Comprehensive explanation of ZZ coupling in superconducting qubits

A major challenge for scaling up superconducting quantum computers is unwanted couplings between qubits, which lead to always-on ZZ couplings that impact gate fidelities by shifting energy levels conditional on qubit states. To tackle this challenge, we introduce analytical and numerical techniques, including a diagrammatic perturbation theory and a state-assignment algorithm, as well as a refined intuitive picture for the workings of the ZZ coupling. Together, these tools enable a deeper understanding of the mechanisms behind the ZZ coupling and facilitate finding parameter regions of weak and strong ZZ coupling. We showcase these techniques for a system consisting of two fixed-frequency transmon qubits connected by a flux-tunable transmon coupler. There, we find three types of parameter regions with zero or near-zero ZZ coupling, all of which are accessible with current technology. We furthermore find regions of strong ZZ coupling nearby, which may be used to implement adiabatic controlled-phase gates. Our methods are applicable to many types of qubits and open up for the design of large-scale quantum computers with improved gate fidelities.