A fast tunable 3D-transmon architecture for superconducting qubit-based hybrid devices

Superconducting qubits utilize the strong non-linearity of the Josephson junctions. Control over the Josephson nonlinearity, either by a current bias or by the magnetic flux, can be a valuable resource that brings tunability in the hybrid system consisting of superconducting qubits. To enable such a control, here we incorporate a fast-flux line for a frequency tunable transmon qubit in 3D cavity architecture. We investigate the flux-dependent dynamic range, relaxation from unconfined states, and the bandwidth of the flux-line. Using time-domain measurements, we probe transmon's relaxation from higher energy levels after populating the cavity with $\approx 2.1\times10^4$ photons. For the device used in the experiment, we find a resurgence time corresponding to the recovery of coherence to be 4.8~$μ$s. We use a fast-flux line to tune the qubit frequency and demonstrate the swap of a single excitation between cavity and qubit mode. By measuring the deviation in the transferred population from the theoretical prediction, we estimate the bandwidth of the flux line to be $\approx$~100~MHz, limited by the parasitic effect in the design. These results suggest that the approach taken here to implement a fast-flux line in a 3D cavity could be helpful for the hybrid devices based on the superconducting qubit.

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