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 [10,11,12,13]). At the macroscopic optical field level non-classical correlations can also be important, as in the case of squeezed light [14], entangled light beams [15,16], and teleportation of continuous quantum variables [17]. 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.As the acronym LASER (Light Amplification by Stimulated Emission of Radiation) indicates, polarizationentangled laser operation would mean that a (spontaneously created) photon pair in two polarizationentangled modes stimulate, inside a non-linear gain medium, the emission of additional pairs. As a gain medium we consider type-II parametric down-conversion [18]. A simplified interaction Hamiltonian [19,20] for the nonlinear interaction between a classical pump field and two polarization-entangled modes a and b is given bŷh andK =â hbv −â vbh are the creation and annihilation operators of polarization entangled photon pairs in modes a and b. Horizontal and vertical polarization are represented by H and V, and κ is a real-valued coupling coefficient. When acting on the vacuum state the time evolution operatorÛ = exp iĤt/h yields:where τ = κt/h is the interaction parameter. The first and second slots in the ket indicate respectively the number of horizontal (n − m) and vertical (m) photons in mode a, and the third and fourth slot indicate the corresponding numbers for mode b. This state represents the general output of type-II parametric down-conversion, but for all experiments reported to date, τ is so small that mainly the first order term (n = 1) has been taken into account and only a few experiments and proposals addressed second order terms [7,21,22,23,24,25]. By analogy with a conventional laser, the idea of an entangled photon laser is to increase τ using a resonator around the gain medium, which enhances the emission of the higher order terms in equation 2. The state shown in equation 2 has the following features. First, modes a and b are entangled in photon number since for any n, the number of photons in each mode The photon number (pair) distribution, P (n), arising from stimulated parametric down-conversion shifts its peak and broadens as the mean number of photons increases. This indicates that for increasing interaction strength (gain), terms with higher numbers of photons obtain a larger amplification factor compared to lower terms, which is a familiar feature of laser operation.is identical. The photon (pair) numbe...