Searching for the optimal compromise between control and scaling-up the energies in cuprate superconducting quantum technologies

High temperature superconducting complex oxides (La₂₋ₓSrₓCuO₄, YBa₂Cu₃O₇₋ₓ, Bi₂Sr₂CaCu₂O₈₊ₓ) are materials where dominant states are not spatially homogeneous [1]. The quantum melted electronic crystal [2] lives in a heterogeneous landscape of "puddles" [3], correlated to a complex phase diagram [4]. Some of the interest in these materials now centers less on nitrogen-range Tc: and is progressively less motivated by achieving higher superconducting critical temperatures than before, but more on controlling this unparalleled and still-mysterious electronic state. Four decades of materials progress have clarified both limitations and enormous potential of these superconducting quantum materials, including their dominant d-wave order nature from grain-boundary junctions [5] and interference experiments [6], flourishing in an early attempt to demonstrate macroscopic quantum tunneling and energy level quantization in bicrystal grain-boundary YBa₂Cu₃O₇₋ₓ junctions [7]. However, the characteristic voltage parameter of cuprate Josephson junctions, IcRn, remained for long-time in the low values of 0.5–2 mV, hindering their potential. Based on recent material science breakthrough with cryogenic stacking technologies [8], it was measured a forward jump to 20–25 mV in a comparable twist-angle junction, using Bi₂Sr₂CaCu₂O₈₊ₓ crystals and a wide d-wave angular dependence was observed. I will therefore discuss our understanding of the intrinsic fragility of these systems, new phenomenology discovered both electronically, optically, and the electrical engineering challenges that we are addressing [9] for potential superconducting quantum technologies [10].