Quantum Computing in Discrete- and Continuous-Variable Architectures

TL;DR

This thesis develops a theoretical framework for hybrid continuous-variable and discrete-variable quantum systems, introducing new protocols and classifications that advance quantum control, state preparation, and error correction for scalable quantum computing.

Contribution

It introduces non-abelian quantum signal processing and the Gaussian hierarchy, advancing control and understanding of hybrid CV-DV quantum systems, with practical protocols for state preparation and error correction.

Findings

High-fidelity CV state control using DV ancillae

Deterministic preparation of GKP states without optimization

Analytical framework for photon loss correction in GKP codes

Abstract

This thesis develops a theoretical framework for hybrid continuous-variable (CV) and discrete-variable (DV) quantum systems, with emphasis on quantum control, state preparation, and error correction. A central contribution is non-abelian quantum signal processing (NA-QSP), a generalization of quantum signal processing to settings where control parameters are non-commuting operators. Within this framework, we introduce the Gaussian-Controlled-Rotation (GCR) protocol, which enables high-fidelity control of CV states using DV ancillae. This approach allows for deterministic preparation of squeezed, cat, and Gottesman-Kitaev-Preskill (GKP) states without numerical optimization. Two previously unpublished contributions are included: (i) Chapter 2.3 introduces the Gaussian hierarchy, a classification of CV operations analogous to the Clifford hierarchy, offering a new lens for understanding CV gate sets; (ii) Chapter 5 presents an analytical framework for correcting photon loss in finite-energy GKP codes, introducing the notion of probabilistic error correction, providing insight into recent GKP experiments surpassing break-even thresholds. Overall, this work lays the groundwork for scalable, fault-tolerant quantum computation in hybrid CV-DV architectures, with applications to logical gate synthesis, readout, and hybrid algorithm design using ancilla oscillators.