Spin blockade in hole quantum dots: Tuning exchange electrically and probing Zeeman interactions

Spin-orbit coupling is key to all-electrical control of quantum-dot spin qubits, and is often much stronger for holes than for electrons. The recent development of high-quality hole nanostructures has generated considerable interest in hole-spin qubit architectures [C. Kloeffel and D. Loss, Annu. Rev. Condens. Matter Phys. 4, 51 (2013)]. Yet hole-spin quantum computing hinges on the ability to discriminate between competing Zeeman terms and on the understanding of the complex interplay between the Zeeman and spin-orbit interactions, which are probed via Pauli spin blockade. Here we investigate spin blockade for two heavy holes in a gated double quantum dot in an in-plane magnetic field B. We find that the leakage period as a function of the field orientation is critically dependent on the relative magnitude of Zeeman interaction terms linear and cubic in B, exhibiting a beat pattern when the two are comparable in magnitude, and providing an effective way to discriminate between the two. Moreover, in certain materials the singlet-triplet exchange splitting is highly tunable by an appropriate choice of field direction, yielding a straightforward control variable for quantum information processing. These findings should stimulate new experiments on hole qubits.