We study the occurrence of multipartite entanglement in spin chains. We show that certain genuine multipartite entangled states, namely $W$ states, can be obtained as ground states of simple XX type ferromagnetic spin chains in a transverse magnetic field, for any number of sites. Moreover, multipartite entanglement is proven to exist even at finite temperatures. A transition from a product state to a multipartite entangled state occurs when decreasing the magnetic field to a critical value. Adiabatic passage through this point can thus lead to the generation of multipartite entanglement.

• Received 1 March 2005

DOI:https://doi.org/10.1103/PhysRevA.72.014301