Long-lived collective Rydberg excitations in atomic gas achieved via ac-Stark lattice modulation

Collective Rydberg excitations are the subject of growing interest in many key domains of physics. In particular, these excitations hold promising potential for applications in fields ranging from quantum information processing, and quantum simulators to ultra-sensitive electrometry. Their vast potential is somewhat limited in real-life scenarios by their short thermal dephasing time, which for typical temperatures in experiments (~100 μK) is not much more than a single microsecond. Some state-of-the-art methods provide means to increase the atomic coherence lifetime, however, they were only ever implemented in ground-state quantum memories. Their application in Rydberg excitations would require a fundamental redesign to work. We propose a novel protocol based on Ac-Stark phase modulation of a quantum memory ensemble, which in principle can freeze the atomic coherence and completely halt the effect of thermal decoherence. Our implementation was realized by interfering two off-resonant laser beams on the atomic medium and demonstrated that a ten-fold increase in lifetime is feasible. We verified our results by simulating the atomic-coherence evolution in phase space, accounting for the finite duration of the modulation pulses, imperfect alignment of the experimental setup, and the movement of atoms in different velocity classes. 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 dephasing. 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.