PhD Defense: Carson Evans

ABSTRACT: Quantum memories are a crucial component in the development of large-scale quantum networks, enabling the synchronization and storage of quantum states for long-distance communication and distributed quantum entanglement. This dissertation explores the experimental realization and characterization of broadband, loop-based quantum memories, with a particular emphasis on their ability to store and preserve entanglement. We present a series of experimental investigations demonstrating the feasibility of these memories in various quantum information applications, including quantum networking and non-local phase correction.

We begin with a detailed study of the key components of our memory system, including high-speed Pockels cells for active storage control, and optics components and their losses limiting maximum efficiencies. Additionally, we analyze alignment conditions using Gaussian beam propagation techniques, ensuring optimal mode-matching for long-term storage stability.

Next, we experimentally demonstrate the storage of polarization-entangled photon pairs using a single active loop-based quantum memory in conjunction with a passive fiber delay line. By performing Bell tests before and after storage, we confirm the preservation of entanglement across multiple storage cycles. This proof-of-concept experiment establishes the viability of loop-based memories for entanglement distribution and provides insights into efficiency limitations due to optical losses.

Building upon this foundation, we extend our system to incorporate two independently controlled active quantum memories, allowing for flexible and tunable storage of both photons of an entangled pair. This dual-memory configuration enables a deeper exploration of entanglement storage, including the effects of differential storage times and fidelity degradation over successive cycles. We introduce new coincidence counting techniques to overcome timing degeneracies inherent in systems with multiple active storage elements, allowing accurate data acquisition and analysis.

Our results validate the practicality of broadband, loop-based quantum memories as a versatile platform for quantum communication. This dissertation represents a significant step toward scalable quantum networking, demonstrating key capabilities necessary for the deployment of high-performance quantum repeaters and entanglement-based communication protocols. The next logical progression involves expanding these experiments to incorporate additional quantum memories, enabling more complex entanglement distribution and multi-node quantum networks. Achieving this will require further technical advancements in the loop-based memory platform, including improved loss mitigation, enhanced phase stabilization, and the integration of additional control mechanisms for manipulating stored qubits. The techniques developed herein lay the groundwork for these future advances in memory-assisted quantum technologies.