Superconducting qubits at elevated frequencies

Quantum processors based on superconducting qubits typically operate in a frequency range between 4-8 GHz. Raising the qubit frequencies beyond this well-established frequency range could result in significant advantages including reduced residual mode populations, enhanced anharmonicities and the potential for operation at elevated cryogenic temperatures. In this work, we study the properties of transmon-type superconducting qubits based on niobium electrodes and standard Al/AlOx/Al junctions with resonance frequencies up to 20GHz. We observe a reduction in qubit lifetimes by two orders of magnitude for high-frequency qubits compared to standard devices operating around 5 GHz. To investigate the source of the additional losses we measure the behavior of niobium (Nb) and aluminum (Al) resonators in the same range. We find that the quality factors of the Nb resonators are nearly constant as a function of frequency, while the losses of Al resonators significantly increase at higher frequencies, suggesting that the lower qubit lifetimes are caused by losses from the Al of the junctions. Finally, we propose strategies to transition to niobium-based junctions to overcome these limitations and explore qubit properties at higher temperatures.