Fluxonium qubits — design, readout and gates

Abstract: Superconducting qubits have enabled recent progress in experiments with large quantum computing circuits. In these, transmon has been the broadly used qubit of choice. To further improve the performance of such circuits, the introduction of the fluxonium qubit can bring added benefits of longer coherence times and larger anharmonicity. One of the most advanced methods for analysing superconducting circuit designs is the energy participation ratio (EPR) method, which constructs quantum Hamiltonians based on the energy distribution extracted from classical electromagnetic simulations. However, the EPR method relies on a low-order expansion of nonlinear terms, which is prohibitive for accurately describing highly anharmonic circuits. In this talk, I will extend the EPR approach to readily deal with highly nonlinear superconducting circuits such as fluxonium qubits. Next, I will discuss how we can perform fast readout of fluxonium qubits using a flux-pulse assisted readout. Increasing the dispersive shift magnitude by almost 20\% through flux pulsing, we achieve an assignment fidelity of 94.3% with an integration time of 280 ns which is significantly faster than previous fluxonium readout schemes. Finally, I'll discuss gates with fluxonium qubits. In our experiment we observe low leakage, fast two qubit gates (< 50ns) and high two-qubit gate fidelities (~99%). We demonstrate experimentally that different pulse schemes can speed up the duration of the gate.