Closed-loop Optimization for high-fidelity Controlled-Z Gates in Superconducting Qubits

Qubit operations must be highly accurate and fast to fully exploit the potential of quantum computing. Here, we use a superconducting qubit architecture with fixed-frequency qubits and tunable coupling elements. In this architecture, adiabatic coupler-activated controlled-phase gates, in combination with single-qubit gates, promise high-fidelity qubit operation. However, using simple calibration routines limits the control complexity of the two-qubit gates and may lead to uncontrolled leakage or decoherence effects during the gate. We use a closed-loop optimization based on two-qubit randomized benchmarking sequences with adaptive sensitivity. We calibrate and compare various pulse shapes ranging from minimal parameter sets to piecewise-constant pulse shapes with high complexity. We find that the choice of pulse parametrization can elevate the gate fidelity, decrease the gate time, and reduce leakage by exploiting the full system dynamics and accommodating the transfer functions of the cryogenic cabling. Using these techniques, we obtain controlled-Z gate fidelities of up to 99.9%.