Beyond the X band: superconducting qubits above 20 GHz

Current state-of-the-art superconducting qubits operate in the microwave range, (typically well below 10 GHz) and require cooling to extremely low temperatures in the mK range to eliminate sources of decoherence. Meanwhile higher frequencies offer a wealth of unexplored opportunities for superconducting experiments, including quantum detection and lower-energy pathways for quantum transduction. Importantly, higher photon energies allow for ground-state operation at higher temperatures, significantly increasing the cooling power available, which is desirable for scaling up quantum computing architectures as well as integrating qubits in hybrid experiments requiring increased heat dissipation. We discuss considerations for circuit parameter estimation, simulation and packaging design required to scale conventional qubit designs to higher frequencies. While not specifically required for higher-frequency devices, we also highlight the benefit of higher critical temperature superconducting elements (such as Nb) for reduced quasiparticle sensitivity, which becomes significant above 160 mK. To investigate the frequency scaling of qubit properties, we present our measurements of transmon qubits near 20 GHz up to 250 mK, as well as a 72 GHz qubit operating near 1 K. These experiments illustrate the feasibility and possibilities for next generations of higher-temperature, higher-frequency superconducting quantum experiments.