A universal set of quantum gates on dynamically protected fluxonium qubits*

Fluxonium qubits are a promising platform for quantum computation since they have one of the highest coherence times among superconducting qubits. But, when choosing their parameters, we face the same problem as we do with all the other types of qubits: we need to make a compromise between coherence and control. If the matrix element of a transition between the ground and excited state is low (that is in a heavy fluxonium), the qubit can become almost insensitive to dielectric losses, but it would be impossible to control it. In the opposite regime, when the matrix element is high (light fluxonium), the qubit can be well-controlled, but the relaxation is high. For example, the fluxon states of the ultra-heavy IST qubit have relaxation times on the order of hours [1], but coherent control is not feasible with a conventional approach. We propose to solve this problem by introducing a dynamically tunable fluxonium architecture that is able to switch between these two regimes in-situ. Base band flux pulses can be used to turn the initially heavy fluxonium into the light regime with suitable interactions to form a universal set of single and two-qubit quantum gates.

[1] F. Hassani, M. Peruzzo, L. N. Kapoor, A. Trioni, M. Zemlicka, and J. M. Fink, A superconducting qubit with noise-insensitive plasmon levels and decay-protected fluxon states (2022).