Engineering Josephson junction arrays for fluxonium qubits resilient to coherent quantum phase-slips

Fluxonium qubits have gained popularity due to their enhanced protection against dielectric loss, enabling qubit T₁ times exceeding 1 millisecond [1]. However, T₂ is often observed to be much shorter than 2T₁ [1,2], and coherent phase slips in the superinductor, typically formed from an array of Josephson junctions, has been identified as a potential source of dephasing [3]. In this work, we design and fabricate arrays of Josephson junction with block-shaped junctions as a modification of conventional Manhattan-style junctions to achieve low junction impedance and suppress coherent phase-slips. By characterizing fluxoniums with varying array junction areas, we demonstrate that the new arrays can substantially enhance the qubit coherence time T₂.