Quantum and classical hardware development for superconducting qubits

Abstract

The journey toward realizing practical quantum computing involves overcoming critical challenges in qubit architecture, robust gate performance, and scalable control systems. This thesis addresses these challenges through three key contributions. The first contribution explores Stimulated Raman Adiabatic Passage (STIRAP) as a robust gate protocol for strongly coupled transmon qubits. Traditional resonant gates are prone to errors due to their sensitivity to pulse amplitude and frequency deviations. In contrast, the STIRAP protocol leverages adiabatic processes to achieve reliable π and π/2 rotations with high fidelity and robustness, using large single-photon detuning. In this thesis, this method is further extended to dual-rail qubits, where quantum information is encoded in two separate physical modes. Dual-rail qubits offer superior resilience against errors, making them an ideal platform for realizing robust, high-fidelity quantum operations in fixed-frequency transmon qubits. The second part of my research focuses on enhancing qubit design to address one of the primary limitations of standard transmon qubits: low negative anharmonicity, which increases the risk of unwanted state transitions during gate operations. To overcome this, I developed a new qubit design, dubbed as ’Linmon qubit’ by introducing an inductive shunt, significantly increasing the energy gap between computational and non-computational states. This enhanced positive anharmonicity reduces leakage errors, resulting in more precise and reliable gate operations. Additionally, the inductive shunt makes the Linmon qubit insensitive to low-frequency charge noise, further improving its stability and coherence properties. Finally, I contributed to the development of SQ-CARS (Scalable Quantum Control and Readout System), a state-of-the-art control system designed to support large-scale quantum computing. As quantum processors grow in size, managing and controllingarrays of qubits becomes increasingly complex. SQ-CARS provides a scalable solution by integrating high-speed data acquisition with real-time control and feedback, enabling precise manipulation and measurement of qubits in large arrays. Unlike traditional analog mixers, SQ-CARS employs a digital mixer, which eliminates the need for calibration, ensuring greater stability and reducing setup complexity. This system is designed to scale seamlessly with the growing number of qubits, ensuring reliable operations in the next generation of quantum processors. Together, these innovations—STIRAP-based gates for dual-rail qubits, the Linmon qubit’s enhanced anharmonicity, and the scalable SQ-CARS control system—address fundamental challenges in quantum computing. By having robust gates, simple qubit design, and scalability, these advancements bring us closer to unlocking the full potential of quantum computation, offering practical solutions for future quantum technologies.