Solid-state optical quantum memory: from large entanglement to telecom wavelengths

Abstract

Quantum mechanics is leading to many modern technologies, e.g. based on laser physics. Emerging fields are for example nonlinear optics, hybrid systems, or quantum communication. Despite these applications, there are still limitations of our understanding of quantum mechanics. Even though it is broadly accepted that decoherence limits the size of entangled states, we currently do not know how to overcome these limitations. How large can commonly entangled states be? Can we observe macroscopic quantum phenomena with coarse-grained detectors or even with our bare eyes? This thesis contributes towards answering these fundamental questions and to technological advances. The experiments are based on a light-matter platform of quantum optical states and atomic ensembles. The ensembles consist of rare-earth ion-doped crystals. Additionally, the atomic frequency comb protocol turns the ensemble into a quantum memory. In the scope of quantum communication we work on a quantum repeater. First, we investigate fundamental quantum mechanical states and their validity at macroscopic scales. We show that the detection of a collectively emitted excitation can be used to measure the size of the entangled ensemble. The entangled ensemble shared this excitation in a coherent superposition. This superposition of an atomic excitation in an ensemble is called W-state. We certify a new lower bound to the possible size of an entangled W-state between at least 16 million atoms. The proof of entanglement of this size relies on two measures: one determines the number of atoms involved in the storage process. The other one determines the state quantumness by examining the re-emitted light. Another experiment studies a macroscopically distinguishable superposition state. The superposition survives the storage in an atomic ensemble. Second, we perform two more experiments that contribute to a quantum repeater. One experiment probes the quantum states simultaneously stored in a multi-mode quantum memory. In this work we also develop the concept of indirect entanglement witness. For the other experiment, a frequency down-conversion platform is set up. The conversion overcomes the wavelength compatibility issue between some quantum memories and the telecommunication C-band. The narrow spectral filtering of the conversion performs better than expected. In summary, our work analyzes large entangled states and building blocks for a quantum repeater. The presented quantum memories are capable of temporal multi-mode storage. Its temporally precise re-emission is due to the re-phasing of the atoms of the W-state. We also show a good way to set up frequency down-conversion for the sake of quantum repeaters. This work emphasizes the atomic frequency comb protocol as a viable and reliable platform for quantum repeaters.