Reflections from Paris: Scaling of Spin Qubits 2025

On May 16th, we hosted the second edition of the Scaling of Spin Qubits Workshop (SSQ25) at the amphithéâtre Jaurès at the École Normale Supérieure at the heart of Paris. Co-organized by C12, Quantum Machines, TU Delft, Delft Circuits, and ENS, the workshop brought together over 130 participants for a day of deeply technical discussion, meaningful connection, and a shared pursuit: building a scalable quantum future with spin qubits.

‍

We welcomed twelve speakers from across academia and industry, including:

• Prof. Giordano Scappucci (TU Delft)
• Inga Seidler (IBM Research Europe – Zurich)
• Dr. Marion Bassi (QuTech)
• Prof. Kristiaan de Greve (IMEC)
• Prof. Fernando González Zalba (Quantum Motion)
• Prof. Arne Laucht (Diraq, UNSW)
• Prof. Guido Burkard (University of Konstanz)
• Dr. Romain Maurand (CEA Grenoble)
• Prof. Pasquale Scarlino (EPFL)
• Prof. Natalia Ares (University of Oxford)
• Dr. Jesse Hoke (HRL Laboratories)
• Dr. Irene Fernandez De Fuentes (QuTech)

Each talk touched on the frontiers of materials, scaling, connectivity, and control. In this blog, we would love to share the key learnings that emerged—and that continue to shape our thinking at C12.
‍

Building the Eiffel Tower of Quantum

The metaphor that resonated most came from Dr. Irene Fernandez De Fuentes:

"To build a quantum processor is like building the Eiffel Tower."

It demands three pillars—solid foundations, modularity, and connectivity.

A Solid Foundation

Materials remain critical for scalable spin qubits:

• Giordano Scappucci emphasized that achieving high-performance 2DEGs requires isotopic purification, channels far from oxide interfaces, and reduced dislocation densities. He has demonstrated great improvement in germanium 2DEG characteristics, with an improved stack of material.
  ‍
• Inga Seidler showed that germanium systems have experienced unexpected spin-orbit effects and g-factor renormalization—highlighting the importance of advanced material characterization at ultra-low temperatures.
  ‍
• Pasquale Scarlino added that microwave resonators can reveal fine spectral features caused by electron–electrons interactions, underlining the sensitivity required in characterising quantum devices at ultra-low temperature.
  ‍
• Complementing this, Natalia Ares demonstrated how machine learning techniques can help mitigate variability—especially disorder—in qubit devices, offering scalable solutions for robust and automated control.

• Kristiaan de Greve reminded us that all the metal stacks are in competition with some pros & cons, so it's important to develop a modular approach for industrialisation development. Spin qubits are advantageously compatible with a foundry process, but some developments are still required to bridge the gap between academia and industrialisation. He highlighted the promising performance of qubits developed at IMEC and measured by the start-up Diraq, with gate set tomography showing fidelities above 99%. 
  ‍
• Arne Laucht further emphasized one of the key strengths of spin qubits—their robustness with temperature—demonstrating high-fidelity operation up to 1K.
  ‍
• Irene Fernandez De Fuentes showcased impressive results on a six-qubit array, where the fidelity of algorithms is now limited by the coherence time of idle qubits.
  
  

The field is moving fast, but materials engineering remains a critical focus—and the very foundation of scalable quantum design.

Modular Architecture and Robust Connectivity

Architectural shifts and strategies to enable long-range coupling are reshaping the landscape:

• Electron shuttling strategies—discussed by Marion Bassi—are improving. She presented a method that overcomes one of the key limitations of electron shuttling: the requirement to shuttle faster than the coherence time. Her results showed that shuttling an electron over 1.2 micrometers in the diabatic regime remains coherent and is equivalent to a Z×Z gate that can be calibrated for each transfer path.
  ‍
• The shift from 2D grids to multi-linear arrays is gaining traction, offering architectural modularity, enabling individual shuttling for each electron—and simplifying the integration of readout components. 
  ‍
• As underlined by Fernando González Zalba, fast and high-fidelity readout will be key for error correction. He presented a range of dispersive and dissipative reflectometry solutions advancing this goal.

• Microwave resonators are one of the most promising solutions to have fast and high-fidelity readout. Romain Maurand presented fantastic progress in this field, with high-impedance resonators enabling fast and coherent control of spin qubits.
  ‍
• Together with Pasquale Scarlino, they showed qubit quality factors close to 400, compatible with single-qubit gate fidelities of 99.9%, and a cooperativity of 100. This architecture, based on microwave links, helps address connectivity challenges.
  ‍
• Guido Burkard further illustrated how such an approach can be extended beyond flip-flop qubits—as used by Romain Maurand—to other types, such as singlet-triplet qubits.
  ‍
• Circuit QED featured prominently, with multiple speakers highlighting its role in advancing scalable, high-fidelity readout.
  
  

Together, these insights suggest that spin qubits are advancing both as individual high-performance units and as part of increasingly integrated, scalable processor architectures.

People, Posters, and Paris

The day included more than talks. A vibrant poster session and shared meals let researchers connect informally, exchange feedback, and spark new collaborations. The day was filled with curiosity and connection, from the registration desk at 9 am to the final toast at dinner.

Thank you to all who joined us—from near or far, on stage or in the audience. You made this event a success.

A special thanks to our scientific committee: Anasua Chatterjee, Fabio Ansaloni, Matthieu Delbecq, Matthieu Desjardins, and Nikolai Drobotun.

Where Spin Qubits Stand—And Where They're Headed

SSQ25 reminded us that scaling quantum technologies is a long game that demands rigor, collaboration, and patience to solve problems layer by layer. From foundational materials to modular designs and new approaches to readout and control, it's clear that progress in spin qubits is accelerating.

This is not just a race for more qubits. It's about engineering systems where every element—materials, architecture, control—holds up under pressure. As Irene Fernandez De Fuentes aptly put it, building a scalable quantum processor is like building the Eiffel Tower: a feat of precision, vision, and trust in the strength of your foundations.

At C12 and across this community, we're committed to that kind of engineering. We're grateful to all speakers and participants who shared their ideas, challenged assumptions, and helped move the field forward.

We'll be back—stronger, clearer, and higher. Until then, stay curious.

Interested in learning more about why C12 is Unique at Scale? Read more on our technology page or contact us at solutions@c12qe.com. 

Work With Us

At C12, we don’t just build quantum computers—we build a culture shaped by excellence, curiosity, care, and scientific integrity. Whether it’s attending international conferences or organizing events like SSQ25, we actively support our team’s growth and visibility in the quantum ecosystem. If you're passionate about building the future of quantum and want to work in the heart of Paris (yes, steps from the Pantheon & ENS), explore our open roles on our careers page.

‍