  Tailored quantum states of light can be created via a transfer of collective
quantum states of matter to light modes. Such collective quantum states emerge
in interacting many-body systems if thermal fluctuations are overcome by
sufficient interaction strengths. Therefore, typically ultracold temperatures
or strong confinement are required. We show that the exaggerated interactions
between giant Rydberg atoms allow for collective quantum states even above room
temperature. The emerging Rydberg blockade allows then only for a single
Rydberg excitation. We experimentally implement a four-wave mixing scheme to
demonstrate an on-demand single-photon source. The combination of glass cell
technology, identical atoms, and operation around room temperature promises
scalability and integrability. This approach has the potential for various
applications in quantum information processing and communication.
