Fabrication, measurement and coherent control of undoped Ge/SiGe quantum dot spin arrays

Tzu-kan Hsiao

Electrostatically-defined quantum dot arrays are a natural and versatile platform for quantum computation and quantum simulations. In particular, spin qubit arrays based on undoped Ge/SiGe quantum wells have attracted tremendous research interest due to their long coherence time and strong spin-orbital coupling. In this talk, I will present our progress in the fabrication and measurement of Ge quantum dot devices.

First, we present a new design of Ge quantum dot devices with a global accumulation gate and depletion-style fine gates. The device fabrication consists of only three metal layers, two oxide layers, and needs only micrometer alignment accuracy. We measured these devices and successfully observed Coulomb blockade and Coulomb diamond both in DC transport and RF reflectometry. In addition, we used the Coulomb peaks to characterize the charge noise and obtained a noise power spectrum density of 1.1μeV/√Hz at 1Hz, confirming the quality of the material and device.

Next, we measure the energy spectrum of two spins in a Ge double dot system. By initializing a singlet state and exciting it into triplet states using microwave burst, we map out the singlet-triplet energy spectrum as a function of external magnetic field. We also achieve the coherent spin control between T ⁻ and T ⁰ triplet states with T₂ ^Rabi~10 μs and T₂^∗~2 μs.

Finally, we propose an experiment to study quantized spin wave (magnon) modes, which play an important role in spintronics and quantum information processing. Their long-range spin order makes the spin waves a good candidate for observing long-range spin correlations. We will describe experimental methods for preparing and probing the quantized spin wave modes in accessible quantum dot simulators.