Multipartite Einstein–Podolsky–Rosen steering and genuine tripartite entanglement with optical networks

The quantum mechanical concept of ‘steering’ refers to the feasibility of one system to nonlocally affect, or steer, another system’s states through local measurements. Multipartite steering is now demonstrated in a programmable optical network. Einstein, Podolsky and Rosen (EPR) pointed out in their famous paradox that two quantum-entangled particles can have perfectly correlated positions and momenta 1 . Such correlations give evidence for the nonlocality of quantum mechanics 2 and form the basis for quantum cryptography 3 and teleportation 4 . EPR steering 5 , 6 , 7 , 8 is the nonlocality associated with the EPR paradox and has traditionally been investigated between only two parties 9 , 10 , 11 , 12 , 13 , 14 . Using optical networks and efficient detection, we present experimental observations of multiparty EPR steering and of the genuine entanglement of three intense optical beams. We entangle the quadrature phase amplitudes of distinct fields, in analogy to the position–momentum entanglement of the original paradox. Our experiments complement tests of quantum mechanics that have entangled small systems 15 , 16 , 17 or have demonstrated tripartite inseparability 18 , 19 . Our methods establish principles for the development of multiparty quantum communication protocols with asymmetric observers, and can be extended to qubits, whether photonic 11 , 12 , 13 , 15 , 17 , atomic 16 , superconducting 20 , or otherwise.