Ultra-cold atomic gases can now be precisely controlled and manipulated in a regime dominated by quantum-mechanical many-body effects. Single photons can be generated deterministically by exploiting the strong interactions of atoms excited into Rydberg levels of principal quantum number n~100. Many-body Rabi oscillations seen in such ensembles hold promise for the simulation of complex quantum systems. The wavelength of single photons can be translated across the optical spectrum using four-wave mixing in a cold atomic gas, while minute-scale storage of light is possible by employing ground-level coherences of atoms stored in optical lattices, laying a foundation for the realization of quantum systems distributed over inter-continental distances. Precise measurements of atomic transition frequencies are at the heart of modern time-keeping. A world-wide effort is now directed towards the measurement of the transition frequency between the nuclear ground doublet states in Th-229. An optical clock based on this transition offers prospects for a test of the temporal variation of fundamental constants at an unprecedented level of precision.