Crosstalk in qubit control lines

As superconducting quantum computing architectures scale up, the increasing density of transmission lines may cause signal leakage through electromagnetic coupling, which induces spurious rotations in neighboring qubits, thereby reducing operation fidelities. In this work, we introduce a new, general model for crosstalk in an arbitrary number of qubit control lines, based on the theory of coupled transmission lines. We apply this to the instructional case of two coupled lines connected to a system of two coupled transmon qubits. We derive a direct relationship between the coupling parameters of the lines and the resulting crosstalk, as well as the Hamiltonian governing state evolution. Through analytical and numerical pulse simulations, we report the fidelity loss induced by cable crosstalk in both arbitrary qubit rotations and the cross-resonance gate, which together comprise a universal gate set. To conclude, we discuss the more general case of an N-qubit system.