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

当涉及到实现大规模量子计算机系统时，远距离耦合量子点中的自旋量子比特是仍然存在的重大挑战之一。到目前为止，只有邻近的量子点系统通过静电方式使用控制栅电极直接耦合。该项目的目标是探索使用超导电极的量子点自旋量子比特的远程耦合的可能性。由于到目前为止，这种方法尚未得到探索，有关这一概念可行性的主要基本问题将在项目中解决。耦合采用了两种不同的机制：第一种是基于两个量子点之间通过超导桥通过超导带隙上方的激发准粒子进行电子的虚拟交换。正如最近所显示的，要在大于超导体相干长度的距离上进行适当的耦合，量子点和超导体之间必须有一个硬超导隙和适度的耦合。作为第二种方法，我们还将研究基于交叉安德烈夫反射的耦合。在这里，来自每个量子点的一个电子隧穿到超导体中，并结合成库珀对，然后再隧穿回来。在这种情况下，期望的耦合长度将在超导相干长度的数量级上，因此比前一种情况小得多。本提案致力于在III-V半导体和硅中实现量子点耦合的实验研究。基于III-V半导体的方法能够覆盖全范围的耦合强度，因为原位制造过程，因此可以评估和评估两种机制，即通过激发准粒子和交叉Andreev反射进行耦合。另一方面，si方法允许在最成熟的材料系统中评估与超导体的远程耦合的适用性，该系统最近重新获得了对量子比特电路的强烈和日益增长的兴趣。量子点远距离耦合的成功演示将对量子比特的远距离耦合有很大的好处，从而向实现可扩展量子信息处理器迈出了重要的一步。

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.