  A linear optical probabilistic scheme for the optimal cloning of a pair of
orthogonally-polarized photons is devised, based on single- and two-photon
interferences. It consists in a partial symmetrization device, realized with a
modified unbalanced Mach-Zehnder interferometer, followed by two independent
Hong-Ou-Mandel interferometers. This scheme has the advantage that it enables
quantum cloning without the need for stimulated amplification in a nonlinear
medium. It can also be modified so to make an optical two-qubit partial SWAP
gate, thereby providing a potentially useful tool to linear optics quantum
computing.
