Crosstalk-Robust Quantum Control in Multimode Bosonic Systems

High-coherence superconducting cavity modes offer an efficient platform for quantum information processing. To realize universal control of these bosonic modes, a transmon ancilla is typically coupled to the cavity, thereby introducing required nonlinearities. However, this configuration is susceptible to crosstalk errors in the dispersive regime, where the ancilla frequency is Stark-shifted by the state of each coupled bosonic mode. This results in a frequency mismatch of the ancilla drive, lowering overall gate fidelities. To mitigate such coherent errors, we employ quantum optimal control to engineer ancilla pulses that are robust to the frequency shifts. These optimized pulses are subsequently integrated into a recently developed two-mode echoed conditional displacement (ECD) protocol. Through numerical simulations, we examine two representative scenarios: the preparation of single-mode Fock states in the presence of spectator modes and the generation of two-mode entangled Bell cat states. Our approach markedly suppresses crosstalk errors, outperforming conventional ancilla control methods by orders of magnitude. These results not only provide guidance for experimentally achieving high-fidelity multimode operations, but also substantiate bosonic systems as competitive candidates for high-performance quantum information processors.