Nature, Published online: 22 January 2025; doi:10.1038/d41586-025-00095-2In certain crystals, quasiparticles called Weyl fermions emerge from the collective quantum behaviour of electrons. However, these quasiparticles typically coexist with undesired conventional electronic states. After a decade-long search, physicists have engineered a crystal without conventional electronic states and with quantum properties governed entirely by Weyl fermions. Such materials could be useful for detecting and generating terahertz light.