  We investigate how decoherence affects the entanglement established between
two quantum dots in micro cavities, and propose a tomographic scheme able to
measure the entangled state. The scheme we consider establishes the
entanglement via the exchange and measurement of a photon. Making the realistic
assumption of noise dominated by pure-dephasing processes, we find that the
photon must be exchanged and measured on timescales shorter than the quantum
dots' characteristic dephasing time for appreciable levels of entanglement to
be achieved. The tomographic scheme is able to reconstruct the full density
matrix of the quantum dots, and requires only single spin rotations and the
injection of an additional photon. Remarkably, we find that the additional
photon need not be exchanged and measured on a timescale shorter than the
dephasing time for accurate tomography, and also allows many to be used in
order to increase the measurement signal.
