Ramsey Frequency Beating in the Higher Level States of a 3D Transmon*

Utilizing higher levels of 3D transmons for computational operations (Qudits) holds promise for more efficient quantum computational architectures. In particular, they give access to an expanded Hilbert space while operating with a minimal number of control lines, and can operate with efficient high-fidelity control gates via optimal control techniques. However, a limiting factor has been the reduced coherence times of the higher levels. Therefore, a full characterization and understanding of the fundamental sources of noise is a key step to mitigating these limitations. We present the observation of a beating phenomenon in the Ramsey fringes of higher qudit levels which is only visible in the overlay of several independent Ramsey measurements. In contrast to the understood deviation in Ramsey fringes caused by parity flips, the beating appears to be mostly coherent and does not correspond to the charge dispersion. We discuss several Ramsey measurements for multiple levels of two 3D transmons, including charge dispersion, phenomenological models, and potential sources for this anomalous noise.