Generation of a high visibility rotationally invariant six-photon entangled state

Invariant multiqubit entangled states, remaining unchanged under simultaneous identical unitary transformations of all their individual constituents, can be applied for decoherence-free encoding, quantum communication without sharing a common reference frame, telecloning and secret sharing. They also have properties which are interesting in studies of foundations of quantum mechanics. We experimentally generate and characterize such an invariant six-photon polarization entangled state. This is done solely with a suitable filtering procedure of triple emissions of entangled photon pairs from a single source, without any need for interferometric overlaps. We report the experimental observation of the invariant six-photon polarization entangled state, given by the superposition of a six-qubit Greenberger-Horne-Zeilinger (GHZ) state and two products of three-qubit states. To verify that we actually are dealing with genuine entanglement we have derived a reduced entanglement witness, capable of affirming genuine six-partite entanglement with only three measurement settings.