Towards 2-Qubit Operations of Hole Spins in Ge/Si Nanowires above 1K

We report on two highly electrically-tunable hole-spin-qubits in a Ge/Si core-shell nanowire device operated at 1.5 K

We present efforts on improving the electrostatic environment of the double quantum dot (DQD) employing low-temperature annealing of the gate dielectric. By comparing crucial qubit parameters such as T₂^Rabi and T₂^* from different devices with annealed and non-annealed gate dielectrics, we can narrow down the location of defects responsible for qubit decoherence, likely to reside within the nanowire or its shell oxide. These results underline the importance of well-controlled nanowire growth and oxidation.

Further, we investigate the effect of the tunnel barrier voltages and microwave driving power on different qubit metrics such as the g-factor, f₂^Rabi, T₂^Rabi, T₂^* and T₂^Hahn, and find potential sweet spots for operation with improved coherence even when operating the qubit at Rabi frequencies above 100 MHz.

Finally, for a particular DQD occupation we can individually address either of the tunnel-coupled hole spin qubits and manage to tune the g-factors from 2 to above 4 without affecting the neighboring qubit’s g-factor, by local control of the electrostatic potential. We also can tune the DQD into a regime in which the exchange coupling J can be ramped from 50 MHz to 200 MHz across a detuning of 15 mV, thus laying the foundation for exchange-based two-qubit operations.