The application of Quantum Mechanics to microscopic systems has already changed our society, where transistors, light-emitting diodes, lasers and many other technologies based on quantum concepts are common in our every-day life. Future quantum technologies that exploit the most fundamental principles of Quantum Mechanics like discreteness, coherence, and entanglement could lead to revolutionary changes in society and industry. All these counter-intuitive ideas are combined in a Cooper pair splitter (CPS) device, where entangled pairs of electrons from a superconductor are coherently split into spatially separated metallic leads [Fig. 1(a)]. To use CPS devices as the building blocks of future quantum applications, Cooper pairs must be split coherently and the entanglement between the electrons must be maintained while they transfer to the leads. However, a proof of the entanglement of the split Cooper pairs is still missing due to the presence of a large number of particles in the electrical current.
To go beyond existing proposals for CPS devices, this project explored the regime of a single or a few emitted pairs by exploiting the unique properties of Dirac materials like graphene and topological insulators [Fig. 1(b)]. The electronic states in Dirac materials have long coherence lengths, comparable to the typical device size, and are robust against material imperfections. Their reduced dimensionality also facilitates the presence of proximity-induced superconductivity, which in Dirac materials becomes unconventional and can be tuned into a topologically nontrivial phase. Therefore, Dirac materials are exciting new candidates for designing novel CPS devices and engineering topological superconductivity.
The project’s objective is to study CPS devices, based on Dirac materials, that work in the regime of a single or a few particles. The project placed special emphasis on the detection and demonstration of the entanglement of the split particles and the conditions for the emergence of exotic topological superconductivity. Specifically,
1. I developed a microscopic theory for the description of junctions between superconductors and Dirac materials in the low particle limit.
2. I suggested alternative signatures of the coherent splitting of Cooper pairs to mean current and noise.
3. I proposed novel design considerations for engineering topological superconductivity and other exotic pairing mechanisms.
A controllable solid-state source of entanglement would be a major scientific breakthrough that substantially advances the development of quantum technologies. This project has demonstrated the advantages of working in the few or single-particle regime to bring this long-sought goal closer. We have developed a theoretical framework that allowed us to (i) identify signatures of the coherent splitting of Cooper pairs in the statistics of charge transfers and (ii) describe the effect of dynamic single-electron sources on mesoscopic conductors. We are now extending our formalism to analyse dynamically driven superconducting hybrid junctions. The novel quantum applications of single particles in superconducting circuits have the promise to unlock new research lines.
The downsizing of modern devices to the nano-scale has made harvesting waste heat into quantum thermoelectric effects an essential feature. Coherent heat transport in superconducting hybrid junctions thus provides an interesting alternative to average current and noise with many potential applications. Specifically, heat currents include unique signatures of the splitting of Cooper pairs and CPS devices can be operated as very efficient quantum heat engines. We also showed that a thermoelectric current provides a very useful tool to explore superconducting correlations.
Topological superconductors have a gap in the bulk but feature gapless Majorana states, which are very promising candidates for fault tolerant quantum computation. Topological superconductivity can be engineered combining topological insulators (Dirac materials featuring strong spin-orbit coupling) with conventional superconductors. This project has explored potential applications of Majorana states for quantum technologies and how CPS devices involving topological superconductors can be used as sources of spin-polarized supercurrents.
