[en] We report an experimental demonstration of effective entanglement in a prepare-and-measure type of quantum key distribution protocol. Coherent polarization states and heterodyne measurement to characterize the transmitted quantum states are used, thus enabling us to reconstruct directly their Q function. By evaluating the excess noise of the states, we experimentally demonstrate that they fulfill a nonseparability criterion previously presented by Rigas et al. [J. Rigas, O. Guehne, and N. Luetkenhaus, Phys. Rev. A 73, 012341 (2006)]. For a restricted eavesdropping scenario, we predict key rates using postselection of the heterodyne measurement results