We present a gradient-based method to construct high-fidelity, two-qubit quantum gates in a system consisting of two transmon qubits coupled via a tunable coupler. In particular, we focus on single flux quantum (SFQ) pulses as a promising and scalable alternative to traditional control schemes that use microwave electronics. We develop a continuous embedding scheme to optimize these discrete pulses, taking advantage of auto-differentiation of our model. This approach allows us to achieve fSim-type gates with average gate fidelities on the order of 99.99% and CZ and CNOT gates with fidelities above 99.9%. Furthermore, we provide an alternative semi-analytical construction of these gates via an exact decomposition using a pair of fSim gates which leads to the reduction in memory required to store the associated pulse sequences.
@article{arxiv.2412.15816,
title = {Optimization of Two-Qubit Gates in Tunable-Coupler Architectures Using Single Flux Quantum Control},
author = {Boyan Torosov and Bohdan Kulchytskyy and Florian Hopfmueller and John Gunderson and Xiangzhou Kong and Pooya Ronagh},
journal= {arXiv preprint arXiv:2412.15816},
year = {2024}
}