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Quantum Large Scale Integretion in Silicon

Periodic Reporting for period 1 - QLSI (Quantum Large Scale Integretion in Silicon)

Reporting period: 2020-09-01 to 2022-02-28

QLSI, Quantum Large Scale Integration in Silicon is a 4-year project, which objective is to demonstrate that silicon spin qubits are a compelling platform for scaling to very large numbers of qubits. Our demonstration relies on four
ingredients:

• Fabrication and operation of 16-qubit quantum processors based on industry-compatible semiconductor technology;
• Demonstration of high-fidelity (>99%) single- and two-qubit gates, read-out and initialization;
• Demonstration of a quantum computer prototype, with online open-access for the community (up to 8 qubits available online);
• Documentation of the detailed requirements to address scalability towards large systems >1000 qubits.

To achieve these results, our consortium brings together an unrivalled multidisciplinary team of European groups in academia, RTOs and industry working on silicon-based quantum devices. These groups are committed to playing an active part in developing the industrial ecosystem in silicon-based quantum technologies.
QLSI is structured in three enabling toolboxes and one demonstration and scalability activity:

- the semiconductor toolbox brings together skills from the semiconductor industry such as fabrication, high throughput test and CAD (computer aided design) with the expertise of the physics community;
- the quantum toolbox gathers skills from the physics community on spin and quantum properties of Si based nanostructures and on quantum engineering from theory and experience perspectives;
- the control toolbox gathers teams with instrumentation skills ranging from RF signal generation, automation and set up of high throughput characterization at low temperature.

The toolboxes will generate stand-alone beyond the state-of-the-art results and will generate inputs to feed the demonstrator and scalability activity, which will integrate devices, hardware and software solutions to create an online open access demonstrator, to perform hybrid computation and to analyze scalability
As planned, during RP1, partners have pursued three paths in parallel: improve the quality and understanding of silicon qubits, establish reliable industrial fabrication processes and develop tools and modules to sustain further developments and assess scalability.
The project has already provided outstanding results towards its goal of demonstrating the ability of silicon technology to fabricate a large number of good quality qubits.
A linear 6-qubit system based on Si/SiGe have been operated by QuTech. In a 2-qubit system, the project has set up the state-of-the-art for two-qubit gates fidelity reaching 99.6%. This expected milestone was reached concomitantly with Australian and Japanese teams.
In addition, record Hahn-echo coherence time up to 88 μs for hole spin qubits have been measured by CEA-IRIG, it exceeds by an order of magnitude the best values reported so far for hole spin qubits
In parallel, the project has already laid solid technological foundations to massively fabricate Si spin qubits. The first step consists in transferring fabrication to industrial facilities to go from the scientific first to consistent and reproducible operation. For this statistical understanding is required to optimize technological parameters, to catch up with the other technologies, the consortium chose to develop three industrial fabrication technologies in parallel at Imec and CEA-Leti: Si/SiGe, Si/SiO2 on bulk devices and Si/SiO2 on SOI featuring at least two level of gates. IHP has developed versatile short loops fabrication flows to qualify materials and layouts.
Having these technologies has allowed for parallel evaluation and optimization of modules with a strong impact on performance: channel material, gate stack, comparison of electrical vs structural confinement, testing of different layouts and critical dimensions. The consortium in the next period will be able to measure, characterize and will help make decisions based on data to define the best technological fabrication process.
To quality the technology, there was a critical need for low temperature statistics. Within the consortium, we have established two kinds of statistical low T° platforms: first one relying on Bluefors 300mm cryoprobers (CEA-Leti one is in production and FhG is hooking its) or multiplexing boards (RF compatible has been developed by HEU).
In this first period, the project has built several tools to improve qubits quality, to provide automatic control and calibration and to help for assessment of the scalability.
On one hand, sophisticated superconducting amplifiers TWPA were developed (CNRS) to account from limitations from first generation of Josephson parametric amplifiers evidenced by HEU.
One the software side, a suite of simulation tools for qubits design relying both on home made and commercial softwares has been established (CEA-IRIG and Imec), collaborative calibration routines have been developed under FHZ responsibility. QM has developed QuEST, a simulator of universal quantum circuits, the objective is that the users will be able to input the desired topology and noise at the qubit level, creating a virtual silicon device that mimics the topology of the QLSI device for testing algorithms on it.
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