Universal nonequilibrium signatures of Majorana zero modes in quench dynamics

The quantum evolution after a metallic lead is suddenly connected to an electron system contains information about the excitation spectrum of the combined system. We exploit this type of "quantum quench" to probe the presence of Majorana fermions at the ends of a topological superconducting wire. We obtain an algebraically decaying overlap (Loschmidt echo) ${\cal L}(t)=| < \psi(0) | \psi(t) > |^2\sim t^{-\alpha}$ for large times after the quench, with a universal critical exponent $\alpha$=1/4 that is found to be remarkably robust against details of the setup, such as interactions in the normal lead, the existence of additional lead channels or the presence of bound levels between the lead and the superconductor. As in recent quantum dot experiments, this exponent could be measured by optical absorption, offering a new signature of Majorana zero modes that is distinct from interferometry and tunneling spectroscopy.Comment: 9 pages + appendices, 4 figures. v3: published versio

Biocapteurs pour le contrôle de la toxicité des eaux : application des bioélectrodes algales

L'amélioration de la qualité de l'environnement passe par la réalisation de contrôles de toxicité in situ et en continu des sources de pollution ou des milieux contaminés, à l'aide de systèmes automatisés à réponse rapide. Les systèmes donnant une réponse en temps réel permettent d'intervenir immédiatement à la source, d'interrompre le rejet d'un flux toxique et de prévenir ainsi les accidents de pollution. Ce type de stratégie ne peut être développé qu'au moyen de biocapteurs : les méthodes d'essais conventionnelles n'autorisent que des contrôles de toxicité épisodiques, en laboratoire, effectués dans des conditions statiques quelque peu éloignées des conditions dynamiques.Nous nous sommes intéressés à la mesure de l'activité photosynthétique d'algues unicellulaires immobilisées. La photosynthèse induite par des stimuli lumineux est en effet un processus dont la réponse est immédiate et aisément mesurable à l'aide de transducteurs électrochimiques. Il apparaît donc intéressant d'utiliser ces réactions photosynthétiques pour la détection des polluants.Deux dispositifs mesurant l'activité photosynthétique d'algues unicellulaires ont été testés. Le premier dispositif mesure le transfert d'électrons le long de la chaîne photosynthétique lors d'une illumination des micro-organismes. Le second système permet de quantifier la production d'oxygène résultant de cette excitation lumineuse.La mesure du transfert d'électrons photosynthétiques nécessite l'addition d'une substance oxydo-réductible (médiateur) dans le milieu pour capter ces électrons. De la série de médiateurs testés, seuls les dérivés à caractère lipophile (2,6-diméthylbenzoquinone et p-benzoquinone) ont permis de mesurer un transfert d'électrons. Toutefois la durée de vie de ce biocapteur s'est révélée limitée à moins de 24 heures, ce qui exclut toute utilisation en continu.Le second dispositif développé présente en revanche une longévité d'une semaine, ce qui le rend intéressant en vue d'une utilisation in situ. Les performances de ce capteur à oxygène ont été testées sur des produits de type herbicides, cyanures, métaux et comparées aux valeurs obtenues à l'aide de tests algues classiques ou de méthodes de détection rapide de la toxicité.Environmental monitoring of pollutants with automatic systems, applied on-line and allowing rapid response constitutes one of the most successful ways to improve the quality of the environment. Real time analysis offers the advantage of detecting rapidly sources of pollution and preventing any accidental release of pollutants. Such a strategy is possible only by means of biosensors : current methods, commonly used far toxicity testing are usually carried out in Laboratory in static conditions, making real lime analysis impractical.Two types of amperometric environmental sensor incorporating eukaryotic algae were investigated for use in monitoring industrial pollution of aquatic systems. Both sensors allowed the monitoring of photosynthetic events.The first sensor follows photosynthetic electron chain events within the cell resulting in the reduction of mediator acting as terminal electron accepter. Reoxidation of the mediator at the biosensor electrode surface rues in a flow of current, the magnitude of which is proportional to the level of photosynthetic activity of the microalgae.In the second approach photosynthetic oxygen evolution by the illuminated biocatalyst is measured by reduction at a cathodic electrode. Enzymic systems associated with the water splitting and oxygen evolution are amongst the most fragile components of the photosynthetic apparatus, and the monitoring of algal oxygen production is therefore a useful approach to early detection of toxic environmental pollutants.Several species of unicellular algae were used for these experiments : Chlorella vulgaris, Scenedesmus subspicatsus and Selenastrum capricomutum. Algal cultures were harvested in the exponential growth phase and diluted to 0.5 O.D. (655 nm); then 1 ml aliquots were centrifuged at 900xg for 3 min. After centrifugation, cells were resuspended in growth medium, LEFEBVRE and CZARDA (LC), and immobilized by aspiration onto a filter disc. This filter disc was placed onto the carbon working electrode surface. Filters were held in place by a fine nylon mesh.This biosensor is a two electrode system comprising a carbon working electrode and Ag/AgCl reference/counter electrode. Solution was continuously flowed through the electrochemical cell at a flow rate of 2 ml min-1. Illumination of the algal biocatalyst was supplied by light emitting diodes with a peak wavelength of 635 nm and a light intensity of 125 millicandellas. Periodicity of illumination was chosen in order to obtain a stable photosynthetic response.Biosensors exploiting direct electron transfer from a biocatalyst to an electrical system are not feasible. Indeed, the tell wall of the biocatalyst act as a barrier to the exchange of electrons between the electrode and the redox intermediates oft the cell. Electroactive compounds (mediators) must be used to shuttle electrons from the photosynthetic electron transfer chain to the electrode. Mediators were added to the flowing solution of LC medium, and a potential of 550 mV applied at the working electrode to reoxidize mediator reduced by the biocatalyst. The mediator must be lipophilic to access the chloroplast electron transport chain of eukaryotic algae. We tested a wide range of mediators but only p-benzoquinone (p-BQ) and 2,6-dimethylbenzoquinone gave measurable responses.A concentration of 0.2 mM p-BQ (21.5 mg/l) was employed to measure photosynthetic activity. Experiments showed that 15 minutes light period followed by a 15 minutes dark period gave a steady photosynthetic response. However, this high concentration of mediator was toxic for the cells. Static algal tests using Chlorella vulgaris have shown that growth is totally inhibited after 72 hours at a concentration of 5.4 mg/l. The working life of this sensor was therefore very short, less than 24 hours : after 16 hours of continuous monitoring, the recorded photosynthetic current was less than 20 % of initial response. Sensor life was not increased when the probe was used alternately with recovery periods in nutrient medium (4 hours of working period/4 hours of recovery period).The same apparatus was used for the oxygen electrode based biosensor. The working electrode was coated with a Teflon gas permeable membrane to protect the sensor against poisoning by electrochemically active compounds. Separation of the working and reference/counter electrode requires addition of electrolyte in the flowing solution. With such a semi-protected oxygen electrode, mass transport controlled oxygen reduction currents were obtained when the Teflon covered cathode was poised at -700 mV.The oxygen biosensor responded more rapidly than the mediated biosensor to changes in the light regime, and alternating light and dark periods of 1 min of light followed by 4 min of dark were used. The sensor also showed good long term stability, with a working life of up to seven days using Chlorella vulgaris or Scenedesmus subspicalus as biocatalysts.The sensitivity of this oxygen electrode based biosensor was tested on herbicides (isoproturon, propanil and atrazine), cyanide and heavy metals (copper and mercury). Results were compared with chose obtained with three toxicity tests : a standard algal growth inhibition test, the inhibition of photosynthetic activity in spinach leaves and the alga Chlamydomonas reinhardii, and the Microtox test using the luminescent bacterium Photobacterium phosphoreum.IC50 obtained for isoproturon and atrazine were very similar for the growth inhibition and the oxygen sensor tests. The inhibition of oxygen production by spinach leaves was less sensitive to atrazine; no toxic affect could be detected with the Microtox test. The oxygen sensor was also very sensitive to cyanide but the response of the probe was quite different if Selenastrum capricornutum or Chlorella vulgaris was used.The sensor allowed metals detection but this detection of toxicity was slow compared to that of herbicides or cyanide. Inhibition growth tests and Microtax test were more sensitive than the algal sensor for copper and mercury

Critical properties of joint spin and Fortuin-Kasteleyn observables in the two-dimensional Potts model

The two-dimensional Potts model can be studied either in terms of the original Q-component spins, or in the geometrical reformulation via Fortuin-Kasteleyn (FK) clusters. While the FK representation makes sense for arbitrary real values of Q by construction, it was only shown very recently that the spin representation can be promoted to the same level of generality. In this paper we show how to define the Potts model in terms of observables that simultaneously keep track of the spin and FK degrees of freedom. This is first done algebraically in terms of a transfer matrix that couples three different representations of a partition algebra. Using this, one can study correlation functions involving any given number of propagating spin clusters with prescribed colours, each of which contains any given number of distinct FK clusters. For 0 <= Q <= 4 the corresponding critical exponents are all of the Kac form h_{r,s}, with integer indices r,s that we determine exactly both in the bulk and in the boundary versions of the problem. In particular, we find that the set of points where an FK cluster touches the hull of its surrounding spin cluster has fractal dimension d_{2,1} = 2 - 2 h_{2,1}. If one constrains this set to points where the neighbouring spin cluster extends to infinity, we show that the dimension becomes d_{1,3} = 2 - 2 h_{1,3}. Our results are supported by extensive transfer matrix and Monte Carlo computations.Comment: 15 pages, 3 figures, 2 table

Les biocapteurs appliqués au contrôle des eaux: Revue - État de l'art

Cet article présente l'ensemble des biocapteurs en cours d'étude et proposés pour le contrôle en continu, automatisé et in situ de la qualité des eaux. Le principe des systèmes, étudiés jusqu'ici majoritairement en laboratoire et sur pilote, sera donné avec leurs performances au plan sensibilité et spécificité de détection des polluants hydriques. Ces performances conditionnent leur domaine d'application : les systèmes très sensibles étant affectés au contrôle des eaux d'alimentation et des eaux souterraines, les moins sensibles au contrôle des effluents très contaminés.Les biocapteurs peuvent se caractériser par deux de leurs composantes principales :- le réactif biologique ou biocatalyseur, sensible au(x) polluant(s); - le détecteur appelé transducteur, qui traduit la réponse biologique du biocatalyseur en un signal électrique. Le transducteur peut être de type optique, électrochimique, ampérométrique principalement, ou piézoélectrique. Trois grands types de biocapteurs peuvent être distingués selon la nature du biocatalyseur :- les bioréacteurs, basés sur l'étude des réponses comportementales des vertébrés (poissons) et d'autres organismes aquatiques (microcrustacés, bivalves): - les biosondes cellulaires reposant sur l'étude des fonctions métaboliques telles que la respiration, la bioluminescence, la photosynthèse de microorganismes immobilisés (bactéries, microalgues, levures) ou libres (boues activées) dans le milieu analysé: - les biocapteurs "d'affinité" basés sur l'utilisation d'enzymes ou d'anticorps, chargés de détecter respectivement les substrats et inhibiteurs enzymatiques spécifiques, ou les substances antigéniques vis à vis desquelles les anticorps ont été développés. Ces systèmes sont, par principe, les plus spécifiques mais aussi les plus sensibles. Ils ne couvrent, cependant, qu'une gamme encore très limitée de micropolluants hydriques. Le degré d'autonomie d'un biocapteur, sa facilité d'utilisation et de maintenance et sa fiabilité, sont des éléments qui rentrent en ligne de compte dans les performances. Ces qualités devront être évaluées lors de la phase de validation in situ, essentielle et déterminante pour juger de l'intérêt du système en conditions de fonctionnement réel.ContextThis paper reviews the use of biosensors for environmental biomonitoring and especially for the detection of water pollutants. These systems are developed in view of on-line applications, continuous and real time analysis. The principle and the design of the different systems proposed for this purpose are described with their performances deduced from pilot or in situ studies carried out up to now. Automation and autonomy, sensitivity and specificity are critical points that will determine the success of their applications in biomonitoring and the kind of application that can be envisaged. It is necessary they require minimal human intervention for maintenance and working . The more sensitive systems can be used for the monitoring of drinking and ground waters, the less sensitive ones for the monitoring of complex effluents, more heavily contaminated.Biosensors can be distinguished on the basis of the type of biocatalyst associated with thetransducer: the biological signal delivered by the biocatalyst is transmitted to a detector, also called transducer. The transducer, which may be an optical, electrochemical or piezoelectrical detector, transforms the biological response into an electric signal. This signal can be easily amplified and interpreted in terms of the toxicity and level of pollution of the analyzed sample.Three categories of biosensors can be defined:- biosensors using aquatic vertebrates and invertebrates: fish, microcrustacea, bivalves. Their behavior in the tested medium is studied as the criterion for toxicity; - cellular sensors, measuring physiological and biochemical functions such as respiration, bioluminescence, and photosynthesis, in microorganisms immobilized on the transducer (bacteria, yeast, microalgae,..) or suspended in the tested medium (activated sludge); - biosensors measuring an "affinity" response and a specific binding between enzyme/substrate or antibody/antigen. These systems use enzymes or antibodies immobilized in close contact with the transducer; they may detect the (analogs of) enzymatic substrates and inhibitors, or the (analogs of) antigenic substances binding to the antibody. These systems appear promising on the basis of their sensitivity. At present they can be applied for the detection of triazines and phenols. Such systems need to be developed and extended to other pollutants in order to cover the wide range of aquatic contaminants. User-friendliness, attendance and maintenance requirements, and service life are other critical aspects affecting the performances of a biosensor. These qualities need to be evaluated during the validation step of the equipment. In situ validation is essential for evaluating the relevance of the system in environmental biomonitoring and its applications. It is probable that among the numerous systems proposed as biosensors, only a few will be considered as suitable tools for on-line monitoring of waters

Mutual interference is common and mostly intermediate in magnitude

<p>Abstract</p> <p>Background</p> <p>Interference competition occurs when access to resources is negatively affected by the presence of other individuals. Within a species or population, this is known as mutual interference, and it is often modelled with a scaling exponent, <it>m</it>, on the number of predators. Originally, mutual interference was thought to vary along a continuum from prey dependence (no interference; <it>m </it>= 0) to ratio dependence (<it>m </it>= -1), but a debate in the 1990's and early 2000's focused on whether prey or ratio dependence was the better simplification. Some have argued more recently that mutual interference is likely to be mostly intermediate (that is, between prey and ratio dependence), but this possibility has not been evaluated empirically.</p> <p>Results</p> <p>We gathered estimates of mutual interference from the literature, analyzed additional data, and created the largest compilation of unbiased estimates of mutual interference yet produced. In this data set, both the alternatives of prey dependence and ratio dependence were observed, but only one data set was consistent with prey dependence. There was a tendency toward ratio dependence reflected by a median <it>m </it>of -0.7 and a mean <it>m </it>of -0.8.</p> <p>Conclusions</p> <p>Overall, the data support the hypothesis that interference is mostly intermediate in magnitude. The data also indicate that interference competition is common, at least in the systems studied to date. Significant questions remain regarding how different factors influence interference, and whether interference can be viewed as a characteristic of a particular population or whether it generally shifts from low to high levels as populations increase in density.</p

Traffic jams and intermittent flows in microfluidic networks

We investigate both experimentally and theoretically the traffic of particles flowing in microfluidic obstacle networks. We show that the traffic dynamics is a non-linear process: the particle current does not scale with the particle density even in the dilute limit where no particle collision occurs. We demonstrate that this non-linear behavior stems from long range hydrodynamic interactions. Importantly, we also establish that there exists a maximal current above which no stationary particle flow can be sustained. For higher current values, intermittent traffic jams form thereby inducing the ejection of the particles from the initial path and the subsequent invasion of the network. Eventually, we put our findings in the broader context of the transport proccesses of driven particles in low dimension