Coupling of quantum dots with superconductors- towards long-range coupling of qubits

在实现大规模量子计算机系统时，在量子点中长距离耦合自旋量子比特是剩下的最大挑战之一。到目前为止，只有相邻的量子点系统通过静电方式使用控制栅电极直接耦合。这个项目的目标是探索使用超导电极实现量子点自旋量子比特远程耦合的可能性。由于这一方法迄今尚未得到探讨，因此将在本项目中主要探讨有关这一概念可行性的基本问题。两种不同的机制用于耦合：第一种是基于通过超导带隙以上的激发准粒子通过超导桥在两个量子点之间的电子的虚拟交换。最近的研究表明，要在大于超导体相干长度的距离上实现适当的耦合，必须有一个硬的超导带隙和量子点与超导体之间的适度耦合。作为第二种方法，我们还将研究基于交叉Andreev反射的耦合。在这里，一个电子从每个量子点隧道进入超导体，并结合到一个库珀对之前，它隧道回来。在这种情况下，预期的耦合长度将在超导相干长度的数量级，因此比前面的情况小得多。本提案致力于在III-V族半导体以及硅中实现的量子点耦合的实验研究。基于III-V族半导体的方法能够覆盖由于原位制造过程而导致的耦合强度的全部范围，并且因此允许评估和评估两种机制，即通过激发准粒子的耦合以及交叉Andreev反射。另一方面，Si方法允许评估最成熟的材料系统中与超导体的长程耦合的适用性，该材料系统最近重新获得了对量子位电路的强烈且日益增长的兴趣。量子点在更长距离上的耦合的成功演示对于量子比特的远程耦合将是非常有益的，因此是朝着实现可扩展的量子信息处理器迈出的重要一步。

Coupling spin qubits in quantum dots over a long distance is one of the big remaining challenges, when it comes to realizing large scale quantum computer systems. So far only neighbouring quantum dot systems were coupled directly by electrostatic means using control gate electrodes. The goal of this project is to explore the possibility of a long-range coupling of quantum dot spin qubits using superconducting electrodes. Since this approach is rather unexplored up to now, mainly fundamental questions concerning the feasibility of this concept will be addressed within the project. Two different mechanisms are employed for the coupling: The first is based on the virtual exchange of an electron between two quantum dots via a superconducting bridge by means of excited quasiparticles above the superconducting band gap. As has been recently shown, for a proper coupling over distances larger than the coherence length of the superconductor a hard superconducting gap and a moderate coupling between quantum dot and superconductor is necessary. As a second method we will also study the coupling based on crossed Andreev reflection. Here, one electron from each quantum dot tunnels into the superconductor and combines to a Cooper pair before it tunnels back. In this case the expected coupling length would be in the order of the superconducting coherence length, thus much smaller than in the previous case. The present proposal is devoted to an experimental investigation of the coupling of quantum dots realized in III-V semiconductors as well as in silicon. The approach based on III-V semiconductors enables to cover the full range of coupling strengths due to an in-situ fabrication process and thus allows assessing and evaluating both regimes, i.e. coupling via excited quasiparticles as well as crossed Andreev reflection. The Si-approach, on the other hand, allows assessing the suitability of the long-range coupling with superconductors within the most mature material system that has recently regained a strong and increasing interest for qubit circuits. A successful demonstration of a coupling of quantum dots over longer distances would be of great benefit for the long-range coupling of qubits and thus of a major step forward towards a realization of a scalable quantum information processor.