A Millimeter-wave Qubit Cooled with Helium-4

Superconducting qubits are a promising platform for quantum experiments studying light-matter interactions in the strong coupling regime. Current microwave qubits are cooled to extremely low temperatures through dilution of helium-3 and helium-4 in order to reduce sources of decoherence. Scaling these superconducting quantum devices to millimeter-wave frequencies (near 100 GHz) can greatly reduce their sensitivity to thermal noise. This enables operation at significantly higher temperatures near 1 K: achievable using simpler methods such as helium-4 evaporation, which provides orders of magnitude higher cooling power. Combining low-loss niobium trilayer junctions with a ground-shielded millimeter-wave coupling design, we realize a qubit at 72 GHz with resolved energy level transitions in a cryogenic system cooled solely with Helium-4. Using high-speed millimeter-wave pulse spectroscopy, we measure decoherence and dephasing times of 15.9 and 17.4 ns respectively, corresponding to qubit quality factors of 7.2x10³. This demonstration of a millimeter-wave quantum emitter enables new opportunities for quantum transduction, integrated experiments with high heat dissipation requirements, and opens the door to higher-frequency, higher-energy quantum experiments.