Magnetic-Field Effects on Error and Leakage in Randomized Benchmarking of a Si/SiGe Triple-dot Qubit

The exchange-only, triple-dot qubit is fully controllable through voltage-based modulation of the exchange interaction between neighboring dot pairs, requiring no magnetic gradients or magnetic resonance. Nevertheless, several magnetic effects play important roles in errors and leakage of the qubit under operation, whether by dephasing for an idle qubit or during a multi-pulse experiment such as randomized benchmarking. This talk discusses the magnetic-field-dependent impacts of three types of magnetic gradients: magnetic screening gradients due to the Meissner effect in superconducting gates, interface-spin-orbit effects, and contact hyperfine interactions including the full vector of nuclear magnetization. While Meissner and spin-orbit gradients are suppressed by operating near zero magnetic field, we find that gradients oriented in directions transverse to the field, in particular the flip-flop terms of the hyperfine component, play a more dominant role. Isolating the effects of magnetic gradients on qubit performance is aided by modifications to the randomized benchmarking protocol that allow reliable extraction of the proportion of total error due to leakage, which is a signature of magnetic gradient effects in exchange-only qubits.