Wir demonstrieren ein optisches, nicht-deterministisches CSIGN-Gatter für Quantencomputer. Das CSIGN-Gatter ist in der Lage ursprünglich nicht verschränkte Qubits zu verschränken und repräsentiert daher ein elementares und wichtiges Gatter für universelles quantencomputing. Die Wirkung des Gatters wurde mit Hilfe von Prozesstomographie vollständig charakterisiert und wir finden eine Prozessgüte von 0.84 +/- 0.01.

We present and demonstrate an all-optical, non-deterministic CSIGN-gate for quantum computation. The CSIGN-gate is capable of entangling previously unentangled qubits and
therefore represents an elementary operation relevant for universal quantum computing. It can also be employed for the generation of novel multi-particle entangled states, among
them the so-called cluster states. The operation of the quantum gate is completely characterized by performing quantum state and process tomography. Reconstructing the process matrix of the CSIGN-gate, we find an average gate fidelity of Favg = 0.84 +/- 0.01. The realized optical CSIGN-circuit is based on the two-photon scheme of References [14, 15], and since it requires only a single optical mode-matching condition, its construction is drasti-
cally facilitated compared to previous schemes. This circuit indeed presents the simplest entangling optical gate realized to date. This thesis is written in a fully self-contained manner, introducing and establishing the required theoretical background and giving a full description of the experimental setup and procedure as well as a thorough discussion of the results and occurring problems. We propose the extension of the above scheme to
generate a genuine 3-photon cluster state, which is equivalent to a Greenberger-Horne-Zeilinger-state (GHZ-state) [16], and give a short outlook on future experiments. In additional experiments the effects of temporal mode-mismatch has been studied. This was achieved with an adapted quantum teleportation experiment, showing that the fidelity of such a quantum communication protocol declines in a Gaussian fashion as a function of the temporal mode-mismatch. A simple theoretical model is developed that explains this behaviour, consistent with the experimental data.