  We use semiconductor quantum dots, "artificial atoms," to implement a scheme
for deterministic generation of long strings of entangled photons in a cluster
state, an important resource for quantum information processing. We demonstrate
a prototype device which produces strings of a few hundred photons in which the
entanglement persists over 5 sequential photons. The implementation follows a
proposal by Lindner and Rudolph (Phys. Rev. Lett. 2009) which suggested
periodic timed excitation of a precessing electron spin as a mechanism for
entangling the electron spin with the polarization of the sequentially emitted
photons. In our realization, the entangling qubit is a quantum dot confined
dark exciton. By performing full quantum process tomography, we obtain the
process map which fully characterizes the evolution of the system, containing
the dark exciton and n photons after n applications of the periodic
excitations. Our implementation may greatly reduce the resources needed for
quantum information processing.
