There are lots of systems that break the reciprocity: https://arxiv.org/pdf/1705.09548.pdf https://arxiv.org/pdf/1705.09548.pdf from a few months ago has some d

There are lots of systems that break the reciprocity: https://arxiv.org/pdf/1705.09548.pdf https://arxiv.org/pdf/1705.09548.pdf from a few months ago has some descriptions. I think (not having read the Science paper) the issue would be that the underlying fundamental physics has reciprocity as a basic feature by way of Hermitian operators.