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Hole spin qubits are gaining popularity in quantum processors because they operate quickly and efficiently using only electric signals. Their strong spin-orbit interaction makes control easy but leads to inconsistencies, meaning qubit properties vary across different locations.
While these variations allow precise control of individual qubits, they must be carefully managed to ensure the system can scale effectively.
Researchers at ISTA, led by Georgios Katsaros, are exploring germanium as a key material for quantum processors. Unlike most materials that rely on electrons to carry charge, germanium uses “holes”—missing electrons that behave like positively charged particles.
This material has been important since the first transistor in 1947, yet some physics remains a mystery. Now, Katsaros and his team have made important discoveries about these holes, which could help improve quantum computing in the future.
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Electrons exist in all atoms, but when one leaves its orbit, it creates a “hole”—a region with a net positive charge. These holes can be deliberately designed and controlled in germanium, allowing them to behave like moving particles under electrical fields.
Interestingly, holes retain the magnetic properties of the missing electrons, known as “spins,” which act like magnetic shadows. These hole spins are essential for quantum computing because they can function as qubits—the basic units of quantum information.
Germanium’s ability to produce “hole spin qubits” makes it a promising material for semiconducting quantum processors.
In quantum systems, spins remain isolated when far apart but interact when brought close together, revealing unique quantum behaviors. Researchers led by Jaime Saez-Mollejo at ISTA explored how to control two-hole spins in germanium. They aligned and tuned these spins by adjusting magnetic and electric fields for better functionality.
Scientists solved a major hurdle in quantum computing
To achieve this, the team collaborated with Polytechnic University of Milan experts to fabricate a specialized chip. This chip allowed them to position two hole spins precisely in germanium, bringing them close enough to interact.
These confined locations, called quantum dots, are tiny, only tens of nanometers wide, similar to molecular dimensions. By creating two quantum dots, they formed a double quantum dot device, which functions as a qubit made from the magnetic “shadows” of missing electrons.
The duration a qubit retains quantum information—its coherence time—depends on how well the hole spins are adjusted and aligned. Scientists can fine-tune quantum states by identifying the factors that affect this alignment, ensuring qubits function reliably. Mastering this control is key to enabling quantum computation.
In a thin germanium layer, a magnetic field can be positioned in-plane (parallel to the material) or out-of-plane (perpendicular to it). Researchers used models, simulations, and experiments to study how hole spins behave under different conditions.
They discovered that spins align well when the magnetic field is out-of-plane, but the alignment shifts by 45 degrees when it’s in-plane. They found that spins respond differently to electric and magnetic fields.
Misalignment depends on the magnetic field’s direction and the electric field’s strength and direction. Collaborating with theorists at QuTech, they developed a method to measure spin misalignment under different conditions.
These findings help answer key questions about the future of quantum processors. Unlike electron spins, hole spin qubits don’t require extra micromagnets, making chip fabrication simpler. This could allow more qubits to be packed onto a processor, improving scalability.
Understanding how hole spins behave in germanium quantum devices could guide the design of advanced semiconducting quantum processors.
“We see this as a small step in the broader development of quantum computing—a tiny contribution that could still make a difference,” says Saez-Mollejo.
Journal Reference:
- Jaime Saez-Mollejo, Daniel Jirovec, Yona Schell, Josip Kukucka, Stefano Calcaterra, Daniel Chrastina, Giovanni Isella, Maximilian Rimbach-Russ, Stefano Bosco & Georgios Katsaros. 2025. Exchange anisotropies in microwave-driven singlet-triplet qubits. Nat Communications. DOI: 10.1038/s41467-025-58969-y
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