Experiments on Multiphoton Entanglement

Entanglement lies at the heart of quantum mechanics and challenged
the intuition of physicists ever since it was discovered. At the
same time, it is a powerful tool that serves as a key resource for
quantum communication and quantum computation schemes. Many of
these applications rely on multiparticle entanglement, whose
description, generation and manipulation became therefore a very
active field in theoretical and experimental quantum information
science. The goals are here to classify and understand the
different types of entanglement, to find new applications and to
control and analyze the quantum states experimentally. In this
thesis, the experimental observation and analysis of two different
types of four-photon polarization entangled states is presented:
The cluster state and the symmetric Dicke state with two
excitations. For this purpose, experimental setups based on
spontaneous parametric down conversion and linear optics with
conditional detection were designed. They allowed to observe the
cluster state with a fidelity of 74.1 % and the symmetric Dicke
state with a fidelity of 84.4 %. The cluster state experiment
included the development of a new instrument that is of interest
for linear optics quantum logic in general: A probabilistic
controlled phase gate that is, due to the simplification of a
previous approach, highly stable and can actually be used in
multiphoton experiments. The quality of the gate is evaluated by
analyzing its entangling capability and by performing full process
tomography. The achieved results demonstrate that this device is
well suited for implementation in various multiphoton quantum
information protocols. In order to study the observed quantum
states, efficient analysis tools are introduced. It was possible to
verify that essential properties of the ideal states are indeed
reproduced in the experiment, among others, the presence of genuine
four-partite entanglement. A particular focus is put on the
behavior of the states under projective measurements and photon
loss. Several new insights in their entanglement structure are
revealed and verified experimentally. We further demonstrate
properties that are characteristic for the entanglement classes of
the states. These can be used to infer the applicability of the
observed states for certain distributed quantum communication
applications. The presented experiments are generic for the design
of setups to observe cluster- and symmetric Dicke states with a
higher number of photons. Furthermore, also the efficient
non-tomographic methods for state analysis we employ can directly
be generalized to experiments with higher qubit numbers, where the
reduction of the experimental effort for state analysis is even
more crucial.