\#1[|[\#1]{}]{}

Introduction
============

Entanglement [@bell87] denotes the nonlocal correlations that exist, even in the absence of direct interaction, between two (spatially separated) parts of a given quantum system. Since the early days of quantum mechanics, understanding the phenomenon of entanglement has been central to the understanding of the foundations of quantum theory. Besides its fundamental importance, a great deal of interest has been brought forth by its role in quantum information [@nielsen00]. Entanglement is believed to be the main ingredient of computational speed-up in quantum information protocols.

Most of the work on entanglement has been performed in optical systems with photons [@zeilinger99], cavity QED systems [@rauschenbeutel00] and ion traps [@sackett00]. Only recently attention has been devoted to the manipulation of entangled states in a solid state environment. This interest, originally motivated by the idea to realize a solid state quantum computer [@makhlin01; @awschalom02], has been rapidly growing and by now several works discuss how to generate, manipulate and detect entangled states in solid state systems. It is probably worth to emphasize already at this point that, differently from the situation encountered in quantum optics, in solid state system entanglement is rather common. What is not trivial is its control and 