Absorb, store, and re-emit: Quantum memory via coherent light-matter interactions and collective atomic excitations

Rooted in established phenomena like superradiance and electromagnetically induced transparency (EIT), the manipulation and storage of light by collective excitations among atoms has expanded beyond the realm of fundamental research and into the domain of applications as a platform for quantum memory, with near-term potential in quantum communications systems. Optical quantum memories coherently absorb, store, and re-emit light, which facilitates the transfer of (quantum) information from a photonic degree of freedom (which can be transmitted over long distances) to an atomic one (which is stationary and long-lived), and back again, sometimes on-demand. Here, we will explore the nature of the atom-light interactions that give rise to collective atomic excitations, how these collective excitations both preserve information and re-emit light, and how quantum information can be coherently transformed from photonic to atomic degrees of freedom. We will, in particular, consider the two-field Λ-configuration used to create a "spin-wave" among ground states, and look at how factors such as efficiency, noise, bandwidth, and multimode capacity can be optimized. Several flavours of atomic quantum memory will be discussed, including those with warm, cold, and ultracold atoms, using protocols including off-resonant Raman, EIT, and Autler-Townes Splitting (ATS). We will, through this exploration of quantum memories, have the opportunity to appreciate the duality of light as a tool to manipulate atomic states, and atomic states as a tool to manipulate the state of light.