Some new experiments leading to observation of multi-photon entanglement will be presented. They utilize
multi-pair emissions in a down-conversion crystal. In configurations that avoid interferometric overlaps this
leads to a high visibility multi-photon interference. General theoretical results concerning the influence of photon
statistics and distinguishability on the interference effects in such experiments is be given. Methods of detection
of entanglement in multi-qubit systems, via Bell's theorem and a non-linear generalization of entanglement
witnesses, will be presented. Finally it will be shown that multi-photon entanglement can be applied to reduce
of communication complexity of some computational tasks.
Multiphoton entanglement is the basis of many quantum
communication schemes, quantum cryptographic protocols, and
fundamental tests of quantum theory. Spontaneous parametric
down-conversion is the most effective source for polarization
entangled photon pairs. Here we show, that a entangled 4-photon
state can be directly created by parametric down-conversion. This
state exhibit perfect quantum correlations and a high robustness
of entanglement against photon loss. We have used this state for
four-particle test of local realistic theories. Therefore this
state can be used for new types of quantum communication. We also
report on possibilities for the experimentally realization of a
3-photon entangled state, the so called W-state, and discuss some
of its properties.
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