Fault-tolerant quantum computing requires quantum gates that combine short duration, high fidelity, and robustness to noise. As quantum processors scale, robustness becomes increasingly important because exhaustive characterization and frequent recalibration of individual gates become impractical. We present a comprehensive optimal-control framework for controlled-Z (CZ) gates between directly coupled, frequency-tunable transmons that systematically accounts for intrinsic Hamiltonian errors and realistic decoherence mechanisms, including energy relaxation and dephasing. Using smooth, single-shot flux pulses, we obtain 12-ns CZ gates---close to the theoretical speed limit of 11.6 ns---with intrinsic errors suppressed to the
2×10−5 level. We construct dephasing noise models, calibrated against published experimental measurements, that capture fast dephasing from room-temperature control electronics and slow dephasing from
1/f flux noise. Using these models, we identify dephasing sweet spots that suppress sensitivity to dephasing noise. We further exploit transmon asymmetry as a design parameter to access distinct noise-resilient operating regimes. These results establish an experimentally applicable framework for balancing gate speed, fidelity, and noise robustness while reducing characterization and recalibration overhead.
Circuit QED elements can facilitate entanglement generation between distant spin qubits, acting as a quantum bus to connect different parts of future spin-based quantum processors. Here, microwave-activated phase gates between two remote, gate-defined spin qubits, where an offset-charge-sensitive (OCS) transmon coupler is driven, are proposed. The spin qubits, specifically resonant-exchange (RX) qubits, are longitudinally coupled to the coupler such that the coupler’s frequency depends on the logical states of the qubits. The capacitance network model is used to describe the coupling between the quantum dots and the coupler, using first-quantized charge operators instead of second quantized field operators. Two pulse sequences are investigated: a faster, off-resonant drive pulse with constant amplitude, and a pulse with envelope engineering that incorporates dynamical decoupling to mitigate charge noise. The main source of decoherence for the system is the 1/f ᵝ charge noise, with β ∼ 1. The non-Markovian time-domain simulations in the presence of charge noise are employed to accurately predict the gate performance. Simulation results suggest that a CZ gate fidelity exceeding 90% is possible with realistic parameters and noise models.