Multi-photon entanglement and applications in quantum information Fakultät für Physik - Digitale Hochschulschriften der LMU - Teil 02/05

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.