Seminar - Valentin John, Delft University of Technology

Qubit characterisation in a ten-quantum dot array in Germanium

Over the past years, semiconductor spin qubits have advanced significantly, enabling demonstrations of small quantum processors across various platforms. To push beyond these systems, larger quantum dot arrays are being developed, though characterizing qubit parameters across such extended arrays remains a critical challenge. Here, we present results from two experiments performed on a two-dimensional ten-quantum-dot array in Ge/SiGe heterostructures.

In the first experiment, we perform spin-coherent shuttling. We initialise two spins in a double dot, and shuttle one of them across the full array. For each quantum dot along the path we observe spin coherent oscillations induced by the difference in quantization axes of the quantum dots. This allows the characterisation of the g-factors of all quantum dots within the ten-quantum dot array solely by means of shuttling. We observe ultra-low g-factors with values of 0 to 0.07.

In the second experiment, we initialise a spin in each quantum dot. We perform an in-depth characterisation of the system obtaining statistics in various properties of each of the ten qubits for different charge occupations.  Our study encompasses g-factors and coherence times (T2*), with the former being significantly larger () than in the previous device. We then obtain the spin susceptibility and Rabi driving strength of each qubit when driven by 22 different gates. Our observations across all qubits suggest that systems with three holes per quantum dot offer significant advantages in terms of qubit control and performance.

Finally, we address the challenges posed by large g-tensor anisotropies and outline future strategies to mitigate their impact.

Bio:

Valentin John, originally from Hamburg/Germany, is a PhD candidate in the Veldhorst lab at QuTech and Delft University of Technology, where he has been working since October 2021. His research focuses on control and characterisation of extended spin qubit arrays in Germanium. He earned a Master’s degree in Nanoscience and Nanotechnology in 2021 from KU Leuven and Université Grenoble Alpes, with a thesis at CEA Grenoble under supervision of Romain Maurand. He completed his Bachelor’s in Physics at the University of Hamburg in 2019.