Controlling observables in time-dependent quantum transport

TL;DR

This paper develops methods to control quantum transport in nanoscale systems by designing time-dependent biases to achieve desired behaviors, such as specific current patterns or suppression of oscillations, in quantum dot junctions.

Contribution

It introduces a novel approach to inverse quantum transport problems, enabling tailored control of observables in quantum dot systems with normal and superconducting leads.

Findings

Successfully optimized current, density, and vibrations to follow target patterns.

Demonstrated suppression of Josephson oscillations via bias tailoring.

Enhanced Cooper pair splitting efficiency using optimized bias.

Abstract

The theory of time-dependent quantum transport addresses the question: How do electrons flow through a junction under the influence of an external perturbation as time goes by? In this paper, we invert this question and search for a time-dependent bias such that the system behaves in a desired way. This can, for example, be an observable that is forced to follow a certain pattern or the minimization of an objective function which depends on the observables. Our system of choice consists of quantum dots coupled to normal or superconducting leads. We present results for junctions with normal leads where the current, the density or a molecular vibration is optimized to follow a given target pattern. For junctions with two superconducting leads, where the Josephson effect triggers the current to oscillate, we show how to suppress the Josephson oscillations by suitably tailoring the bias. In a second example involving superconductivity, we consider a Y shaped junction with two quantum dots coupled to one superconducting and two normal leads. This device is used as a Cooper pair splitter to create entangled electrons on the two quantum dots. We maximize the splitting efficiency with the help of an optimized bias.