Two-qubit gates constitute fundamental building blocks in the realization of large-scale quantum devices. Using superconducting circuits, two-qubit gates have been implemented in various ways, with each method aiming to maximize gate fidelity. Another important goal of a new gate scheme is to minimize the complexity of gate calibration. In this work, we demonstrate a high-fidelity two-qubit gate between two fluxonium qubits, enabled by an intermediate capacitively coupled transmon. The coupling strengths between the qubits and the coupler are designed to minimize residual crosstalk while still allowing for fast gate operations. The gate is based on frequency selectively exciting the coupler using a microwave drive to complete a 2π rotation, conditional on the state of the fluxonium qubits. When successful, this drive scheme implements a conditional phase gate. Using analytically derived pulse shapes, we minimize unwanted excitations of the coupler and obtain gate errors of 10−2 for gate times below 60 ns. At longer durations, the gate performance is limited by relaxation of the coupler. Our results show how carefully designed control pulses can speed up frequency-selective entangling gates. ... Read more

Quantum gates play an indispensable role in quantum computing, serving as the foundation for executing efficient quantum algorithms. In particular, the development and optimization of two-qubit gates have emerged as a significant challenge in the field. In this work, we propose an innovative structure, termed as the FXTFX (fluxoniumtransmon-fluxonium) system, leveraging a tunable transmon as the coupler for two fluxonium qubits to construct a two-qubit gate. Our FXTFX configuration demonstrates the capability to achieve a strong coupling strength between the coupler and fluxonium (g /2π = 226 MHz), while maintaining the Z Z interaction below 10 kHz. Utilizing this setup, I designed and simulated the CZ gate and show through rigorous simulations that a gate time of 66 ns produces a high gate fidelity of 99.88%. ... Read more