Hole Spin Qubits in Ge/Si Core/Shell Nanowires

Spins in semiconductor quantum dots are among the most promising candidates for the realization of a scalable quantum bit (qubit), the basic building block of a quantum computer. Quantum dots in the common semiconductors Si and Ge profit from the compatibility with industrial microelectronic technologies, a small footprint, and thermal stability. Moreover, the low amount of isotopes with nuclear spin is beneficial for the qubit coherence. The rich physics in the valence band of Ge gives rise to particular properties which make holes attractive for the implementation of hole spin qubits. In particular, the strong spin-orbit interaction, termed direct Rashba spin-orbit interaction, that arises in one-dimensional Ge/Si core/shell nanowires due to the admixture of heavy hole and light hole states is promising for very fast qubit gates and all-electrical qubit control. In order to implement a spin qubit, a large degree of control over quantum dots and the spins confined in it is essential. We demonstrate the formation of single, double and triple quantum dots in Ge/Si core/shell nanowires. In a single quantum dot, we observe indications for single hole occupation. Furthermore, the transport through a double quantum dot at an effective (1,1)-(0,2) charge transition is governed by Pauli spin blockade, which leads to current rectification. In presence of spin-orbit interaction, the blockade is lifted at finite magnetic field and leads to a leakage current. The study of the leakage current as a function of external magnetic field and double quantum dot detuning yields information about the dominant lifting mechanisms. Here, we observe pronounced orbital effects and a renormalization of the g-factor which arises in presence of strong spin-orbit interaction. A spectroscopic model accounts for all these effects and allows to extract a spin-orbit interaction length of lSO = 65nm in a Ge/Si core/shell nanowire quantum dot. Finally, spin-orbit interaction is used to drive electric dipole spin resonance of a hole spin qubit in a Ge/Si core/shell nanowire. We demonstrate coherent Rabi oscillations and two-axis single qubit control. Important qubit parameters such as the Rabi frequency and the g-factor can be tuned over a wide range by changing the gate voltages. This tunability arises from the electric field dependent spin-orbit interaction in Ge/Si core/shell nanowires. In an optimal configuration, the Rabi frequency increases to 435 MHz at a drive frequency of fMW = 3.4 GHz, thus almost entering the strong driving regime. The results shown here demonstrate the suitability of Ge/Si core/shell nanowires to implement a hole spin qubit which can be electrically switched between a control state, enabling fast qubit gates, and an idle state, prolonging qubit coherence.