Contemporary quantum computers encode and process quantum information in binary qubits (d = 2). However, many architectures include higher energy levels that are left as unused computational resources. We demonstrate a superconducting ququart (d = 4) processor and combine quantum optimal control with efficient gate decompositions to implement high-fidelity ququart gates. We distinguish between viewing the ququart as a generalized four-level qubit and an encoded pair of qubits, and characterize the resulting gates in each case. In randomized benchmarking experiments we observe gate fidelities greater 95% and identify coherence as the primary limiting factor. Our results validate ququarts as a viable tool for quantum information processing.
@article{arxiv.2304.11159,
title = {Exploring Ququart Computation on a Transmon using Optimal Control},
author = {Lennart Maximilian Seifert and Ziqian Li and Tanay Roy and David I. Schuster and Frederic T. Chong and Jonathan M. Baker},
journal= {arXiv preprint arXiv:2304.11159},
year = {2024}
}