Controlling Bosonic States in Multimode Josephson-Array Cavities

We implement and characterize compact, high-impedance multimode microwave cavities based on Josephson junction arrays. Our cavities are sections of a high-impedance transmission line built from two parallel arrays of Josephson junctions. The arrays' large kinetic inductance reduces the wave velocity, shrinking the cavity's physical footprint by two orders of magnitude relative to 50 Ω coplanar waveguide resonators. We exploit inter-mode cross-Kerr couplings to measure the coherence of the cavity modes with modes' T1 and T2 times exceeding 100 µs. Additionally, we inductively couple a cavity to a fluxonium qubit, enabling the creation and characterization of non-Gaussian states. The combination of small footprint, built-in nonlinearity, and relatively high coherence makes Josephson junction arrays a compelling planar platform for multimode bosonic encoding.