Group IV Fully Crystalline Josephson Junctions for Scalable Superconducting Qubits

Material-induced losses remain a primary limitation to coherence in superconducting qubits. Standard tunnel junctions rely on amorphous oxides to define the barrier between the two superconducting leads, which introduce poorly defined interfaces and parasitic loss channels. Building on our recent demonstration of epitaxial growth of superconducting hyperdoped Ga:Ge films with low disorder [1], we propose to exploit the atomically sharp, lattice-matched interfaces of the GaGe/Si platform to realize a fully crystalline group IV Josephson junction (JJ). We report the first demonstration of such a JJ, featuring a semiconductor barrier with a lower superconducting gap, offering a novel parameter space for engineering the Josephson-to-charging energy ratio (Ej/Ec). This enables implementation of the merged element Transmon qubit geometry [2], a compact design favorable for increasing qubit density and facilitating flip-chip integration. Furthermore, the semiconducting nature of the barrier opens pathways to gate-tunable mergemon devices and scalable Andreev spin qubit arrays, presenting a promising route for next-generation quantum processor architectures.

[1] J. A. Steele, P. J. Strohbeen et al., arXiv:2412.15421 (2025) 

[2] R. Zhou et al., Phys. Rev. Applied 14, 064006 (2020).