1. Centre for Quantum Computation, Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, UK

Correspondence to: D. Bouwmeester Correspondence and requests for materials should be addressed to A.L.-L. (e-mail: Email: a.lamas@qubit.org).

Entangled photon pairs—discrete light quanta that exhibit non-classical correlations—play a crucial role in quantum information science (for example, in demonstrations of quantum non-locality^{1, 2, 3, 4, 5, 6, 7}, quantum teleportation^{8, 9} and quantum cryptography^{10, 11, 12, 31}). At the macroscopic optical-field level non-classical correlations can also be important, as in the case of squeezed light¹³, entangled light beams^{14, 15} and teleportation of continuous quantum variables¹⁶. Here we use stimulated parametric down-conversion to study entangled states of light that bridge the gap between discrete and macroscopic optical quantum correlations. We demonstrate experimentally the onset of laser-like action for entangled photons, through the creation and amplification of the spin-1/2 and spin-1 singlet states consisting of two and four photons, respectively. This entanglement structure holds great promise in quantum information science where there is a strong demand for entangled states of increasing complexity.