Engineering Low-Loss Optical Interconnects for Rare-Earth Quantum Memory

Abstract

Erbium-doped rare-earth crystals are promising candidates for atomic quantum memory due to their long optical and hyperfine coherence times. These properties have been utilised in the Rephased Amplified Stimulated Emission (RASE) protocol, a DLCZ-like quantum repeater scheme that enables both on-demand generation and storage of photons using a single crystal. Non-classical storage was demonstrated with RASE, achieving recall efficiencies of up to 30%. Furthermore, erbium-RASE operates at wavelengths compatible with existing long-haul fibre optic networks, making it an ideal foundation for a quantum repeater network.

To scale from a single erbium-RASE system to a distributed network of quantum memory nodes, two complementary requirements must be met. First, the distribution of entangled photons requires highly efficient classical coupling between the crystal and the interconnecting fibre pathways. This is challenging because the erbium-doped crystal must be maintained at cryogenic temperatures and within a substantial magnetic field. Due to losses from scattering and mode mismatches, complete fibre-to-fibre collection efficiency through the windows of a cryostat is only approximately 20%. Second, the recall efficiency of RASE is directly influenced by the optical depth of a rare-earth crystal, and high optical depths cause control pulse distortion and the reabsorption of emitted photons. Improving recall efficiency requires placing the crystal into a low-loss, impedance-matched optical cavity.

This thesis outlines the design and development of a fibre-coupling system that delivers light directly into an erbium-doped crystal, achieving collection efficiencies of up to 74% while being highly insensitive to mechanical noise. This coupling system requires no additional optics or alignment, allowing for the immediate integration of fibre-based optical cavities using Bragg gratings or ring resonators. The high beam quality also enhances the response of erbium ions to optical control pulses, with the effective Rabi frequency of the atomic ensemble empirically measured to be 6.67 times higher than in previous free-space experiments. This suggests the potential of this system to support a large number of temporal modes without requiring modification to existing experimental setups.

The thesis also addresses the limitations and constraints of mechanical systems for cavity enhancement in cryogenic conditions, proposing several novel design configurations based on the developed full-fibre coupling system to overcome these challenges. Show more