Exploring ququart computation on a transmon using optimal control
Lennart Maximilian Seifert, Ziqian Li, Tanay Roy, David Schuster, Frederic T. Chong, Jonathan M. Baker
Abstract
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.