A Photon-Photon Quantum Gate Based on Rydberg Polaritons (S. Schmidt-Eberle)
Rydberg polaritons offer a unique way to create strong
interactions for photons. We utilize these interactions to demonstrate a
photon-photon quantum gate. To achieve this, a photonic control qubit is stored
in a quantum memory consisting of a superposition of a ground state and a
Rydberg state in an ultracold atomic gas. This qubit interacts with a photonic
target qubit in the form of a propagating Rydberg polariton to generate a
conditional pi phase shift, as in Ref. . Finally, the control photon is retrieved.
We measure two controlled-NOT truth tables and the two-photon state after an
entangling-gate operation. This work is an important step toward applications
in optical quantum information processing, such as deterministic photonic
Bell-state detection which is crucial for quantum repeaters. - Reference:  D. Tiarks et al., Science Advances 2, 1600036
Coherent spin dynamics in Rydberg-dressed Ising chains (J. Zeiher)
Off-resonant optical coupling of an atomic ground state to a Rydberg state, so called "Rydberg-dressing", has been proposed as a versatile method to implement various long-range interacting spin models with ultracold atoms. In our experiment, we realize Rydberg-dressed Ising spin interactions in an atomic Mott insulator of Rubidium-87 by off-resonant optical coupling to a Rydberg p-state. First interferometric experiments in a two-dimensional sample demonstrated versatile control of the induced interactions, however collective loss processes reduced the lifetime of the system. Here, we present recent experimental results for a Rydberg-dressed 1d spin chain with long-range Ising interactions. Contrary to the 2d case, the collective loss can be avoided and lifetimes increase significantly. We substantiate the improved lifetimes by showing purely interaction driven coherent collapse and revival dynamics of the magnetization in a 1d spin chain.