Ultralong-distance quantum correlations in three-terminal Josephson junctions

Abstract

The production of entangled pairs of electrons in ferromagnet-superconductor-ferromagnet or normal metal-superconductor-normal metal three-terminal structures has aroused considerable interest in the last twenty years. In these studies, the distance between the contacts is limited by the zero-energy superconducting coherence length. Here, we demonstrate nonlocality and quantum correlations in voltage-biased three-terminal Josephson junctions over the ultralong distance that exceeds the superconducting coherence length by orders of magnitude. The effect relies on the interplay between the time-periodic Floquet-Josephson dynamics, Cooper pair splitting, and long-range coupling similar to the two-terminal Tomasch effect. We find crossover between the “Floquet-Andreev quartets” (if the spatial separation is smaller than the superconducting coherence length) and the “ultralong-distance Floquet-Tomasch clusters of Cooper pairs” if the separation exceeds the superconducting coherence length, possibly reaching the same ≃30μm as in the Tomasch experiments. The effect can be detected with DC-transport and zero-frequency quantum current-noise cross-correlation experiments, and it can be used for fundamental studies of superconducting quasiparticle quantum coherence in the circuits of quantum engineering.

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