly interacting atomic Bose-Bose mixtures with attractive interspecies interaction are stabilized by beyond-mean-field effects. Here we consider the peculiar properties of these systems in a strictly one-dimensional configuration, taking also into account the nontrivial role of spin-orbit and Rabi couplings. We show that when the value of inter- and intraspecies interaction strengths are such that mean-field contributions to the energy cancel, a self-bound bright soliton fully governed by quantum fluctuations exists. We derive the phase diagram of the phase transition between a single-peak soliton and a multipeak (striped) soliton, produced by the interplay between spin-orbit, Rabi couplings and beyond-mean-field effects, which also affect the breathing mode frequency of the atomic cloud. Finally, we prove that a phase imprinting of the single-peak soliton leads to a self-confined propagating solitary wave even in the presence of spin-orbit coupling.Comment: 6 pages, 4 figures, published in Phys. Rev.

Condensation and superfluidity of dilute Bose gases with finite-range interaction

We investigate an ultracold and dilute Bose gas by taking into account a finite-range two-body interaction. The coupling constants of the resulting Lagrangian density are related to measurable scattering parameters by following the effective-field-theory approach. A perturbative scheme is then developed up to the Gaussian level, where both quantum and thermal fluctuations are crucially affected by finite-range corrections. In particular, the relation between spontaneous symmetry breaking and the onset of superfluidity is emphasized by recovering the renowned Landau's equation for the superfluid density in terms of the condensate one.Comment: 18 pages, 4 figures, invited contribution to New Journal of Physics Focus Issue on Quantum Transport in Ultracold Atom

The use of information theory in evolutionary biology

Information is a key concept in evolutionary biology. Information is stored in biological organism's genomes, and used to generate the organism as well as to maintain and control it. Information is also "that which evolves". When a population adapts to a local environment, information about this environment is fixed in a representative genome. However, when an environment changes, information can be lost. At the same time, information is processed by animal brains to survive in complex environments, and the capacity for information processing also evolves. Here I review applications of information theory to the evolution of proteins as well as to the evolution of information processing in simulated agents that adapt to perform a complex task.Comment: 25 pages, 7 figures. To appear in "The Year in Evolutionary Biology", of the Annals of the NY Academy of Science

What caused what? A quantitative account of actual causation using dynamical causal networks

Actual causation is concerned with the question "what caused what?" Consider a transition between two states within a system of interacting elements, such as an artificial neural network, or a biological brain circuit. Which combination of synapses caused the neuron to fire? Which image features caused the classifier to misinterpret the picture? Even detailed knowledge of the system's causal network, its elements, their states, connectivity, and dynamics does not automatically provide a straightforward answer to the "what caused what?" question. Counterfactual accounts of actual causation based on graphical models, paired with system interventions, have demonstrated initial success in addressing specific problem cases in line with intuitive causal judgments. Here, we start from a set of basic requirements for causation (realization, composition, information, integration, and exclusion) and develop a rigorous, quantitative account of actual causation that is generally applicable to discrete dynamical systems. We present a formal framework to evaluate these causal requirements that is based on system interventions and partitions, and considers all counterfactuals of a state transition. This framework is used to provide a complete causal account of the transition by identifying and quantifying the strength of all actual causes and effects linking the two consecutive system states. Finally, we examine several exemplary cases and paradoxes of causation and show that they can be illuminated by the proposed framework for quantifying actual causation.Comment: 43 pages, 16 figures, supplementary discussion, supplementary methods, supplementary proof

L’Oulipo ou la Nouvelle Vague du Classicisme

The Oulipo establishes a new relationship with Antiquity, one which submits to a reevaluation and not to a process of destruction: by means of an analytic tendency, the Oulipo relives forgotten works or rereads the literature so dear to Classical tradition. With regards to its relationship with contemporary avant-garde, the Oulipo, which upholds the idea of a voluntary literature, opposes Surrealism, but shares with Tel Quel the playful notion of literature and the intent of reevaluating text as a practice founded on an handcrafted perception of a writer’s creation. The Oulipo’s answer to Nineteenthcentury Realism’s desire to break from narrative conventions animating Tel Quel and the Nouveau Roman is replaced by a new approach of perceived reality in its numerous and continuous transformations.El Oulipo establece una nueva relación con los Antiguos, que no somete a un método destructivo sino a una revalorización: por medio de la tendencia analítica, el Oulipo vuelve a proponer las obras olvidadas o a releer las obras ya consagradas por la tradición. Con respecto a las vanguardias contemporáneas, el Oulipo, que sostiene la noción de literatura voluntaria, se opone al Surrealismo. Sin embargo, comparte con Tel Quel el concepto de literatura lúdica y también el mismo propósito de una revalorización de la escritura como práctica que se basa en el carácter artesanal del trabajo del escritor. Al deseo de ruptura de las convenciones narrativas del realismo del siglo XIX, que anima Tel Quel y el Nouveau Roman, y que propone sólo una reforma de los mecanismos narrativos, el Oulipo opone una nueva aproximación a la realidad, percibida como continua transformación.L’Oulipo établit un rapport nouveau avec les Anciens qu’il ne soumet pas à une dynamique destructive mais à une réévaluation : par son orientation analytique l’Oulipo repropose les oeuvres oubliées et relit les oeuvres déjà consacrées par la tradition. En ce qui concerne son rapport aux avant-gardes contemporaines, l’Oulipo, qui soutient l’idée d’une littérature volontaire, s’oppose au Surréalisme mais partage avec Tel Quel la conception ludique de la littérature et le propos d’une même réévaluation de l’écriture en tant que pratique fondée sur une conception artisanale du travail de l’écrivain. Au désir de rupture des conventions narratives du réalisme du XIXe siècle qui anime Tel Quel et le Nouveau Roman et qui ne propose qu’une innovation des procédés narratifs, l’Oulipo substitue une nouvelle approche du réel perçu dans ses transformations nombreuses et continues

G\ue9n\ue9tique des textes et syst\ue8me chaotique

Textual genetics and chaotic system Although we share Louis Hay's idea that it is impossible to homologate and systematize the operations which can be observed in manuscripts, it is however possible to interpret whatever creation process in terms of the passage from entropy to order. The possibility of an analogy between text and chaotic system had been advanced by No\ueblle Batt according to whom each work actually satisfies the principles of the chaotic system. It is starting from such principles that we will try to show the analogy between chaotic system and textual genetics: 1. The fact that we are dealing with an evolving system (with variables and permanent features) endowed with a temporal dynamics. 2. The paradoxical association of determinism and unpredictability. 3. The alternation of ordered and disordered sentences. 4. The sensitivity to initial inner situations. 5. Strange attractor.Tout en partageant l\u2019impossibilit\ue9 remarqu\ue9e par Louis Hay d\u2019une syst\ue9matisation et d\u2019une homologation des op\ue9rations qui s\u2019\ue9talent dans les manuscrits, il faut toutefois interpr\ue9ter tout processus de cr\ue9ation en tant que transition de l\u2019entropie \ue0 l\u2019ordre. La possibilit\ue9 d\u2019une analogie entre le texte et le syst\ue8me chaotique a \ue9t\ue9 d\ue9j\ue0 soutenue par No\ueblle Batt selon laquelle toute \u153uvre satisfait les facteurs du syst\ue8me chaotique. C\u2019est \ue0 partir de ces principes que l\u2019on d\ue9montrera l\u2019analogie entre syst\ue8me chaotique et g\ue9n\ue9tique des textes : 1. Le fait d\u2019\ueatre un syst\ue8me \ue9volutif (avec variables et constantes) dou\ue9 d\u2019une dynamique temporelle. 2. L\u2019association paradoxale de d\ue9terminisme et d\u2019impr\ue9visibilit\ue9. 3. L\u2019alternance de phases ordonn\ue9es et de phases d\ue9sordonn\ue9es. 4. La sensibilit\ue9 aux conditions initiales internes. 5. Le fait d\u2019avoir pour attracteur un attracteur \ue9trange qui est une courbe fractale

When is an action caused from within? Quantifying the causal chain leading to actions in simulated agents

An agent's actions can be influenced by external factors through the inputs it receives from the environment, as well as internal factors, such as memories or intrinsic preferences. The extent to which an agent's actions are "caused from within", as opposed to being externally driven, should depend on its sensor capacity as well as environmental demands for memory and context-dependent behavior. Here, we test this hypothesis using simulated agents ("animats"), equipped with small adaptive Markov Brains (MB) that evolve to solve a perceptual-categorization task under conditions varied with regards to the agents' sensor capacity and task difficulty. Using a novel formalism developed to identify and quantify the actual causes of occurrences ("what caused what?") in complex networks, we evaluate the direct causes of the animats' actions. In addition, we extend this framework to trace the causal chain ("causes of causes") leading to an animat's actions back in time, and compare the obtained spatio-temporal causal history across task conditions. We found that measures quantifying the extent to which an animat's actions are caused by internal factors (as opposed to being driven by the environment through its sensors) varied consistently with defining aspects of the task conditions they evolved to thrive in.Comment: Submitted and accepted to Alife 2019 conference. Revised version: edits include adding more references to relevant work and clarifying minor points in response to reviewer

Plasticity in the structure of visual space

Visual space embodies all visual experiences, yet what determines the topographical structure of visual space remains unclear. Here we test a novel theoretical framework that proposes intrinsic lateral connections in the visual cortex as the mechanism underlying the structure of visual space. The framework suggests that the strength of lateral connections between neurons in the visual cortex shapes the experience of spatial relatedness between locations in the visual field. As such, an increase in lateral connection strength shall lead to an increase in perceived relatedness and a contraction in perceived distance. To test this framework through human psychophysics experiments, we used a Hebbian training protocol in which two-point stimuli were flashed in synchrony at separate locations in the visual field, to strengthen the lateral connections between two separate groups of neurons in the visual cortex. After training, participants experienced a contraction in perceived distance. Intriguingly, the perceptual contraction occurred not only between the two training locations that were linked directly by the changed connections, but also between the outward untrained locations that were linked indirectly through the changed connections. Moreover, the effect of training greatly decreased if the two training locations were too close together or too far apart and went beyond the extent of lateral connections. These findings suggest that a local change in the strength of lateral connections is sufficient to alter the topographical structure of visual spac