The Coherence Challenge: Foundations and Advancements in Superconducting Quantum Circuits

Over the past two decades, intense efforts have focused on developing quantum computing hardware to solve problems intractable for classical computers. Unlike classical computing, quantum computing is based on entirely different principles of quantum mechanics, specifically superposition and entanglement. These quantum states can be created in various hardware platforms, including optical systems, trapped ions, superconducting quantum circuits, and quantum defects in materials, among others. However, none of these platforms has yet achieved a fault-tolerant quantum computer due to limitations in scalability and coherence. Planar superconducting quantum circuits, based on circuit quantum electrodynamics (cQED), show significant promise for quantum computing due to their fabrication scalability. A major bottleneck, however, is decoherence caused by material imperfections, particularly two-level system (TLS) defects at various interfaces. Significant research over the last two decades has focused on understanding and mitigating these TLS losses in planar superconducting circuits. These efforts have explored approaches such as design optimization, material selection for substrates and devices, and interfacial engineering. While these strategies have successfully improved coherence times by more than six orders of magnitude, further advancements are crucial for developing commercially viable superconducting circuit-based computing devices. This review provides a comprehensive overview of current strategies for mitigating TLS losses at various interfaces. It covers the fundamentals of cQED related to loss mechanisms in superconducting circuits and examines recent developments in reducing TLS defects, all with the goal of improving the coherence time of quantum circuits.