Room temperature single-photon sources and atomic quantum memories for broadband quantum networks

Quantum networks are envisioned to overcome current limitations in quantum communication and computation. The building blocks that enable the realization of such networks are quantum memories and single-photon sources.
A promising path to realize these networks is to make use of heterogeneous interconnects. By interfacing quantum memories based on atomic ensembles with solid-state sources, the best of two worlds could be exploited. However, achieving compatibility between different systems, in order to realize a hybrid quantum network node, has remained an open challenge.
In this thesis I report on broadband quantum memories implemented in warm rubidium vapor and on a compatible single-photon source based on non-degenerate cavity-enhanced spontaneous parametric downconversion. The goal is to implement an elementary interconnect where the experimental complexity is kept low by operating all components at or above room temperature. Choosing a monolithic cavity design, the source is inherently robust and reaches high efficiencies. It generates heralded single photons with hundreds of \si{\mega\hertz} bandwidth and reaches high heralding efficiencies of $\geq 40\%$, measured after coupling into a single-mode optical fiber.
The memories presented here are based on electromagnetically induced transparency (EIT) in a lambda-level scheme in warm rubidium vapor. To suppress the read-out noise, which is a limiting factor in common ground-state memory implementations, two separate approaches are followed.
In the first one, the atomic structure is modified by applying a tesla-order magnetic field and working in the hyperfine Paschen-Back regime. This results in large splittings between the energy levels, which allows us to optically address individual sublevels in the warm vapor.
A spectroscopic study of EIT and optical pumping in this regime is presented.
Our proof-of-principle implementation of a quantum memory in a miniaturized vapor cell has delivered promising results. This setup was capable of storing and retrieving weak coherent pulses attenuated to the single-photon level, yielding an end-to-end efficiency of $\eta_{e2e} \approx 3\%$ and a SNR of up to $7.9(8)$.
The second approach to suppressing read-out noise relies on exploiting polarization selection rules in a Zeeman-pumped vapor. This memory implementation was successfully interfaced with \SI{370}{\mega\hertz}-broad single photons from the heralded downconversion source.
The stored photons maintained their non-classical signature after retrieval, yielding a $g^{(2)}(0) = 0.177(23)$. This constitutes the first demonstration of single photon storage and retrieval from the ground state of a warm atomic vapor.
The developed platform operates in a technologically relevant regime for future experiments, paving the way for the exploration of promising quantum-network protocols at high bandwidth.