Mastrovito, Pasquale (2024) Alternative approaches to readout and control in circuit quantum electrodynamics. [Tesi di dottorato]

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Abstract

Readout and control are essential operations for processing quantum information in systems that leverage the quantum behavior of their constituent elements. Superconducting circuits offer an ideal platform to observe and control quantum mechanics phenomena on a chip-level architecture. This peculiarity distinguishes superconducting circuits from most of the quantum platforms not only from a structural point of view but also from a physical one. Compared to the sub-nanoscale dimension of natural atoms, the micron dimension of artificial superconducting atoms allows to easily reach strong atom-light coupling regimes, which made superconducting circuits rapidly emerge as a promising candidate to build quantum computers. Nonetheless, this platform also presents some relevant drawbacks. While strong atom-light coupling enables effective control, it can also lead to unwanted couplings, that result in cross-talk and decoherence. Searching for a trade-off between these two effects is one of the fundamental engineering challenges of this platform, which is to simultaneously have robust control of the system while keeping the various elements as isolated from each other as possible. The diverse dynamics of superconducting circuits provide opportunities to develop innovative approaches that address the limitations affecting core quantum operations. This thesis explores alternative methods for qubit readout and harmonic element control. This work covers the experimental characterization of a superconducting phase detector for in-situ qubit readout, and multiplexed readout using a superconducting traveling-wave parametric amplifier, and explores the dynamics of a cryogenic coherent photon source.

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