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TOPOLOGICALLY PROTECTED AND SCALABLE QUANTUM BITS

Periodic Reporting for period 3 - TOPSQUAD (TOPOLOGICALLY PROTECTED AND SCALABLE QUANTUM BITS)

Reporting period: 2022-09-01 to 2023-08-31

Our vision is to enable the world of quantum computing through an unprecedented stable and scalable many-qubit system. Quantum computers will be able to process enormous amounts of data within an incredibly short time vs. millions of years that a classical computer would require. This long-awaited innovation can help solve many global challenges of our time related to health, energy and the climate, such as quantum chemistry problems in order to design new medicines, material property prediction problems for efficient energy storage, big data handling problems, needed for complexity of climate physics. Such a quantum computer has not yet been realized. There are two key reasons:
1. Qubit fragility: current qubit architectures are too fragile to withstand interference from the environment (e.g. noise). This ‘quantum decoherence’ means destruction of the information of the qubits and thus poses a tremendous challenge to build a system of thousands of coherent logical quantum bits.
2. Qubit scalability: the qubit architecture is not scalable enough: the required number of qubits for a universal quantum computer is far out of reach for all proposed quantum systems, including superconducting qubits, ion trap qubits and spin qubits. Depending on the qubit type and architecture, the estimated minimum number of building blocks varies from 10 thousand up to 100 million. The most successful platforms can make between 10 and 100 quantum bits.
In TOSPQUAD, we studied two scalable platforms: germanium-silicon wires and Ge quantum wells. We addressed the qubit fragility by aiming to create topological states - stable states of matter with properties that are not destroyed by local perturbances. We addressed the qubit scalability by developing waferscale fabrication technology, using CMOS-compatible processes.
We successfully realized all necessary ingredients for an engineered topological protected qubit and gained significant insights in the study the underlying physics, such as Majorana or Andreev bound states.
We obtained remarkable results and achievements in all work packages. In the high-quality wires grown at our TUE partner we demonstrated strong spin-orbit coupling and hole-hole interactions and we observed superconducting proximity effects. TUE in collaboration with nanoPHAB explored the growth of Ge/Si core/shell channels in Si(111) and has revealed the optimum parameters allowing the start of the electronic properties tests. Superconductors have been grown on the Ge/Si core/shell nanowires. The measurements on these nanowires performed at UT, UBAS and IST have shown high-quality transport. The UBAS partner developed a radio-frequency (RF) charge sensing technique with exploitation potential: a voltage-tunable capacitor (varactor) suitable for single-shot charge and spin readout of quantum dots. In a joint effort involving the UT, UBAS and IST partners, we have made significant progress in understanding and maximizing the magnetic field resilience of the induced superconductivity and we demonstrated gate tuneable transmon devices in both platforms studied in TOPSQUAD: Ge-Si core-shell nanowires and the Ge quantum wells. The very large tunability of the qubit frequency, strong coupling with high-quality superconducting resonators and relaxation and coherence times that are on par with state-of-the-art type III-V gatemon devices constitute major achievements.
The consolidation of results, novel findings and productive interactions between all the partners led to 16 articles published in high impact journals like Nature Nanotechnology, Nano Letters, Nature Review Materials and Physical Review Letters. Recently, another 2 manuscripts have been submitted and 7 are in preparation.
The potential of the two young high-tech SMEs, Basel Precision Instruments (BASPI) and nanoPHAB, the main suppliers of the TOPSQUAD technologies, has been boosted thanks to the exploitation activities that took place within the project. BASPI focusses on high-sensitivity experiments and nanoPHAB provides unprecedented nanophotonic technologies. At the final stage of the project, multiple categories of exploitation are foreseen: (1) nanofabrication services for quantum physics applications (lead nanoPHAB), (2) nanopatterning of silicon on germanium devices (lead nanoPHAB) (3) high precision instruments for quantum transport experiments - Gate Leakage Current Measurement Box and Low-Noise High-Stability Voltage Preamplifier (lead BASPI), and at longer term (4) proof of concept of topological qubits for quantum computing applications.
The universal quantum computer is a long-awaited innovation that can help solve many global challenges of our time related to health, energy, and the climate. Combining qubit fragility and qubit scalability in a single system as a tremendous step towards the realization of a quantum computer, has not been realized. In TOPSQUAD we addressed the corresponding challenges by aiming to obtain topological protection in two scalable platforms: germanium-silicon wires and Ge quantum wells. In order to obtain reproducible Ge-Si wires grown on Si wafers we combined the waferscale nanofabrication skills of the consortium with our individual research. We aimed to validate the potential and utility of this system, by showing that it can answer current and emerging scientific questions. We investigated fundamental issues such as proximity-induced superconductivity in semiconductor wires, and one of the hottest topics in modern condensed matter physics: Majorana Bound States.
TOPSQUAD was exceptionally successful in terms of conquering new grounds, most notably proximity effect in Ge, fast electrical spin qubit control with sweet spots, gatemons and preparing the grounds for Andreev spin qubits. We delivered a major achievement in quantum computation by demonstrating gate tuneable transmon devices in both type-IV platforms: in Ge quantum wells and GeSi core-shell nanowires.
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