Cuprate Twistronics Enables Quantum Hardware Via Nanoscale Engineering of Superconducting Films

The pursuit of robust and scalable quantum technologies receives a significant boost from recent advances in manipulating complex oxide materials, particularly atomically thin films of cuprate superconductors. Tommaso Confalone, Flavia Lo Sardo, and colleagues at the Leibniz Institute for Solid State and Materials Research Dresden, alongside Yejin Lee, Sanaz Shokri from the Max Planck Institute for Chemical Physics of Solids, and Giuseppe Serpico and Alessandro Coppo from the University of Naples Federico II and Istituto dei Sistemi Complessi respectively, demonstrate how twisting these layered materials creates entirely new electronic properties. This emerging field, termed ‘cuprate twistronics’, builds on over three decades of research and offers a pathway to control superconductivity at the nanoscale, potentially unlocking exotic quantum states and accelerating the development of next-generation quantum hardware. The team’s work highlights methodologies that promise to overcome current limitations and fully realise the potential of these complex materials for both fundamental science and technological innovation.

Protected Qubits and Heterostructure Fabrication

Researchers are actively investigating superconducting qubits and the materials that underpin their functionality, with a focus on creating more robust and protected quantum systems. Approaches include parity-protected qubits, 0-π qubits, two-Cooper-pair tunneling, and stabilizer qubits, employing quantum error correction techniques to enhance stability. The foundation of many of these qubits relies on Josephson junctions, and scientists are investigating novel materials, particularly two-dimensional materials, to improve their performance. This work extends to the fabrication and integration of superconducting circuits for precise qubit control and readout, including designs utilizing the phase of the superconducting wavefunction.

A key enabling technology is the creation of van der Waals heterostructures, carefully stacking two-dimensional materials to tailor their properties. High-temperature superconductors, such as cuprates and iron-based materials, are being integrated with other two-dimensional materials to enhance qubit performance, employing techniques like dry transfer and ultrahigh vacuum fabrication. Specific material combinations, like cuprate/FeSe heterostructures and twisted bilayer materials, are under intense scrutiny, aiming to combine the strengths of different materials and create new electronic properties. Achieving reproducibility in two-dimensional material fabrication demands reliable and scalable techniques, minimizing contact resistance, maintaining material quality, and developing scalable fabrication processes. This research aims to create hybrid qubit designs, explore novel superconducting materials, and develop scalable fabrication techniques for large-scale qubit arrays, ultimately leading to more robust and high-performance quantum computers.

Nanoscale Cuprate Control and Heterostructure Fabrication

Researchers are pioneering techniques to investigate and manipulate cuprate superconductors, materials exhibiting the highest critical temperature under ambient pressure, to enhance coherence and unlock novel functionalities for quantum technologies. Scientists employ molecular beam epitaxy to fabricate atomically thin cuprate films, allowing precise control over material composition and structure, and cryogenic stacking techniques to create twisted heterostructures inducing unique quantum states. Mechanical exfoliation isolates thin flakes of cuprate materials for further investigation. These advanced fabrication methods address the inherent complexity and sensitivity of cuprates, which exhibit strong electronic correlations and unconventional superconducting characteristics.

Researchers investigate materials including Bismuth Strontium Calcium Copper Oxide, Lanthanum Strontium Copper Oxide, and Yttrium Barium Copper Oxide, seeking to understand their unique electronic behaviors and potential for quantum applications. The study prioritizes understanding and controlling nanoscale electronic structures within cuprates, aiming to improve coherence of superconducting qubits and harness entanglement for quantum information processing, integrating these materials into nanocircuits to overcome limitations of current quantum hardware. Investigations into nickelates further expand understanding of unconventional superconductivity and potential mechanisms driving these quantum phenomena.

Twisted Cuprate Junctions Demonstrate Superconductivity Transitions

Scientists have made significant progress in fabricating twisted cuprate heterostructures, exploring how nanoscale engineering governs superconductivity and related electronic properties. Initial experiments focused on creating twisted junctions from BSCCO flakes, followed by annealing in either flowing oxygen or ozone, revealing transitions in superconducting characteristics. Subsequent work utilized a polymer adhesive stamp to cleave single BSCCO flakes, forming junctions within an argon-filled glovebox, restoring superconductivity after removing residual polymer with acetone and annealing. Investigations using X-ray nanobeams created Josephson junctions, inducing a slight increase in resistance and a shift in the superconducting transition, indicating structural changes likely due to oxygen loss.

Studies of La2CuO4+y revealed that interstitial oxygen dopant ordering influences structural openings, forming stripe-like patterns, mapped with X-ray microdiffraction, quantifying it with a power-law distribution indicative of a fractal nature. Thermal treatments altered the superconducting phases and Q2 intensity, demonstrating a strong link between oxygen ordering and superconducting properties. Further research on YBa2Cu3O6+x samples revealed superconductivity emerging within nanoscale networks of oxygen-ordered puddles separated by oxygen-depleted zones, identified with high-energy X-ray diffraction, exhibiting periodic ordering along the a-axis. These findings demonstrate that oxygen interstitial ordering is inhomogeneous at the micrometer scale, yet exhibits fractal characteristics beneficial to superconductivity, and can be tuned by external stimuli.

Cuprate Twistronics Reveals Unexpected Superconducting Behaviour

Over three decades of research into cuprate twistronics has yielded significant advances in understanding how nanoscale engineering influences superconductivity and related electronic properties. Investigations into twisted heterostructures of cuprate superconductors demonstrate the potential to manipulate these materials and explore exotic quantum states. Initial experiments utilizing naturally formed whisker junctions revealed a surprising independence of critical current density from twist angle, suggesting a dominant s-wave superconducting pairing component. Subsequent work focused on fabricating cleaner, more controlled twisted interfaces, employing techniques like mechanical exfoliation and polymer-assisted cleavage, alongside careful environmental control to mitigate material degradation.

These studies consistently demonstrate the necessity of post-fabrication annealing to restore superconductivity at the twisted interface. While early attempts suffered from interface instability and potential chemical contamination, improvements in fabrication techniques, such as lower temperature processing and shorter fabrication times, have enabled the observation of anisotropic angular dependence, mirroring results from whisker junctions. Researchers acknowledge that achieving atomically clean interfaces remains a significant challenge, and that further refinement of fabrication processes is crucial to fully isolate and characterize the intrinsic properties of twisted cuprate heterostructures. Future work will likely focus on optimizing interface quality and exploring a wider range of twist angles and material combinations to unlock the full potential of these complex systems.