Scalable quantum information processing has become one of the most sought-after disruptive technology, owing to the prospect of providing solutions exponentially faster to various mathematical problem classes, relevant in the field of cryptography, materials science, optimization problems and artificial intelligence. The building block of quantum computers, the quantum bit (qubit) is a microscopic two-level system which harnesses the laws of quantummechanics enabling a massive parallelism in comparison to its classical counterpart. Because of the vast opportunities of technical applications, several physical approaches are already being investigated in both the academic and industrial sector, such as superconducting qubits, spin qubits in semiconductor quantum dots, or trapped ion systems. However, thus far, no clear single platform has emerged with a well-defined technical roadmap towards scalability, therefore it is a timely challenge to investigate alternative, novel platforms for quantum computation.
The AndQC consortium set out to demonstrate the Andreev qubit as a scalable platform in hybrid nanodevices consisting of superconducting quantum circuits and semiconductor nanostructures. This platform is implemented by utilizing the discrete superconducting quasiparticle levels (Andreev levels) that appear in weak links between superconductors, and can be occupied by zero, one or two electrons. These Andreev levels therefore can host quantum bits, both based on their occupation number (zero or two) and by utilizing the spin degree of freedom of the single, localized electron. This configuration has the unprecedented functionality of coupling a single localized spin to the dissipationless supercurrent, and enables the so far experimentally unexplored scheme of fermionic quantum computation, which has the potential of efficiently simulating electron systems in complex molecules and novel materials. To achieve these scientific goals, the consortium combines the expertise of leading groups in the fields of nanodevice growth, quantum theory and experiment.
Our project has demonstrated the viability of Andreev levels as a novel quantum computation platform with several key improvements in materials science, theoretical description and experimental protocols for quantum information control. Working on the various combinations of superconductor and semiconductor nanomaterials, we showed that several superconductors of higher critical temperature than aluminium are suitable in combination with narrow bandgap III-V semiconductors, for instance InAs. This breakthrough allows future research to explore novel regimes of intrinsic parameters, such as the spin-orbit coupling. Progressing with planar superconducting quantum circuits, we demonstrated that planar resonator and control structures can be tailored for Andreev qubits leading to the potential of high level integration, and thus, scalability of quantum processors. During the project execution, our theory workpackage allowed a better understanding of these hybrid quantum devices on the basis of fundamental Hamiltonian as well as realistic self-consistent modeling.
