Abstract
The sixth lesson continued the description of trapped ion entanglement experiments. The definition of GHZ and W states was given, as well as an analysis of the decoherence of these states by unread measurement of one of the ions. We have described two studies by the Innsbruck group in which GHZ and W entangled states were generated by individual ion addressing and studied by tomography. The first, limited to N = 3, shows how the unread measurement of one of the ions affects the density operator of the ensemble (decoherence). It also demonstrates simple quantum eraser procedures. The second study generated and performed the complete W-state tomography comprising up to 8 ions. We then gave the principle of the topological gate, based on the loop displacement in its phase space of a collective vibrational mode accumulating a phase dependent on the ions' internal state. We described a Boulder Group experiment illustrating this principle on two Be+ ions .
