There is a free PDF on the nature site, but the paper is also available on arxiv (https://arxiv.org/abs/1407.4437 https://arxiv.org/abs/1407.4437), the first ve

There is a free PDF on the nature site, but the paper is also available on arxiv (https://arxiv.org/abs/1407.4437 https://arxiv.org/abs/1407.4437), the first version on the arxiv dates back to 2014. It's strange, but I find the arxiv version much more readable in terms of typesetting than the polished version in nature. Also there are additional appendices in the arxiv version.