High-T_C cuprate superconductors are characterized by a d-wave order parameter. However, the exact microscopic origin of unconventional superconductivity, the nature of charge-transport mechanisms, and the superconducting order at engineered interfaces between such states remain largely unexplored [1]. The recent ability to fabricate twist-controlled van der Waals heterostructures provides a powerful platform to probe interfacial superconducting order.

Here, 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-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 states 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 difference, 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.