Collaborative Research: EAGER: Quantum Manufacturing: Vertical Coupling and Cross-Talk Shielding of Superconducting Quantum Devices

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One of the main challenges facing the development of next-generation superconducting quantum devices is high-density three-dimensional (3D) integration of large numbers of individual superconducting quantum bits (qubits). While superconducting quantum devices have reached a high level of maturity as a technology, coupling of qubits to one another to enable large-scale circuits for practical computation still remains a challenge. This project seeks to address this challenge by exploring a new approach for coupling superconducting circuits using a thin electromagnetic coupler interposed between qubit layers. The proposed work will be based on an innovative integration of quantum elements that are by themselves well-established in the PI’s and Co-PIs’ labs. Successful performance of the proposed work will expand our knowledge of quantum states in superconducting devices and will result in the development of improved quantum manufacturing approaches of broad interest for practical quantum information processing technologies. The team is committed to mentoring graduate and undergraduate students and to broadening the participation of under-represented groups in quantum engineering. In addition, the PIs will be involved in outreach efforts aiming to raise awareness about physics, materials science, and mathematics to school students. A substantial part of this effort will reach out to school students from the local Native American community. Developing new approaches to 3D integration of superconducting qubits is of crucial relevance for realizing high-depth circuits suitable for running practically relevant algorithms. Current coupling techniques for transmon qubits typically involve relatively large (millimeter-sized) coplanar resonators, while for phase qubits a variety of different capacitive or inductive coupling approaches are being investigated. No optimal solution has yet been identified for vertical expansion. Existing approaches are typically planar, due in part to limitations stemming from the technology used to create the qubit Josephson junctions (JJs) typically angle-deposition and controlled oxidation of the tunnel barrier. This results in low spatial densities for JJ circuits and coherence-limiting cross-talk. In order to address this challenge, the PIs will fabricate high-quality JJ array chips and link them vertically via waveguide arrays operating in the microwave frequencies. While each enabling component and manufacturing method has been demonstrated, their integration is a daunting task with high-risk and the co-PIs are uniquely positioned to tackle this challenge. Intellectual significance: the team’s vision of full-3D integration of JJs and cross-talk shielding is in its early stages and untested experimentally, yet presents a potentially transformative approach to solve, in a single stroke, the triple challenge of efficient superconducting circuit coupling, high-density 3D vertical integration, and cross-talk mitigation. Additionally, the project will train graduate students in state-of-the art quantum device nanofabrication, advanced materials growth, and quantum transport. The PIs will also develop courses on quantum information sciences aimed at undergraduates and quantum engineering/manufacturing aimed at graduate students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.