Production of Electronic Quartets in Superconducting Josephson Bijunctions – NANOQUARTETS

..Théorie : méthodes de transport électronique hors équilibre (fonctions de Green de Keldysh), étude de circuits classiques contenant une bijonction.
Expérience : transport Josephson dc dans des structures à 3 terminaux, mesure de courants par SQUIDS. Réflectométrie ac en cavité microonde

..Théorie : calcul non perturbatif des résonances de quartets à des potentiels commensurables, pour des bijonctions à double dot ou métalliques balistiques.
Expérience : détection des 3 modes de quartets dans une trijonction métallique diffusive, avec des tensions plus grandes que l'énergie de Thouless, ce que seuls les quartets peuvent expliquer.

..La bi- ou trijonction Josephson st un nouvel élément de circuit dans lequel coexistent les deux signatures de l'effet Josephson, continu (Jdc) et alternatif (Jac) qui sont incompatibles dans des jonctions simples gouvernées ou bien par la phase (Jdc) ou par la tension (Jac). Ceci implique de revisiter toute la physique Josephson des jonctions tunnel aux jonctions métalliques ou contacts ponctuels (et points quantiques); et permet d'envisager des applications en termes d'intrication quantique et de corrélations de photons microondes. un défi expérimental est de contrôler indépendamment (notamment dans des géométries SQUID) les deux variables «potentiels« et «phase« dont dépend la physique des quartets et autres résonances multipaires. Un défi technique est de calculer les caractéristiques V(I) pour des potentiels quelconques et pas seulement commensurables.

.1. Multipair Josephson resonances in a biased all-superconducting bijunction (T. Jonckheere, J. Rech, T. Martin, B. Douçot, D. Feinberg, R. Mélin, Phys. Rev. B, sous presse)
2. Subgap structure in the conductance of a 3-terminal Josephson junction (A. Pfeffer, J.E. Duvauchelle, H. courtois, R. Mélin, D. Feinberg, F. Lefloch, en préparation)

Four-electron correlations will be induced in a Josephson bijunction formed by two junctions separated by a distance smaller than the coherence length. In addition to the usual Josephson effect in each junction, nonlocal Andreev processes between the junctions produce simultaneous transfer of two pairs (a nonlocal quartet), one in each side of the bijunction. The quartet current depends on the sum of the phases of the closeby junctions. This project aims at creating and detecting such nonlocal quartets by transport experiments, and extending the theoretical understanding as well as modelizing the experiments. The superconducting samples will have three current terminals and will involve either all-metallic junctions or tunable junctions made of carbon nanotube quantum dots. Both kinds of samples are already available or will be easily provided from previous experience with similar devices. The most striking prediction is the existence of a dc Josephson component due to quartets even in presence of voltage biases, provided both junctions are biased at opposite voltages. The detection of this dc quartet resonance will be achieved either by direct I(V) measurements or by Shapiro steps observation. A second set of experiments aims at detecting the dependence of the quartet current with the sum of the phases of the two junctions. This will involve generalizations of the usual SQUID for instance with two-loop circuits. All experiments will reveal the charge 4e of quartets. At last, a more exploratory topic is that of four-particle entanglement induced by quartet formation. The whole project relies on : i) a solid theoretical background; ii) existing samples; iii) existing experimental set-ups aimed at measuring three-terminal transport and the corresponding correlations. Promising results, both in theory and experiments, have been obtained in very preliminary studies, guaranteeing future observation of electron quartets in Josephson bijunctions. Beyond such experiments, generalization to arrays of Josephson junctions or mesoscopic grains open a vast field of research and potential applications, due to the nontrivial coupling of local phases, and the coexistence of dissipative and nondissipative currents, a new feature in superconductivity.