Cross Resonant CNOT Gates in Hybrid Fluxonium-Transmon Systems

We analyze a dual-species, capacatively coupled fluxonium-transmon system. Our coherent simulations of selective darkening CNOT gates [1,2] with fluxonium control and transmon target qubits achieve fidelity exceeding 0.9999 and gate durations below 50 ns. We present applications of such fluxonium-transmon gates as building blocks for facilitating high-fidelity fluxonium-fluxonium gates and performing fluxonium readout. Additionally, the effective coupling between fluxoniums is weak and generates ZZ phase accumulation rates below 1 kHz, resulting in minimal unwanted entanglement between the fluxoniums. Further, we demonstrate how the transmons can be used as ancilla qubits to perform parity checks of their nearest neighbor fluxoniums, providing an essential element for quantum error correction protocols with fluxonium subsystems.

[1] K.N. Nesterov et al., Phys. Rev. Appl. 18, 034063 (2022).

[2] E. Dogan et al., Phys. Rev. Applied 20, 024011 (2023).