Germanium-based quantum dots attract attention thanks to high hole mobility and strong spin-orbit coupling, which allows fast spin control. However, charge noise caused by charge trapping at the SiGe-dielectric interface limits the coherence time of spin qubits. Previous attempts to solve the problem included surface treatment and changing the barrier thickness, but their effectiveness remained insufficient. This work is in line with the search for low-noise materials for scalable quantum processors.
The researchers used molecular beam epitaxy to grow SiGe/Ge heterostructures with a graded composition of the top barrier. The composition changed from Si0.2Ge0.8 to Si0.8Ge0.2 over a thickness of 25 nm, which smoothly lowered the valence band edge. For comparison, a structure without composition modulation was grown. Characterization was performed using transmission electron microscopy, X-ray diffraction, and Hall bar measurements at 2 K. Quantum dots were fabricated using electron-beam lithography and measured at 60 mK in a dilution refrigerator.
Hall measurements showed that composition modulation widens the range of stable gate voltage by a factor of 2.75 compared to a conventional structure. In the quantum dots, the average charge noise at 1 Hz was 0.46 µeV/√Hz, which is the lowest value for Ge quantum wells on silicon substrates. At 10 mHz the noise was 6.77 µeV/√Hz, and when tracking the Coulomb peak at 1 mHz it was 46.48 µeV/√Hz. Stability diagrams in the few-hole regime showed no drift of the transition voltage over 100 minutes.
The work demonstrates that barrier composition modulation effectively suppresses charge noise in Ge quantum dots. This differs from previous approaches that focused on surface treatment or barrier thickness but did not solve the problem energetically. This method opens a path to scalable spin qubits with long coherence times.
Further improvement is possible by using germanium substrates to reduce lattice mismatch and increase the barrier thickness. It is also promising to combine composition modulation with optimization of the dielectric and gate geometry. In the future, such structures could be integrated into multi-qubit arrays.
The results will influence the development of quantum processors based on germanium hole spin qubits, as well as technologies for growing low-noise semiconductor heterostructures.
The next step will be to reproduce the technique on germanium substrates and verify spin coherence in qubits fabricated from such structures.
The work is directly related to the problem of decoherence in semiconductor spin qubits, where charge noise is the main limiting factor. Suppressing noise at the material level opens a path to increasing coherence time without complex magnetic field calibration.
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What the work does
The work proposes a SiGe/Ge heterostructure with a graded composition in the upper barrier to suppress charge noise in germanium quantum dots. Measurements showed a record-low charge noise of 0.46 µeV/√Hz at 1 Hz and stability in the few-hole regime over several hours.
Where its strength is
Modulating the barrier composition smoothly lowers the valence band edge, which energetically solves the problem of charge accumulation at the interface with the dielectric, unlike previous approaches that focused on surface treatment or barrier thickness.
Where it could go next
Worth trying
Why this may matter
The developed method for suppressing charge noise could become the foundation for scalable spin qubits with long coherence times, bringing the practical realization of quantum processors based on germanium quantum dots closer.
Nearby works in our archive
The archive is sparse around this work: the closest review is only 0.59, against a usual 0.64. It looks like few have drilled nearby.
The charge noise in these devices is so low that the transition voltage between the zero-hole and one-hole states does not change for nearly two hours, even though such transitions usually drift within minutes.