Entanglement between quasiparticles in superconducting islands mediated by a single spin

TL;DR

This paper demonstrates that a single spin in superconducting islands can mediate long-range entanglement between quasiparticles, challenging the notion that impurity spins cannot form such correlations in superconductors.

Contribution

It introduces a method to overscreen a spin with two quasiparticles in superconducting islands, enabling long-range entanglement in solid-state systems.

Findings

Long-range ferromagnetic correlations between quasiparticles over micrometer distances.

Overscreened spin state requires large Coulomb repulsion and exchange binding.

Potential for controllable large-scale entanglement in quantum dot-superconductor systems.

Abstract

Condensed matter is composed of a small set of identical units, yet it shows an immense range of behaviour. Recently, an array of cold atoms was used to generate long-range quantum entanglement, a property of topological matter. Another approach to strong non-local correlations employs the macroscopic coherence of superconductors. Impurity spins in superconductors are thought to be unamenable to the formation of long-range spin entanglement because each spin tends to be screened by binding to a quasiparticle from the superconductor to form a local singlet. Here we demonstrate that it is possible to attach a second quasiparticle to the spin, overscreening it into a doublet state carrying ferromagnetic correlations between two quasiparticles over a micrometer distance. To demonstrate this effect, which is strongest for equal binding, we symmetrically couple the spin of a quantum dot to two ultrasmall superconducting islands. The overscreened state requires sufficiently large Coulomb repulsion and exchange binding to become well defined. We predict that this state will carry long-range correlations for an alternating chain of quantum dots and superconducting islands, opening a new route to controllable large-scale entanglement in the solid state.