Andreev qubits for scalable quantum computation

Project Information

AndQC

Grant agreement ID: 828948

DOI

10.3030/828948

Project closed

EC signature date 17 December 2018

Start date 1 April 2019

End date 31 March 2024

Funded under

• EXCELLENT SCIENCE - Future and Emerging Technologies (FET)

Total cost

€ 3 636 822,50

EU contribution

€ 3 484 107,50

Coordinated by

CHALMERS TEKNISKA HOGSKOLA AB
Sweden

Periodic Reporting for period 3 - AndQC (Andreev qubits for scalable quantum computation)

Reporting period: 2021-10-01 to 2024-03-31

Summary of the context and overall objectives of the project

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.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

Over the course of the research program, the consortium reached its key targeted technological and scientific breakthroughs, and thus established that Andreev levels are a viable host for quantum information processing. Our material platform is based on semiconductor nanostructures with high quality superconducting contacts, and two leading geometries have been investigated: semiconductor nanowires and planar semiconductor heterostructures. Our researchers used special, ultra-high vacuum techniques to grow these nanostructures, including chemical beam epitaxy, molecular beam epitaxy and in-situ junction forming. Crucially for future devices, we have showcased the viability of several different superconductor-semiconductor material combination. These unique nanodevices have been distributed in the consortium where the quantum devices have been built and measured. We conclude that both InAs and InSb devices can host ballistic transport, which we characterized both in the nanowire and in the planar device geometry.

To access the quantum information encoded in the Andreev level excitation or the Andreev spin, we have used planar superconducting resonators cooled down to millikelvin temperatures. This control and measurement architecture is very similar to that established in the context of conventional superconducting quantum bits (qubits), and thus with similar measurement techniques we could establish quantum lifetimes up to the 10 microsecond scale, in par with state of the art transmon qubits. We identified the main source of dephasing to be spin fluctuations for the Andreev spin qubits, which result underlines the importance of materials engineering for these devices.

The experimental and materials growth activities of the consortium have been in a synergetic connection with the theory activities including Hamiltonian modeling and three dimensional simulations with the material stack. Our theory efforts have provided crucial guidance to understand the role of disorder, spin fluctuations and electrostatic confinement for device operation. Furthermore, we have suggested novel device geometries to exploit various aspects of Andreev physics in superconductor - semiconductor hybrid devices.

Our scientific activities have resulted in 88 peer-reviewed publications over the course of the project. In addition, we have organized and co-organized three topical workshops both online and on-site to establish scientific connections and provide a dialog around the physics of Andreev bound states.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

Our work performed in the first year demonstrated the feasibility of the longer term objectives of the scientific work program, adding a radically new platform to quantum technologies, the scalable Andreev quantum bit. Combining semiconductor and superconductor nanostructures, our work complements the technological development of pure superconductor and semiconductor quantum bits.
Our goal is to combine the best of these two, thus far distinct worlds, in an effort to provide an alternative qubit platform for scalable, fault-tolerant quantum computers. This effort fits well in the European innovation ecosystem, which will benefit not only from the scientific knowledge built by the consortium, but also from our interactions with the leading suppliers of specialized electronic components, cryogenic systems and clean materials.
At the conclusion of the project, we anticipate most of the utilization of the project results in synergy with the quantum technology efforts in Europe, and also world-wide. The material platform of superconductor-semiconductor hybrids, which our project established as a scalable platform for quantum technologies, have the prospect for wide impact in various fields within quantum technologies.

Andreev qubit coupled to a superconducting resonator. Credit: University of Basel

InAs nanowire with in-situ defined junction. Credit: University of Copenhagen