Ultralong-distance quantum correlations in three-terminal Josephson junctions

TL;DR

This paper demonstrates ultralong-distance quantum correlations in three-terminal Josephson junctions, surpassing the superconducting coherence length, through Floquet dynamics and Cooper pair splitting, with potential applications in quantum engineering.

Contribution

It reveals nonlocal quantum correlations over distances much greater than the coherence length in three-terminal Josephson junctions, introducing the ultralong-distance Floquet-Tomasch effect.

Findings

Quantum correlations extend over ~30 μm, exceeding coherence length.

Identification of crossover from Floquet-Andreev quartets to Floquet-Tomasch clusters.

Detection methods include DC-transport and quantum noise measurements.

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 cross-over 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 $\simeq 30\,\mu$m 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.