Stimulated emission of polarization-entangled photons

Entangled photon pairs-discrete light quanta that exhibit nonclassical correlations-play a crucial role in quantum information science (for example, in demonstrations of quantum non-locality(1-7), quantum teleportation(8,9) and quantum cryptography (10-12,31)). At the macroscopic optical-field level non-classical correlations can also be important, as in the case of squeezed light(13), entangled light beams(14,15) and teleportation of continuous quantum variables(16). 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.