Multi-photon entanglement and applications in quantum information

Since the awareness of entanglement was raised by Einstein,
Podolski, Rosen and Schrödinger in the beginning of the last
century, it took almost 55 years until entanglement entered the
laboratories as a new resource. Meanwhile, entangled states of
various quantum systems have been investigated. Sofar, their
biggest variety was observed in photonic qubit systems. Thereby,
the setups of today's experiments on multi-photon entanglement can
all be structured in the following way: They consist of a photon
source, a linear optics network by which the photons are processed
and the conditional detection of the photons at the output of the
network. In this thesis, two new linear optics networks are
introduced and their application for several quantum information
tasks is presented. The workhorse of multi-photon quantum
information, spontaneous parametric down conversion, is used in
different configurations to provide the input states for the
networks. The first network is a new design of a controlled phase
gate which is particularly interesting for applications in
multi-photon experiments as it constitutes an improvement of former
realizations with respect to stability and reliability. This is
explicitly demonstrated by employing the gate in four-photon
experiments. In this context, a teleportation and entanglement
swapping protocol is performed in which all four Bell states are
distinguished by means of the phase gate. A similar type of
measurement applied to the subsystem parts of two copies of a
quantum state, allows further the direct estimation of the state's
entanglement in terms of its concurrence. Finally, starting from
two Bell states, the controlled phase gate is applied for the
observation of a four photon cluster state. The analysis of the
results focuses on measurement based quantum computation, the main
usage of cluster states. The second network, fed with the second
order emission of non-collinear type II spontaneous parametric down
conversion, constitutes a tunable source of a whole family of
states. Up to now the observation of one particular state required
one individually tailored setup. With the network introduced here
many different states can be obtained within the same arrangement
by tuning a single, easily accessible experimental parameter. These
states exhibit many useful properties and play a central role in
several applications of quantum information. Here, they are used
for the solution of a four-player quantum Minority game. It is
shown that, by employing four-qubit entanglement, the quantum
version of the game clearly outperforms its classical counterpart.
Experimental data obtained with both networks are utilized to
demonstrate a new method for the experimental discrimination of
different multi-partite entangled states. Although theoretical
classifications of four-qubit entangled states exist, sofar there
was no experimental tool to easily assign an observed state to the
one or the other class. The new tool presented here is based on
operators which are formed by the correlations between local
measurement settings that are typical for the respective quantum
state.