In high-Tc cuprate superconductors, the precise microscopic origin of unconventional superconductivity, the nature of charge-transport mechanisms, and the symmetry of the superconducting order at engineered interfaces remain largely unknown [1]. Recent advances in fabrication have significantly improved the ability to engineer artificial systems through controlled twisting and stacking.

In our work, we investigate the superconducting order at the twisted interface of van der Waals Bi₂Sr₂CaCu₂O_8+δ (BSCCO) using a c-axis SQUID geometry. By realizing twist-controlled SQUID devices comprising two Josephson junctions (JJs) in parallel, we access phase-sensitive information inaccessible in single Josephson junction measurements. Near a twist angle of 45°, where single Cooper pair tunneling is strongly suppressed, we observe clear signatures of Cooper pair co-tunneling. Furthermore, we observe clear evidence of time-reversal symmetry breaking, indicating the emergence of chiral superconducting order of the form d+id or d-id at the twisted interface. We measure an anomalous phase shift between the two JJ junctions: identical chiralities produce zero phase shift, whereas opposite chiralities yield a phase shift approaching π. Additionally, these SQUIDs are well-suited for use as state-of-the-art flux sensors at ~77 K, achieving a flux-noise sensitivity of ~1.5 μΦ0/√Hz [2].

Our SQUID architecture provides a versatile platform for probing charge-transport mechanisms and the symmetry of superconducting order at interfaces, extending its applicability well beyond cuprate superconductors.

[1] New frontiers in quantum science and technology using van der Waals Josephson junctions, arXiv:2604.15276.

[2] Quantum interference in a twisted high-Tc SQUID senses emergent interfacial order, arXiv:2603.12092v1.