An international team from École polytechnique, Collège de France, and HZDR has experimentally realized an all-optical photonic time crystal. Using HZDR’s TELBE terahertz source, they strongly modulated the optical properties of a plasmonic metamaterial on picosecond timescales. This enables ultrafast control of light in the terahertz range and opens paths toward new lasers and optical technologies. Published in Nature.