Entanglement of Orbital Angular Momentum States of Photons

Entanglement contains one of the most interesting features of quantum mechanics, often named quantum non-locality[1, 2]. This means entangled states are not separable regardless of the spatial separation of their components. Measurement results on one particle of a two-particle entangled state define the state of the other particle instantaneously with neither particle enjoying its own well-defined state before the measurement. So far experimental confirmation of entanglement has been restricted to qubits, i.e. two-state quantum systems including recent realization of three[3, 4] and four-qubit [5, 6] entanglements. Yet, an ever increasing body of theoretical work calls for entanglement in quantum system of higher dimensions[7, 8]. For photons one is restricted to qubits as long as the entanglement is realized using the photons polarization. Here we report the first realization of entanglement exploiting the orbital angular momentum of photons,