内容：

Ultra-cold atomic gases can now be precisely controlled and manipulated in the quantum regime. In this talk, I will present our results on using ensembles of cold atoms, especially high-lying Rydberg atoms to study novel quantum effects and their applications in quantum optics. We have developed experimental techniques and protocols to push the limits on achieving long-lived (seconds) coherence in ground states and rapid (sub-µs) quantum operations with strongly-interacting Rydberg states. With these capabilities, we have demonstrated long-term storage of photons, Rydberg blockade and many-body Rabi oscillations, deterministic single photon source, magic trapping of Rydberg atoms, Rydberg atom-light entanglement, and strongly-interacting quantum memory.