Qubits Based on Hole Quantum Dots in Strained Ge.

We argue, supported by ab initio calculations, that holes in a SixGe1-x/Ge/ SixGe1-x quantum well possess highly desirable properties as qubits, including a large (>100 meV) intrinsic splitting between the light and heavy hole bands, a very light (0.05m0) effective mass parallel to the Ge well interfaces (potentially leading to high mobilities and tunnel rates, as well as relaxed fabrication demands regarding dot size), and a high natural abundance of nuclear spin-0 isotopes. Compared to electrons in quantum dots, such hole qubits benefit from larger size, and do not suffer from the presence of nearby quantum levels (e.g., valley states) that can compete with electron spins as qubits. The strong spin-orbit coupling in Ge quantum wells may be harnessed to implement electric dipole spin resonance, leading to gating times of several nanoseconds for single-qubit rotations. The microscopic mechanism of such spin-orbit coupling is discussed, stressing the relevance of coupling terms stemming from the underlying cubic crystal field.