We show that the condition of no faster-than-light signalling restricts the number of quantum states that can be cloned in a given Hilbert space. This condition leads to the constraints on a probabilistic quantum cloning machine (PQCM) recently found by Duan and Guo.
@article{arxiv.quant-ph/9905024,
title = {No Signalling and Probabilistic Quantum Cloning},
author = {Lucien Hardy and David D. Song},
journal= {arXiv preprint arXiv:quant-ph/9905024},
year = {2009}
}