Observation of collective excitation of two individual atoms in the   Rydberg blockade regime
  The dipole blockade between Rydberg atoms has been proposed as a basic tool in quantum information processing with neutral atoms. Here we demonstrate experimentally the Rydberg blockade of two individual atoms separated by 4 $\mu$m. Moreover, we show that, in this regime, the single atom excitation is enhanced by a collective two-atom behavior associated with the excitation of an entangled state. This observation is a crucial step towards the deterministic manipulation of entanglement of two or more atoms using the Rydberg dipole interaction. 
A. Ga\"etan, Y. Miroshnychenko, T. Wilk, A. Chotia, M. Viteau, D.   Comparat, P. Pillet, A. Browaeys, and P. Grangier