Parametric $\mathrm{CZ}$ Gates in a Tunable Tripod Architecture with Fixed-Frequency Superconducting Qubits
Efficient implementation of the operations required for quantum error correction in superconducting quantum computers requires high gate fidelity, adequate qubit connectivity, and selective control of interactions. Sharing a single coupler among multiple qubits can improve connectivity, but also requires accounting for its effects on qubits outside the target pair. In this work, we numerically investigate the feasibility of implementing parametric-resonance $\mathrm{CZ}$ gates in a tunable tripod structure consisting of three fixed-frequency transmon qubits connected to a single tunable coupler. We modulate the external flux applied to the coupler and optimize the pulse parameters for each qubit pair to construct $\mathrm{CZ}_{01}$, $\mathrm{CZ}_{02}$, and $\mathrm{CZ}_{12}$ gates. We calculate the time evolution of a Hamiltonian that includes three energy levels for each circuit element and evaluate gate fidelity and leakage after virtual Z correction. The fidelities obtained for the target-pair computational blocks, whose input and output basis states have both the coupler and spectator qubit in their ground states, are 0.9943, 0.9834, and 0.9907, respectively. We also find that population preservation at the end of the gate depends on the initial state of the spectator qubit. These results demonstrate the possibility of constructing $\mathrm{CZ}$ operations for each qubit pair through flux modulation of a shared coupler while maintaining fixed data-qubit frequencies. They also highlight the need to evaluate performance and optimize control over the full computational space, including the spectator qubit.