Abstract (eng)
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.