Highly-sensitive superconducting quantum interference proximity transistor

We report the design and implementation of a high-performance superconducting quantum interference proximity transistor (SQUIPT) based on aluminum-copper (Al-Cu) technology. With the adoption of a thin and short copper nanowire we demostrate full phase-driven modulation of the proximity-induced minigap in the normal metal density of states. Under optimal bias we record unprecedently high flux-to-voltage (up to 3 mV/$\Phi_0$) and flux-to-current (exceeding 100 nA/$\Phi_0$) transfer function values at sub-Kelvin temperatures, where $\Phi_0$ is the flux quantum. The best magnetic flux resolution (as low as 500 n$\Phi_0/\sqrt{Hz}$ at 240 mK, being limited by the room temperature pre-amplification stage) is reached under fixed current bias. These figures of merit combined with ultra-low power dissipation and micrometer-size dimensions make this mesoscopic interferometer attractive for low-temperature applications such as the investigation of the magnetization of small spin populations.Comment: 7 pages, 5 color figure

Interacting electrodynamics of short coherent conductors in quantum circuits

When combining lumped mesoscopic electronic components to form a circuit, quantum fluctuations of electrical quantities lead to a non-linear electromagnetic interaction between the components that is not generally understood. The Landauer-B\"uttiker formalism that is frequently used to describe non-interacting coherent mesoscopic components is not directly suited to describe such circuits since it assumes perfect voltage bias, i.e. the absence of fluctuations. Here, we show that for short coherent conductors of arbitrary transmission, the Landauer-B\"uttiker formalism can be extended to take into account quantum voltage fluctuations similarly to what is done for tunnel junctions. The electrodynamics of the whole circuit is then formally worked out disregarding the non-Gaussianity of fluctuations. This reveals how the aforementioned non-linear interaction operates in short coherent conductors: voltage fluctuations induce a reduction of conductance through the phenomenon of dynamical Coulomb blockade but they also modify their internal density of states leading to an additional electrostatic modification of the transmission. Using this approach we can account quantitatively for conductance measurements performed on Quantum Point Contacts in series with impedances of the order of $R_K = h / e^2$. Our work should enable a better engineering of quantum circuits with targeted properties

Balanced double-loop mesoscopic interferometer based on Josephson proximity nanojunctions

We report on the fabrication and characterization of a two-terminal mesoscopic interferometer based on three V/Cu/V Josephson junctions having nanoscale cross-section. The junctions have been arranged in a double-ring geometry realized by metallic thin film deposition through a suspended mask defined by electron beam lithography. Although a significant amount of asymmetry between the critical current of each junction is observed we show that the interferometer is able to suppress the supercurrent to a level lower than 6 parts per thousand, being here limited by measurement resolution. The present nano-device is suitable for low-temperature magnetometric and gradiometric measurements over the micrometric scale.Comment: 5 pages, 4 figure

Collective phenomena in social animal dynamics

Treballs Finals de Grau de Física, Facultat de Física, Universitat de Barcelona, Curs: 201, Tutora: M. Carmen MiguelCollective motion is a fascinating and well-proved behavior of social animals. Bird flocks, fish schools or sheep herds are common examples of that phenomenon. In the following paper, we analyze that kind of behavior of social animals, reproducing on that purpose the so called Vicsek model. In the model, animals copy their neighbors, averaging their direction of motion in each step with more or less precision depending on the intensity of a noise term. This gives raise to a phase transition between polarized and unpolarized states. We also analyze the effect of leadership and show that this new ingredient eliminates the phase transition

Quantum Properties of the radiation emitted by a conductor in the Coulomb Blockade Regime

We present an input-output formalism describing a tunnel junction strongly coupled to its electromagnetic environment. We exploit it in order to investigate the dynamics of the radiation being emitted and scattered by the junction. We find that the non-linearity imprinted in the electronic transport by a properly designed environment generates strongly squeezed radiation. Our results show that the interaction between a quantum conductor and electromagnetic fields can be exploited as a resource to design simple sources of non-classical radiation.Comment: 14 pages, 4 figures, includes Supplementar

Phase-driven collapse of the Cooper condensate in a nanosized superconductor

Superconductivity can be understood in terms of a phase transition from an uncorrelated electron gas to a condensate of Cooper pairs in which the relative phases of the constituent electrons are coherent over macroscopic length scales. The degree of correlation is quantified by a complex-valued order parameter, whose amplitude is proportional to the strength of the pairing potential in the condensate. Supercurrent-carrying states are associated with non-zero values of the spatial gradient of the phase. The pairing potential and several physical observables of the Cooper condensate can be manipulated by means of temperature, current bias, dishomogeneities in the chemical composition or application of a magnetic field. Here we show evidence of complete suppression of the energy gap in the local density of quasiparticle states (DOS) of a superconducting nanowire upon establishing a phase difference equal to pi over a length scale comparable to the superconducting coherence length. These observations are consistent with a complete collapse of the pairing potential in the center of the wire, in accordance with theoretical modeling based on the quasiclassical theory of superconductivity in diffusive systems. Our spectroscopic data, fully exploring the phase-biased states of the condensate, highlight the profound effect that extreme phase gradients exert on the amplitude of the pairing potential. Moreover, the sharp magnetic response observed near the onset of the superconducting gap collapse regime can be exploited to realize ultra-low noise magnetic flux detectors.Comment: 7 pages, 5 color figures plus supporting inf

Tuning Energy Relaxation along Quantum Hall Channels

The chiral edge channels in the quantum Hall regime are considered ideal ballistic quantum channels, and have quantum information processing potentialities. Here, we demonstrate experimentally, at filling factor 2, the efficient tuning of the energy relaxation that limits quantum coherence and permits the return toward equilibrium. Energy relaxation along an edge channel is controllably enhanced by increasing its transmission toward a floating ohmic contact, in quantitative agreement with predictions. Moreover, by forming a closed inner edge channel loop, we freeze energy exchanges in the outer channel. This result also elucidates the inelastic mechanisms at work at filling factor 2, informing us in particular that those within the outer edge channel are negligible.Comment: 8 pages including supplementary materia

Plasmon scattering approach to energy exchange and high frequency noise in nu=2 quantum Hall edge channels

Inter-edge channel interactions in the quantum Hall regime at filling factor nu= 2 are analyzed within a plasmon scattering formalism. We derive analytical expressions for energy redistribution amongst edge channels and for high frequency noise, which are shown to fully characterize the low energy plasmon scattering. In the strong interaction limit, the predictions for energy redistribution are compared with recent experimental data and found to reproduce most of the observed features. Quantitative agreement can be achieved by assuming 25 % of the injected energy is lost towards other degrees of freedom, possibly the additional gapless excitations predicted for smooth edge potentials.Comment: 4 pages, 4 figure

Dynamical Coulomb Blockade of Shot Noise

We observe the suppression of the finite frequency shot-noise produced by a voltage biased tunnel junction due to its interaction with a single electromagnetic mode of high impedance. The tunnel junction is embedded in a quarter wavelength resonator containing a dense SQUID array providing it with a characteristic impedance in the kOhms range and a resonant frequency tunable in the 4-6 GHz range. Such high impedance gives rise to a sizeable Coulomb blockade on the tunnel junction (roughly 30% reduction in the differential conductance) and allows an efficient measurement of the spectral density of the current fluctuations at the resonator frequency. The observed blockade of shot-noise is found in agreement with an extension of the dynamical Coulomb blockade theory

La utilidad de los folletos de información de, medicamentos en pacientes cronicos hospitalizados

ResumenPara conocer cómo los pacientes perciben los folletos informativos sobre medicamentos, se ha realizado un estudio en una muestra de 65 pacientes ingresados en un hospital por enfermedades respiratorias y cardiacas crónicas.El 47,7% de los pacientes declaró que la información principal obtenida de los folletos se refería a la finalidad del fármaco, seguidos por el 18,5%, que resaltó el manejo de situaciones imprevistas y los efectos secundarios. El 63% declaró que no le había aportado información nueva. Una proporción importante de pacientes desconocía el uso correcto de algunos términos empleados con frecuencia en folletos y prospectos (80% de errores con el término «fuera de las comidas»).Se concluye que los folletos informativos pueden ser útiles en el marco de una buena relación médico-paciente, como recordatorio y para reforzar los conocimientos que ya posee el paciente. El momento idóneo para la educación sobre los medicamentos en las enfermedades crónicas es al iniciar su tratamiento.SummaryTo know how patients perceive the information leaflets about medicines, it has been carried out a study in a sample of 65 patients admitted to a hospital for chronic respiratory or cardiac diseases.A 47.7% of patients expressed that the main information provided by the leaflet was such concerning to the purpose of the drug, whereas a 18.5% pointed out such concerning to side effects and management of unexpected circumstances. Sixty three percent of patients expressed that leaflets did not provide them new information. A relevant percentage of patients did not know the meaning of some terms frequently used in leaflets and package-inserts (80% of mistakes in the interpretation of «take the drug on an empty stomach»).It is concluded that the information leaflets can be useful in the framework of a good physician-patient relationship, as a reminder, and to reinforce the knowledge that the patient already has. It is considered that the most appropriate time for the education of chronic patients about drugs is when the treatment is prescribed for the first time