  We report on tomographic means to study the stability of a qubit register
based on a string of trapped ions. In our experiment, two ions are held in a
linear Paul trap and are entangled deterministically by laser pulses that
couple their electronic and motional states. We reconstruct the density matrix
using single qubit rotations and subsequent measurements with near-unity
detection efficiency. This way, we characterize the created Bell states, the
states into which they subsequently decay, and we derive their entanglement,
applying different entanglement measures.
