Angular entanglement of two photons

Most protocols for quantum communication rely on entanglement between qubits. In this contribution we report on an experiment on angular entanglement of two photons. We introduce a quantifier of the dimensionality of the entanglement, using concepts from classical information theory. This Shannon dimensionality gives the effective number of information channels. We exploit the orbital-angular-momentum degree of freedom of photons that emerge from a parametric down-conversion source. The number of entangled modes, known as the Schmidt number, is of order 40. The entanglement is analyzed with a pair of angular state projectors, each consisting of a phase plate, a single-mode fiber and a single-photon detector. The phase plates are rotated about their axes and we record the photon coincidence rate as a function of their independent orientations. The coincidence count rates depend only on the relative orientation of the analyzers. This is a key signature of entanglement.