Quartet correlations between Cooper pairs in a mesoscopic double Josephson junction

Tuesday 7 February 2012 at 3pm
Room "Rémy Lemaire" K 223 (1st floor) Building K, Institut Néel / CNRS

Régis MELIN (Institut Néel)

Two Josephson junctions separated by less than the superconducting coherence length involve higher order processes that can be quite sizeable for more transparent junctions. Those processes originate from crossed Andreev reflections (CAR), where Cooper pairs are split into two spin-entangled electrons. In a Sa-S-Sb bijunction, double CAR simultaneously produces two Cooper pairs, one in each junction. This amounts to producing nonlocal quartets in the superconductors Sa and Sb [1]. Cooper pairs being pseudo-bosons, this mechanism bears some similarity with the emission of pairs of time-correlated photons in Quantum Optics, thus the name "superconducting beam splitter" given to the basic bijunction set-up. Energy conservation implies that a coherent dc quartet current can flow even if Sa and Sb are biased at respective voltages V and -V, thus offering the possibility of dc Josephson effect in biased junction arrays.
We will review several consequences and possible detection of quartet currents. They rely on a superconducting circuit having 3 (and not 2) current terminals. Dc quartet current can be detected by the synchronization of the ac Josephson oscillations in each contact a and b, either with Va=V and Vb=-V, or adding a small dc and ac components such as to achieve Shapiro steps for quartet motion. The potential of quartet production in superconducting circuits includes entanglement, which can be continuously controlled by a superconducting phase.

[1] A. Freyn, B. Douçot, D. Feinberg and R. Mélin, Phys. Rev. Lett. 106, 257005 (2011).