Self-consistent dynamics of a Josephson junction in presence of an arbitrary environment

We derive microscopically the dynamics associated with the d.c. Josephson effect in a superconducting tunnel junction interacting with an arbitrary electromagnetic environment. To do so, we extend to superconducting junctions the so-called P(E) theory (see e.g. Ingold and Nazarov, arXiv:cond-mat/0508728) that accurately describes the interaction of a nonsuperconducting tunnel junction with its environment. We show the dynamics of this system is described by a small set of coupled correlation functions that take into account both Cooper pair and quasiparticle tunneling. When the phase fluctuations are small the problem is fully solved self-consistently, using and providing the exact linear admittance Y({\omega}) of the interacting junction.Comment: 7 pages, 1 figur

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

Room-temperature tunnel current amplifier and experimental setup for high resolution electronic spectroscopy in millikelvin STM experiments

The spectroscopic resolution of tunneling measurements performed with a scanning tunneling microscope is ultimately limited by the temperature at which the experiment is performed. To take advantage of the potential high spectroscopic resolution associated with operating an STM in a dilution refrigerator we have designed a room temperature tunnel current amplifier having very small back-action on the tunnel contact and allowing to nearly reach the predicted energy resolution. This design is a modification of the standard op-amp based tip-biasing current-voltage converter which implements differential voltage sensing and whose back action on the tip voltage is only ~2 $\mu$V rms for a 14 MV/A transimpedance and 22 kHz bandwidth.Comment: Available at http://www-spht.cea.fr/articles/s06/03

Spectroscopie des états liés d'Andreev dans un nanotube de carbone

La supraconductivité est un ordre électronique fascinant dans lequel les électrons s apparient par le biais d une interaction attractive et condensent dans un état quantique macroscopique pouvant porter un courant non dissipatif, i.e. un supercourant. Dans les structures hybrides où des supraconducteurs (S) sont mis en contact avec des matériaux non supraconducteurs (X), les paires se propageant de S contaminent X lui conférant des propriétés supraconductrices à proximité de l interface, parmi lesquels la possibilité de porter un supercourant. La transmission d un supercourant à travers n importe quelle structure S-X-S s explique par l interférence constructive de paires d électrons traversant X. En effet, à la manière d un résonateur Fabry-Perot, une telle interférence a seulement lieu pour certains états électroniques résonants appelés Etats Liés d Andreev (ELA). Récemment, il est devenu possible de fabriquer une variété de nanostructures dans lesquelles X peut être par exemple un nanofil, un nanotube de carbone ou même une molécule. Ces dispositifs ont en commun que leur X contient seulement quelques électrons de conduction ce qui implique que les ELA sont aussi en petit nombre. Dans ce cas, pour comprendre quantitativement l effet de proximité dans ces systèmes, il est nécessaire de comprendre en détail la formation des ELA individuellement. Ceci peut être vu comme la question centrale du développement d électronique supraconductrice à l échelle nanométrique. Dans cette thèse, nous avons observé des ELA résolus individuellement par spectroscopie tunnel dans un nanotube de carbone.PARIS-BIUSJ-Biologie recherche (751052107) / SudocSudocFranceF

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

What are Landauer's conduction channels in an atomic-size metallic contact?

Using microfabricated break junctions, we have produced Al and Au contacts consisting of a small number of atoms. These metallic quantum point contacts accomodate a small number of conduction channels. We have determined the individual transmission coefficients in contacts containing up to six conduction channels. The determination is based on a comparison of the highly nonlinear current voltage characteristics in the superconducting state with the predictions of the theory for a single channel superconducting contact (in the case of Au contacts superconductivity is induced by means of the proximity effect). We find that in the smallest achievable contacts usually three channels contribute to the transport in the case of Al, whereas only one does in the case of Au. These findings are consistent with the quantum chemistry point of view that the channels are determined by the valence atomic orbitals at the narrowest section of the contacts

Conduction Channel Transmissions of Atomic-Size Aluminum Contacts

We have determined the individual transmission coefficients of Al quantum point contacts containing up to six conduction channels. The determination is based on a comparison of the highly nonlinear current-voltage characteristics in the superconducting state with the predictions of the theory for a single channel superconducting contact. We find that at least two channels contribute to the transport even for contacts with conductance lower than the conductance quantum

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