Nonequilibrium Cooper quartet generation in superconducting devices

TL;DR

This paper proposes a method to generate and detect Cooper quartets—four-electron states—in a double-quantum-dot system, revealing signatures of multifermion correlations through transport measurements.

Contribution

It introduces a novel experimental platform for isolating and studying Cooper quartets in a solid-state device, advancing understanding of multifermion states.

Findings

High bias voltage resonance involves two-Cooper pair exchange.

A peak in Andreev current scales with quartet coupling strength.

Equal auto- and cross-correlations indicate coherent two-Cooper-pair oscillations.

Abstract

Cooper quartets are aggregates of four electrons that generalize the concept of Cooper pairs, and their study can unfold unexplored perspectives in correlated matter and many-body physics. We propose a method to isolate them in a double-quantum-dot system coupled to conventional superconducting and normal leads. By driving the system out of equilibrium, we show that a resonance between the vacuum $|0\rangle$ and the four-electron state $|4e\rangle$ emerges in the high bias voltage regime, which involves a two-Cooper pair exchange process and is characterized by finite quartet correlations. We study the transport properties of the system and show that a peak in the Andreev current at high bias voltage has a width that scales with the magnitude of the quartet coupling $\Gamma_{4e}$, which can be tuned by the phase of additional superconducting leads, yielding distinctive signatures. By further studying the current-current correlations and the Fano factor, we establish a regime characterized by equal auto- and cross-correlations, which we interpret as a definitive signature of fast coherent two-Cooper-pair oscillations between the dots and the superconducting leads. The proposed platform, experimentally accessible in a quantum solid-state laboratory, enables exploration of quartet correlations and multifermion-correlated states of matter.