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

• Applicants: Professor Dr. Joachim Knoch; Professor Dr. Thomas Schäpers
• Subject Area: Experimental Condensed Matter Physics ; Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
• Term: from 2017 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 387743155

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.

• DFG Programme: Research Grants