A High-Fidelity Gateset for Exchange-Coupled Singlet-Triplet Qubits

A key ingredient for a quantum computer is the accurate manipulation of qubits in order to generate high-fidelity gates. For S-T₀ qubits in semiconductor quantum dots, which allow purely electric control with moderate bandwidth requirements, single-qubit gates with fidelities above the error correction threshold were demonstrated, whereas two-qubit operations have not reached the required fidelity [1-2].
Here, we numerically optimize a complete two-qubit gate set under realistic experimental constraints, exploiting exchange coupling while also accounting for capacitive coupling. We obtain fidelities of 99.9% for GaAs, while about 99.99% are achieved with vanishing magnetic field noise as in Si. We suppress leakage to 10⁻⁵ by choosing high inter-qubit magnetic field gradients.
For optimized parallel single-qubit gates, we find that inter-qubit capacitive coupling needs to be considered to avoid undesired entanglement, but is mitigated by interleaved operation. Realistic levels of residual inter-qubit exchange coupling are compensated for by our gates.
We will also report on progress of the realization of such gates in a GaAs device with two exchange-coupled S-T₀ qubits.
[1] Unpublished results relating to Cerfontaine et al., arXiv:1606.01897
[2] Nichol et al., npj Quantum Inf. 3 (2017)