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

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

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

Mécanismes inélastiques dans des circuits mésoscopiques réalisés dans des gaz bidimensionnels d'électrons

This thesis experimentally investigates the inelastic mechanisms at work in mesososcopic electronic circuits. The measured circuits were tailored in the 2D electron gas obtained at the interface of a GaAs/AlGaAs heterostructure. We explored the inelastic dynamics of edge channels implemented in the integer and fractionnel quantum Hall regimes, as well as the dynamics resulting from the interaction between a quantum conductor and the macroscopic circuit embedding it.Cette thèse est consacrée à l'étude expérimentale des processus d'interaction inélastiques à l'oeuvre dans les circuits mésoscopiques. Les circuits mesurés sont réalisés dans des gaz bidimensionnels d'électrons crées à l'interface d'une hétérojonction semi-conductrice GaAs/Ga(Al)As. Nous avons exploré les régimes de l'effet Hall quantique entier et fractionnaire, ainsi que les interactions entre un conducteur cohérent et le circuit dans lequel ils'insère

Inelastic mechanisms in mesoscopic circuits realized in two dimensional electron gases

Cette thèse présente des expériences sondant les processusd'intéraction inélastiques dans des cirucuits mésoscopiques réalisés dans des gaz bidimensionnels d'électrons. Nous explorons les régimes de l'effet Hall quantique entier et fractionnaire, ainsi que les intéractions entre un conducteur cohérent et le circuit dans lequel il s'insère; Nous abordons le régime de l'effet Hall quantique sous l'angle du transport d'énergie; Nous démontrons la spectroscopie hors équilibre de la distribution en énergie d'un canal de bord, utilisant une boîte quantique comme filtre à énergie. Des situations hors équilibre sont générés grâce à un contact ponctuel quantique polarisé en tension. Avec ces outils nous soudons la dynamique hors équilibre pour deux canaux co-propageants (nu=2). Nous observons une forte relaxation en énergie qui défie l'image usuelle sans intéractions et identifions l'interaction inter-canal comme mécanisme dominant. Nous démontrons aussi deux schémas permettant d'augmenter et diminuer la relaxation d'un canal. Des mesures similaires dans le régimefractionnaire à nu=4/3, révèlent un flux d'énergie antichiral, alors que le courant électrique reste chiral. Nous montrons que le flux d'énergie antipropageant est porté par des excitations de volume neutres. L'impédance d'un conducteur cohèrent est modifée par son environnement, modifiant les lois usuelles de composition d'impédances. c'est effet, connu comme blocage de Coulomb dynamique, est prédit d'être pondéré par le même facteur de Fano apparaissant dans le bruit de granaille. Nous avons démontré ce fort lien en mesurant la conductance d'un contact ponctuel quantique inséré dans un circuit modifiable in-situ.This thesis presents an experimental investigation of inclastic interaction effects in mesocopic circuits, realized in two dimensional electron gases. We explore the regimes of the integer and fractional quantum Hall effect, and the interactions between a coherent conductor and the circuit in whish it is embedded. We investigate the quantum Hall regimes under the point of view of energy transport. We demonstrate the non-equilibrium spectroscopy of th electronic energy distribution of an edge channel, using a quantum dot as an energy filter, while the edge channel is driven out of equilibrium with a voltage biased quantum point contact. With these tolls, we probe the non equilibrium dynamics along two co-propagating edge channels (nu=2). We observe a strong energy relaxation, challenging the widespread picture of non-interacting channels, and identify inter-channelinteractions, without particleexchanges, as the dominant inclastinc mechanism. We further demonstrate two techniques permitting us to increase, and freeze, the energy relaxation. Similar experiments performed in the fractional regime at nu=4/3, reveal an antichiral energy flow , while the electric current is found to follow the expected chirality. Weshow that the antichiral energy flow is carried by neutral bulk excitations. The impedance of coherent conductor is modified by its environment, modifying the usual impedance composition laws. This effect, know as dynamical Coulomb blockade, has been predicted to be renormalized by the Fano factor emerging in the conductorés shot noise. We demonstrate this strag link by measuring the condtance of aquentum point contact embedded in a tunable circuit.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Micro-superconducting quantum interference devices based on V/Cu/V Josephson nanojunctions

We report on the fabrication and characterization of microSQUID devices based on nanoscale vanadium/copper/vanadium Josephson weak links. Magnetically driven quantum interference patterns have been measured for temperatures in the 0.24 - 2 K range. As magnetometers, these devices obtain flux-to-voltage transfer function values as high as 450 microV/Phi_0 leading to promising magnetic flux resolution Phi_N < 3 microPhi_0 / Hz^(1/2), being here limited by the room temperature preamplification stage. Significant improvements in the flux noise performance figures, which are already competitive with existing state-of-the-art commercial SQUIDs systems, are expected either with cryogenic preamplification or with the adoption of optimized electronic readout stages based on active feedback schemes.Comment: 4.5 pages, 4 color figure

Absence of a dissipative quantum phase transition in Josephson junctions: Theory

We obtain the reduced density matrix of a resistively shunted Josephson junction (RSJ), using the stochastic Liouville equation method in imaginary time - an exact numerical scheme based on the Feynman-Vernon influence functional. For all parameters looked at, we find a shunted junction is more superconducting than the same unshunted junction. We find no trace of Schmid's superconducting-insulating quantum phase transition long believed to occur in the RSJ. This work confirms theoretically a similar conclusion drawn in 2020 by Murani et al., based on experimental observations. We reveal that predictions of an insulating junction in previous works were due to considering Ohmic environments with no UV cutoff

Parametric amplification and squeezing with an ac- and dc-voltage biased superconducting junction

Accepted for publication at the Physical Review Applied. 12 pages and 9 figuresInternational audienceWe theoretically investigate a near-quantum-limited parametric amplifier based on the nonlinear dynamics of quasiparticles flowing through a superconducting-insulator-superconducting junction. Photon-assisted tunneling, resulting from the combination of dc- and ac-voltage bias, gives rise to a strong parametric interaction for the electromagnetic modes reflected by the junction coupled to a transmission line. We show phase-sensitive and phase-preserving amplification, together with single- and two-mode squeezing. For an aluminum junction pumped at twice the center frequency, $\omega_0/2\pi=6$~GHz, we predict narrow-band phase-sensitive amplification of microwaves signals to more than 20 dB, and broadband phase-preserving amplification of 20 dB over a 1.2 GHz 3-dB bandwidth. We also predict single- and two-mode squeezing reaching more than -12 dB over 5.3 GHz 3-dB bandwidth. Moreover, with a simple impedance matching circuit, we demonstrate 3 dB bandwidth reaching 4.3 GHz for 20 dB of gain. A key feature of the device is that its performance can be controlled in-situ with the applied dc- and ac-voltage biases