Full characterization and universal control of a superconducting 3D transmon qudit*

While high fidelity single qubit and two-qubit gates have been realized for transmons, use of higher levels as a quantum resource has not yet been systematically studied. Coherent control of these higher energy states offers a direct route to increasing the available quantum volume of a given quantum circuit. In addition, the increasing charge dispersion present in higher level states offers a unique tool for studying charge noise. Here, we use the lowest four levels of a 3D transmon as the quantum register, qudit, which is computational equivalent to two qubits. We measure the decoherence times for each of the excited state. We also demonstrate multiplexing readout and universal control of the qudit with a single drive line.