Gates, Readout, and Simulation with Spin Qubits

The ability for optically active media to rotate the polarization of light is the basis of polarimetry, an illustrious technique responsible for many breakthroughs in fields as varied as astronomy, medicine and material science.  In our work, we recast the primary mechanism for spin readout in semiconductor-based quantum computers, Pauli spin-blockade (PSB), as the natural extension of polarimetry to the third dimension.

We perform polarimetry with spins through a silicon quantum dot exchanging a hole with a Boron acceptor, illustrating the role of spin-orbit coupling in giving rise to spin misalignment. Akin to the humble polarimeter, the misalignment may be tuned by varying an external degree of freedom, the applied magnetic field direction, and we demonstrate how it can be rotated to a magic angle to recover perfect spin alignment and re-establish PSB.

Finally, we discuss the effect of spin misalignment on spin readout, which sets a fundamental upper limit for the fidelity of PSB-based spin readout. Its dependence on sample-to-sample and device-to-device variability poses serious challenges for the feasibility and scaling of quantum computing architectures in the presence of strong spin-orbit coupling.