Cold Rydberg Gases



A quantum many-particle system in which interactions can be tuned over a vast range may enable profound changes in the way we understand and explore physics of the microscopic realm. For example, it may lead to previously unknown phases of matter and aid in the discovery of new phenomena. Strong coupling allows for fast, controllable many-body dynamics, whereas the weakly interacting mode can be used for precise external manipulations and measurements. In the context of quantum information science, strong interactions are required to implement fast quantum gates, and long-term storage is achieved in the noninteracting regime.


Cold atomic gases are a fruitful platform for such studies. They permit one to perform experiments under well-understood and controlled conditions. Resonant optical driving of an atomic gas between the ground level and a high-lying Rydberg level is a particularly promising setting for studies of many-body systems with strong, long-range interactions. Highly excited Rydberg atoms have many exaggerated properties. In particular, the interaction strength between such atoms can be varied over an enormous range.

An important manifestation of Rydberg-level interactions is excitation blockade, in which an atom promoted to a Rydberg level shifts the energy levels of nearby atoms, suppressing their excitation.

Recently, we create a quasi-two-level system in a regime of Rydberg excitation blockade for a mesoscopic Rb ensemble of several hundred atoms confined in a magic-wavelength optical lattice. We observe many-body Rabi oscillations between the ground and collective Rydberg state. In addition we use Ramsey interference techniques to obtain the light shifts of both the lower and upper states of the collective qubit.

The strongly pronounced oscillations indicate a nearly complete excitation blockade of the entire mesoscopic ensemble by a single excited atom. The results pave the way towards quantum computation and simulation using ensembles of atoms.

The interaction-induced dephasing of collective atomic states is often the dominant contribution to the entanglement generation process in atomic ensembles. Here we report a study of the temporal evolution of an initially unentangled Rydberg spin wave into an (entangled) Dicke state. These results have relevance to broad classes of applications for collective atomic systems, including driving of collective atomic qubits, on-demand generation of single photons, and preparation of entangled states involving atoms or light.