The observation of strongly interacting many-body phenomena in atomic gases typically requires ultracold samples. Here we show that the strong interaction potentials between Rydberg atoms enable the observation of many-body effects in an atomic vapor, even at room temperature. We excite Rydberg atoms in cesium vapor and observe in real time an out-of-equilibrium excitation dynamics that is consistent with an aggregation mechanism. The experimental observations show qualitative and quantitative agreement with a microscopic theoretical model. Numerical simulations reveal that the strongly correlated growth of the emerging aggregates is reminiscent of soft-matter type systems.

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 ondemand 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.Single-photon emitters are of interest for manifold applications in quantum computation (1), simulation (2), sensing (3), and especially in quantum secure communication (4). In particular, the latter will demand efficient quantum repeaters (5) that require efficient single-photon creation and storage schemes, ideally at the same platform. On the one hand, room temperature alkali gases can perfectly serve as such storage media and second-long storage times have been already demonstrated for weak coherent states (6). On the other hand, the generation of single photons with stable wavelength and adapted bandwidth is still a challenge.The first observation of anti-bunched photon counting statistics from single-atom fluorescence dates back to 40 years ago (7). Today very efficient on-demand solid state sources based on quantum dots (8), color centers (9), and single molecules (10) are available. Although in principle most of these systems are working at room temperature as well (11 -13), they do require cryogenic temperatures for optimal performance. As they are solid-state embedded, they suffer from interactions with phonons, spin noise, strain, and drifting electric fields. These result in large variations in frequencies, spectral wandering, and additional phase noise causing spectral diffusion, which is a fundamental limitation for scalability.

We present a pulsed four-wave mixing (FWM) scheme via a Rydberg state to create, store and retrieve collective Rydberg polaritons. The storage medium consists of a gas of thermal Rb atoms confined in a 220 {\mu}m thick cell, which are heated above room temperature. The experimental sequence consists of a pulsed excitation of Rydberg polaritons via the D1 line, a variable delay or storage time, and a final retrieval pulse via the D2 line. The lifetime of the Rydberg polaritons is around 1.2 ns, almost entirely limited by the excitation bandwidth and the corresponding motional dephasing of the atoms. The presented scheme combined with a tightly confined atomic ensemble is a good candidate for a deterministic single-photon source, as soon as strong interactions in terms of a Rydberg blockade are added.Comment: 5 pages, 3 figure