Scalable Qubit Readout to Resolve Superconducting Quantum Computing’s Skeleton in the Closet

Project Information

SQELETON

Grant agreement ID: 101214315

DOI

10.3030/101214315

EC signature date 26 March 2025

Start date 1 May 2025

End date 31 October 2027

Funded under

• The European Innovation Council (EIC)

Total cost

No data

EU contribution

€ 2 479 570,25

Coordinated by

SILENT WAVES
France

Periodic Reporting for period 1 - SQELETON (Scalable Qubit Readout to Resolve Superconducting Quantum Computing’s Skeleton in the Closet)

Reporting period: 2025-05-01 to 2026-04-30

Summary of the context and overall objectives of the project

Quantum computing is widely recognized as one of the most transformative technologies of the coming decades, with the potential to revolutionize fields ranging from drug discovery and materials science to cryptography and optimization. Among the various hardware approaches being pursued, superconducting quantum computers have emerged as one of the most promising platforms, with heavy investments from major technology players and research institutions. However, scaling these systems beyond a few hundred quantum bits (“qubits”), a prerequisite for achieving practical quantum advantage, remains an enormous engineering challenge.
One of the most critical yet underappreciated bottlenecks lies in the qubit readout chain. In current superconducting architectures, the readout chain spans from the output of the quantum processing unit (QPU) at temperatures close to absolute zero, all the way up to room temperature electronics. Along this chain, multiple bulky and expensive components hamper larger scale integration. As the number of qubits grows, so does the complexity and physical footprint of the readout chain, making it one of the principal obstacles to building truly scalable quantum computers.
This SQELETON project, which stands for Scalable Qubit Readout to Resolve Superconducting Quantum Computing’s Skeleton in the Closet, directly addresses this challenge. Funded by the European Innovation Council under its Transition program, the project is led by Silent Waves, a deep-tech startup based in Grenoble, France, that has spun out of the Institut Néel lab (CNRS) more than four years ago, in January 2022. Silent Waves has developed a pioneering device called the Travelling Wave Parametric Amplifier and Isolator (TWPAI), which combines forward amplification and backward isolation into a single compact component at cryogenic temperatures. This has the potential to drastically alleviate the size, complexity, and cost of qubit readout infrastructure, paving the way for a scalable generation of readout hardware.
The ultimate ambition of SQELETON is to bring the TWPAI from laboratory demonstration to validation in a real use case by the end of the first project year, then progressing towards a validated market-ready prototype by 2028. In doing so, the project contributes directly to Europe’s strategic objectives in quantum technologies, supporting the goals of the European Quantum Flagship and reinforcing the continent’s technological sovereignty.

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

In the first year of the project, Silent Waves focused primarily on pushing its TWPA/I development workflow towards better performance tracking and reproducibility.
Test structures for low-temperature characterization have been developed and added systematically on fabricated wafers to monitor their quality. A low-frequency sample holder was developed to characterize material properties like superconducting gap or critical current and temperature. Besides, high-frequency resonators were developed to monitor microwave loss properties of the materials. These test structures allowed to demonstrate a 20-fold improvement in dielectric loss thanks to a newly developed dielectric deposition process for silicon nitride (SiN) in place of usual alumina (Al2O3), relieving a key limitation of Silent Waves TWPAs & TWPAIs.
On the software side, Silent Waves established a comprehensive modeling and simulation framework for TWPAI development and validated it thanks to the abundant data available from commercial products. Different techniques were implemented and compared, resulting in the adoption of an open-source frequency-domain package as the most relevant tool for TWPA/I development. Aside from modeling and simulation, new data management software has been developed to centralize all information between fabrication and characterization, aggregating data from different instruments and formats into a single unified database. This infrastructure is expected to improve device yield and accelerate design cycles.
Finally, the low-temperature TWPAI performance has been verified in a real use case, where it enabled improved readout fidelity in a two-qubit multiplexed configuration, without critical reduction of qubit coherence times despite the absence of isolator. This ticks the first key milestone of the project by demonstrating that the TWPAI, characterized in a standalone fashion before the start of project, actually performs as expected in a more realistic environment.

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)

The most significant result is the successful demonstration of two-qubit readout using the TWPAI without any external isolator. In state-of-the-art superconducting quantum computers, isolators are indispensable components at the coldest stage of the readout chain, each adding bulk, cost, and integration complexity. Demonstrating that the TWPAI can simultaneously provide amplification and isolation in a single compact device, in a multiplexed configuration without amplifier re-tuning, while preserving qubit coherence, represents a meaningful step forward for the field and validates the core premise of the SQELETON project.
A second notable result is the 20-fold improvement in microwave loss achieved through a combination of material innovations, including a new dielectric deposition process and a new material for the top ground plane. Reducing microwave loss at cryogenic temperatures is a long-standing challenge in the fabrication of superconducting quantum devices, and the magnitude of this improvement is significant in the context of the field, especially for Silent Waves TWPAs where it was one of the main limitations.
Looking ahead, the main needs to ensure further uptake and impact of these results are threefold. First, continued technical development is required to integrate the TWPAI with other on-chip components and demonstrate system-level performance in a realistic quantum computing environment, which is the focus of the remaining project efforts. Second, access to commercial quantum processors for validation will be critical to establish the technology’s credibility with potential customers. Finally, as the quantum computing industry moves towards standardization of hardware interfaces and components, early engagement with standardization bodies and key industry players will be important to position the TWPAI as a reference solution for scalable qubit readout.