Light fields can be amplified by measuring the field amplitude reflected at abeam splitter of reflectivity R and adding a coherent amplitude proportional tothe measurement result to the transmitted field. By applying the quantumoptical realization of this amplification scheme to single photon inputs, it ispossible to clone the polarization states of photons. We show that optimalcloning of single photon polarization is possible when the gain factor of theamplification is equal to the inverse squareroot of 1-R.
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