Exploring ququart computation on a transmon using optimal control
Lennart Maximilian Seifert, Ziqian Li, Tanay Roy, David Schuster, Frederic T. Chong, Jonathan M. Baker · Physical Review A · 2023
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 $\ensuremath{\ge}95%$ and identify coherence as the primary limiting factor. Our results validate ququarts as a viable tool for quantum information processing.