  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 and quantum cryptography). At the
macroscopic optical field level non-classical correlations can also be
important, as in the case of squeezed light, entangled light beams and
teleportation of continuous quantum variables. 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. This entanglement structure holds great promise in quantum information
science where there is a strong demand for entangled states of increasing
complexity.
