Long-lived collective Rydberg excitations in cold atomic gas*

Collective Rydberg excitations provide applications ranging from quantum information processing to ultra-sensitive electrometry. However, their short lifetime is an immense obstacle in real-life scenarios. The state-of-the-art methods of prolonging the lifetime were mainly implemented for ground-state quantum memories and would require a redesign to effectively work on different atomic transitions. We propose a scheme for extending the Rydberg excitation lifetime by modulating the stored spinwave with two interfering beams, which allows for complete cancelation of the effects of thermal dephasing. Our experimental setup achieved more than a 10-fold improvement in lifetime, reaching very close to the natural lifetime of the Rydberg state determined by spontaneous and radiative decay. With this, our coherence times approach the coherence of single atoms, as predicted by fundamental effects. We envisage applications of our superatom generator for both quantum metrology and in optically-interconnected Rydberg quantum computers. The presented protocol makes measurements of Rydberg interactions possible on longer timescales, which is often crucial in electrometry or for usage of long-lived collective qubits for quantum simulations and computations interfaces with light.