Many-particle entanglement with Bose--Einstein condensates
  We propose a method to produce entangled states of several particles starting from a Bose-Einstein condensate. In the proposal, a single fast $\pi/2$ pulse is applied to the atoms and due to the collisional interaction, the subsequent free time evolution creates an entangled state involving all atoms in the condensate. The created entangled state is a spin-squeezed state which could be used to improve the sensitivity of atomic clocks. 
Anders Sorensen, Lu-Ming Duan, Ignacio Cirac, and Peter Zoller