Characterization and Modelling of the Quantum-classical Interface for Scalable Superconducting Quantum Computing

Quantum Computing holds promise to solve complex computational problems which are intractable by classical calculators. To date, the most advanced solid-state implementation of a Qubit (or a system of Qubits) is realized with Superconducting circuits arranged in the so-called circuit-Quantum-Electro-Dynamics (cQED) architecture. Although this technology has proven to be mature for the implementation of basic Quantum Algorithms, it presents unique challenges in term of integration of a large (millions) array of qubits, necessary for error-correction. The primary objectives of this research are to enhance the interface between classical control electronics and superconducting quantum processors and to provide a scalable solution to improve the performance of these quantum processors.

The study introduces two significant contributions. First, it develops an equivalent circuit model that allows the co-simulation of superconducting qubits with classical control electronics. This model enables a detailed analysis of the behavior of superconducting qubits interacting with superconducting resonators, taking into account various factors such as coherent control, readout, and the impact of non-idealities like relaxation and decoherence. This modeling is essential for designing, optimizing, and scaling superconducting quantum processors.

Second, the study explores the integration of radio-frequency (RF) multiplexers based on complementary metal-oxide-semiconductor (CMOS) technology in close proximity to superconducting qubits. These multiplexers operate at extremely low temperatures (below 15 mK) and are designed to minimize cross-coupling between electronic and thermal noise. The results show that these multiplexers have a minimal impact on the qubit's performance, maintaining high gate fidelities exceeding 99.9% for single-qubit gates, meeting the threshold for surface-code-based quantum error correction. The study also demonstrates the potential of time-division multiplexing for two-qubit operations using a single control line.

This research significantly contributes to the practical and scalable implementation of superconducting quantum computing architectures. It addresses crucial challenges related to the quantum-classical interface and provides an innovative solution for improving the performance of superconducting qubits. These advancements have the potential to accelerate the development of practical quantum computing systems and advance the field of quantum information processing.