Light-mass hole-spins in a Ge quantum well as qubits*

Electrons bound to quantum dots provide well defined (spin 1/2) physical qubits. Under strong spin-orbit coupling spin qubits are expected to suffer rapid dephasing from charge noise; however, recent experiments have effectively employed spin-orbit coupling in coherent quantum gates. Here we show that hole spins in a Si_1-xGe_x/Ge/ Si_1-xGe_x quantum well possess highly desirable properties as qubits, including a high natural abundance of nuclear spin-0 isotopes, a large (~100 meV) intrinsic splitting between the light and heavy hole bands, and a very light (~0.05m₀) effective mass. We employ ab initio methods to determine the band structure of strained Ge quantum wells as a function of x, and we discuss the advantages and challenges of using Ge hole spins as qubits.