Microwave-activated entangling gates in high coherence superconducting qubits

We report experimental progress on microwave-activated entangling gates with capacitively coupled fluxonium qubits. When biased at the flux sweet-spot, individual qubit transition has long coherence (the best device has T₂ > 400 us) [1]. A control-Z gate can be implemented by sending a short 2π-pulse at the frequency near the 1→2 transition of the target qubit [2]. The gate transition has higher frequency and larger matrix element than the qubit transition, resulting in fast gate and minimal spurious phase errors. Another microwave entangling gate, similar to the cross-resonance gate in transmon [3], can be applied to the computational subspace. We discuss qubits' design and fabrication, initialization, readout, and benchmarking of the gates.

[1] Long B. Nguyen, et. al., Phys. Rev. X (2019).
[2] Konstantin N. Nesterov, et. al., Phys. Rev. A 98, 030301 (2018).
[3] Jerry M. Chow, et. al., Phys. Rev. Lett. 107, 080502 (2011).