Tunable coupler to fully decouple and maximally localize superconducting qubits

Enhancing the capabilities of superconducting quantum hardware, requires higher gate fidelities and lower crosstalk, particularly in larger-scale devices, in which qubits are coupled to multiple neighbors. Progress towards both of these objectives would highly benefit from the ability to fully control all interactions between pairs of qubits. Here we propose an alternative coupler model that allows dispersively detuned transmon qubits to be fully decoupled from each other, i.e., ZZ crosstalk is completely suppressed while maintaining a maximal localization of the qubits' computational basis states. We further reason that, for a dispersively detuned transmon system, this can only be the case if the anharmonicity of the coupler is positive at the idling point. A simulation of a 40-ns CZ gate for a lumped-element model suggests that achievable process infidelity can be pushed below the limit imposed by state-of-the-art coherence times of transmon qubits. On the other hand, idle gates between qubits are no longer limited by parasitic interactions. We show that our scheme can be applied to large integrated qubit grids, where it allows a pair of qubits to be fully isolated, that undergoes a gate operation, from the rest of the chip while simultaneously pushing the fidelity of gates to the limit set by the coherence time of the individual qubits.