Ultrastrong dispersive coupling between a superconducting qubit and a millimeter-wave resonator

Strong dispersive coupling between a superconducting qubit and a microwave resonator creates a versatile system for realizing quantum computation, hybrid quantum systems, and quantum-enhanced sensing. Increasing the resonator frequency to the millimeter-wave range (> 30 GHz) opens the door to new possibilities such as higher operating temperatures for the qubit, coupling to neutral atoms, and millimeter-wave astronomical detectors. At the same time, it is desirable to keep the qubit frequency in the few-gigahertz range to take advantage of state-of-the-art qubit coherence. Here, we demonstrate an ultrastrong dispersive coupling between a 3.4-GHz transmon qubit and a 32.8-GHz millimeter-wave resonator using a multimode circuit based on a hairpin resonator. We observe a dispersive shift of −120 MHz, which is equivalent to a qubit–resonator coupling strength of 6.5 GHz and places our system in the ultrastrong-coupling regime. This system provides a platform for exploring the large-detuning limit of circuit quantum electrodynamics, increasing the efficiency of bolometric qubit readout, and observing quasiparticle dynamics near the superconducting gap frequency.