The two-qubit device used in the experiments, mounted on a Printed Circuit Board (PCB) set up in its dilution refrigerator. Credit: CEA-Irig.

Quantum computers, systems that process information leveraging quantum mechanical effects, could reliably tackle various computational problems that cannot be solved by classical computers. These systems process information in the form of qubits, units of information that can exist in two states at once (0 and 1).

Hole spins, the intrinsic angular momentum of holes (i.e., missing electrons in semiconductors that can be trapped in nanoscale regions called quantum dots), have been widely used as qubits. These spins can be controlled using electric fields, as they are strongly influenced by a quantum effect known as spin-orbit coupling, which links the motion of particles to their magnetism.

Unfortunately, due to this spin-orbit coupling, hole spin qubits are also known to be highly vulnerable to noise, including random electrical disturbances that can prompt decoherence. This in turn can result in the loss of valuable quantum information.

Researchers at Pheliqs, a joint laboratory between CEA and Grenoble University, devised a strategy that could enable the optimal operation of hole spin qubits.

Their proposed approach, outlined in a paper in Nature Physics, relies on the implementation of unique magnetic field directions, dubbed sweet spots, which make qubits resistant to electrical noise without affecting the ease with which they can be controlled.

"Across many qubit platforms, operation fidelities remain inherently limited by noise arising from coupling to the surrounding environment, posing major ordeals for the realization of large-scale quantum computers," said Drs. Vivien Schmitt and Marion Bassi, senior and first author of the paper, respectively.

"Understanding these main sources of noise and their implications on the qubit properties is a central interrogation motivating this work, which we could summarize in two points: can we apprehend how the qubit reacts to certain types of noise and can we exploit the anisotropy of the qubit properties to our advantage?"

Device and measurement of longitudinal spin-electric susceptibility. Credit: Nature Physics (2025). DOI: 10.1038/s41567-025-03106-1

Supporting the reliable tuning of spin hole qubits

Drs. Schmitt, Bassi and their colleagues set out to study how the operation speed and coherence of a hole spin qubit are influenced by electrical noise at different magnetic field orientations. To do this, they created a qubit comprised of a single hole trapped inside a silicon quantum dot, then applied a magnetic field to it in different directions and observed the effects of these differences in direction.

"The system we studied consists of a silicon nanowire, overlapped by gates very similar to a small MOSFET transistor, and voltages allow us to trap a single charge in the nanowire, at deep cryogenic temperatures," explained Dr. Schmitt.

"Under an external magnetic field, the spin of this charge is our qubit. A central question for a hole spin qubit is to know for how long and how fast we can perform quantum operations (gates) on our qubit in the presence of electrical noise."

The researchers uncovered the existence of magnetic field orientations that they call "sweet spots." At these orientations, a qubit becomes insensitive to electrical fluctuations, which in turn results in the enhancement of its coherence time.

"Our results demonstrate that this enhancement can be achieved while preserving a high driving efficiency, two key metrics usually antagonistic." said Dr. Bassi. "Motivated by these results, we further investigated to what extent changing the way the hole is trapped (i.e. changing the voltages applied on the near gates) would influence such resilient points with enhanced capabilities."

Drs. Schmitt, Bassi and their colleagues subsequently used the same experimental methods to tune a second qubit to the same sweet spot as the first. This test was successful, suggesting that their approach could be applied to larger systems containing a greater number of qubits.

The future upscaling of the team’s approach

In initial experiments, the team’s approach was found to reliably optimize the operation of two qubits, reducing their vulnerability to electrical noise without affecting their responses to electrical control signals.

While the researchers have so far applied their proposed strategy to silicon-based qubits, it could also be applied to qubits hosted in other superconducting materials, such as Germanium.

"Achieving insensitivity to the major noise source while boosting driving efficiency is the ideal scenario for any qubit system," said Dr. Schmitt. "In these experiments, we were able to tune our spin qubit in a win-win regime that can be seen as the equivalent of the transmon regime for superconducting qubits. We then demonstrated that multiple (2) qubits can be tuned to this regime simultaneously, both with high-fidelity operations."

In the future, this recent study could contribute to the realization of highly performing quantum computers based on hole spin qubits that can be reliably deployed in real-world settings.

Meanwhile, Dr. Schmitt and his laboratory are working on further strategies that address the susceptibility of qubits to noise and could reduce associated errors.

"Now that charge noise has been dealt with, we need to tackle the next source of noise, magnetic noise arising from nuclear spin in the material," added Dr. Schmitt.

"Two approaches can be envisioned, either brute-force by removing them, using isotopically purified silicon, or by carefully engineering the nuclear spin bath in which the qubits are living."

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Publication details

M. Bassi et al, Optimal operation of hole spin qubits, Nature Physics (2025). DOI: 10.1038/s41567-025-03106-1. On arXiv: DOI: 10.48550/arxiv.2412.13069

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Citation: Magnetic ‘sweet spots’ enable optimal operation of hole spin qubits (2026, January 22) retrieved 22 January 2026 from https://phys.org/news/2026-01-magnetic-sweet-enable-optimal-hole.html

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