Optimal encoding of quantum information into a propagating mode

Abstract

This thesis studies how quantum information can be encoded into a propagating microwave mode in a superconducting-circuit setting, with particular emphasis on traveling two-component cat states and cat qubits. The work focuses on how such states can be prepared, released into a transmission line, and characterized after emission. To do this, numerical protocols were developed for state preparation and release, identification of the dominant temporal output mode, and reconstruction of the captured propagating state using a cascaded virtual-cavity approach. Two different confinement mechanisms were examined: a Kerr-nonlinear parametric oscillator (KPO) and an alternative model based on engineered two-photon dissipation. The captured traveling mode was evaluated using fidelity to ideal cat-qubit target states, state purity, and the efficiency of the dominant temporal mode; in particular, fidelities above 0.96 was obtained for cat states with photon numbers up to 5. For the Kerr model, the results show that high-fidelity propagating cat-qubit states can be generated while still remaining strongly concentrated in a single temporal mode. Across the target photon numbers considered here, the low-pass-filter order pair (np, nε) = (3, 2) gave the best overall performance, and shortcut-to-adiabaticity was found to noticeably improve the preparation-and-release protocol. The dominant extracted mode also showed good agreement with the intended reference mode shape. The two-photon dissipative model was also able to generate traveling cat-like states, although its performance depended strongly on the strength of the two-photon loss. Increasing the dissipative confinement improved fidelity, purity, and mode selectivity. This indicates that the dissipative-confinement model provides a comparatively robust state-generation process relative to the Kerr-confinement model, possibly due to quantum-Zeno-like suppression of unwanted dynamics. In addition, noise analyses including pure dephasing and one-photon loss showed a systematic decrease in performance in both models, while also indicating that the protocols remain qualitatively functional in the weak-noise regime. Overall, the results show that high-quality propagating cat-qubit encodings can be generated numerically in superconducting-circuit-inspired models, and that both pulse design and the choice of confinement mechanism play an important role in the quality of the transferred state.