A Warm Atomic Vapour Quantum Memory in the Context of Space Research

Abstract

Quantum memories serve as indispensable enabling components for possible satellite-based quantum repeaters in future intercontinental and global quantum communication networks. These memories play a crucial role in synchronising and buffering incoming photons at network nodes. Additionally, they have wide-ranging applications in distributed quantum computing, optical machine learning, testing of fundamental physics in space, and storage of quantum tokens for secure online authentication, etc. The landscape of possible platforms and implementation protocols for quantum storage is vast. We choose a simple and scalable storage platform: a warm atomic vapour cell. This choice eliminates the need for large technical overhead and can be operated without the need for cryogenics, strong magnetic fields, or vacuum chambers, making it highly suitable for space deployment. In this thesis, we experimentally realise a warm vapour quantum memory based on electromagnetically induced transparency (EIT) on the Cs D1 line and focus on simultaneously optimising two key parameters crucial for various applications: the end-to-end efficiency, ηe2e, and the signal-to-noise ratio (SNR). For the storage and retrieval of attenuated coherent pulses containing |α|^2 = 1.0(1) photons on average, our memory achieves an end-to-end efficiency of ηe2e = 13(2)% and an internal memory efficiency of ηmem = 33(1)%. We simultaneously obtain an SNR= 14(2), equivalent to a noise level corresponding to μ1 = 0.07(2) signal photons. The 1/e memory lifetime is approximately 1 μs, as determined using macroscopic pulses. Through a comprehensive noise analysis, we identify spontaneous Raman scattering and fluorescence as the primary limiting noise sources, while four-wave mixing noise, often problematic in warm vapour memories, has negligible contributions in our experiment. Moreover, we leverage our memory to realise a random-access spatially-multimode memory for coherent macroscopic pulses. Additionally, we implement control pulse optimisation using a genetic algorithm, offering a novel technique for in-experiment optimisations to enhance performance metrics. Furthermore, we propose an experiment to demonstrate, for the first time quantum token storage in a warm vapour memory using time-bin encoding and coherent single-photon pulses, and perform preliminary experiments in this direction. Operating within a technologically relevant regime for future applications, our simple and scalable system has proven to be a promising candidate for satellite deployment and integration into quantum repeaters for future quantum networks.