Wepresent an experimental state-independent violation of an inequality for noncontextualtheories on single particles. We show that 20 different single-photonstates violate an inequality which involves correlations between results ofsequential compatible measurements by at least 419 standard deviations. Ourresults show that, for any physical system, even for asingle system, and independent of its state, there is auniversal set of tests whose results do not admit anoncontextual interpretation. This sheds new light on the role ofquantum mechanics in quantum information processing.

Entanglement is a key resource in many quantum information schemes andin the last years the research on multi-qubit entanglement has drawn lots ofattention. In this thesis the experimental generation and characterization ofmulti-qubit entanglement is presented. The qubits are implemented in the polarizationdegree of freedom of photons and we have prepared genuine entangledstates of two, four and six photons. We emphasize that one type of states thatwe produce are invariant entangled states, remaining unchanged under simultaneousidentical unitary transformations of all their individual constituents.Such states can be applied to e.g. decoherence-free encoding, quantum communicationwithout sharing a common reference frame, quantum telecloning,secret sharing and remote state preparation schemes.In the experimental implementation we use a single source of entangledphoton pairs and extract the first, second and third order parametric downconversion.The multi-order processes are not entirely spontaneous, as we getthe right states utilizing bosonic emission enhancement due to indistinguishability.Despite the achievement of six-photon entangled states, is the setupcompletely free from interferometric overlaps making it robust and contributingto high fidelities of the generated states. The analysis results of our experimentalstates are in very good agreement with theory and also show very highvisibilities in their correlations.

Entanglement is a key resource in many quantum information schemes and in the last years the research on multi-qubit entanglement has drawn lots of attention. In this thesis the experimental generation and characterisation of multi-qubit entanglement is presented. Specifically we have prepared entangled states of up to six qubits. The qubits were implemented in the polarisation degree of freedom of single photons. We emphasise that one type of states that we produce are rotationally invariant states, remaining unchanged under simultaneous identical unitary transformations of all their individual constituents. Such states can be applied to e.g. decoherence-free encoding, quantum communication without sharing a common reference frame, quantum telecloning, secret sharing and remote state preparation schemes. They also have properties which are interesting in studies of foundations of quantum mechanics.

In the experimental implementation we use a single source of entangled photon pairs, based on parametric down-conversion, and extract the first, second and third order events. Our experimental setup is completely free from interferometric overlaps, making it robust and contributing to a high fidelity of the generated states. To our knowledge, the achieved fidelity is the highest that has been observed for six-qubit entangled states and our measurement results are in very good agreement with predictions of quantum theory.

We have also performed another novel test of the foundations of quantum mechanics. It is based on an inequality that is fulfilled by any non-contextual hidden variable theory, but can be violated by quantum mechanics. This test is similar to Bell inequality tests, which rule out local hidden variable theories as possible completions of quantum mechanics. Here, however, we show that non-contextual hidden variable theories cannot explain certain experimental results, which are consistent with quantum mechanics. Hence, neither of these theories can be used to make quantum mechanics complete.

Invariant^{ }entangled states remain unchanged under simultaneous identical unitary transformations of^{ }all their subsystems. We experimentally generate and characterize such invariant^{ }two-, four-, and six-photon polarization entangled states. This is done^{ }only with a suitable filtering procedure of multiple emissions of^{ }entangled photon pairs from a single source without any interferometric^{ }overlaps. We get the desired states utilizing bosonic emission enhancement^{ }due to indistinguishability. The setup is very stable and gives^{ }high interference contrasts. Thus, the process is a very likely^{ }candidate for various photonic demonstrations of quantum information protocols.

We experimentally generate and characterize a six-photon polarizationentangled state, which is usually called ‘9+6 ’. This is realized with a filteringprocedure of triple emissions of entangled photon pairs from a single source,which does not use any interferometric overlaps. The setup is very stable and weobserve the six-photon state with high fidelity. The observed state can be usedfor demonstrations of telecloning and secret sharing protocols.

Quantum^{ }multiphoton interferometry has now reached the six-photon stage. Thus far,^{ }the observed fidelities of entangled states never reached 2/3. We^{ }report a high fidelity (estimated at 88%) experiment in which^{ }six-qubit singlet correlations were observed. With such a high fidelity^{ }we are able to demonstrate the central property of these^{ }“singlet” correlations, their “rotational invariance,” by performing a full set^{ }of measurements in three complementary polarization bases. The patterns are^{ }almost indistinguishable. The data reveal genuine six-photon entanglement. We also^{ }study several five-photon states, which result upon detection of one^{ }of the photons. Multiphoton singlet states survive some types of^{ }depolarization and are thus important in quantum communication schemes.