We construct the unique optimal quantum device for turning a finite number of d-level quantum systems in the same unknown pure state \sigma into M systems of the same kind, in an approximation of the M-fold tensor product of the state \sigma.
@article{arxiv.quant-ph/9804001,
title = {Optimal Cloning of Pure States},
author = {R. F. Werner},
journal= {arXiv preprint arXiv:quant-ph/9804001},
year = {2009}
}