Efficient multiparticle entanglement via asymmetric Rydberg blockade
  We present an efficient method for producing $N$ particle entangled states using Rydberg blockade interactions. Optical excitation of Rydberg states that interact weakly, yet have a strong coupling to a second control state is used to achieve state dependent qubit rotations in small ensembles. On the basis of quantitative calculations we predict that an N=8 Schr\"odinger cat state can be produced with a fidelity of 84% in cold Rb atoms. 
M. Saffman and K. Molmer