Comment to "Nonreciprocal Light Propagation in a Silicon Photonic Circuit"

In the article "Nonreciprocal Light Propagation in a Silicon Photonic Circuit" (Science 333, 729-733 (2011) a nonreciprocal waveguide system based on a combination of silicon, dielectrics and metals is reported. In this Comment it is explained that the interpretation with respect to nonreciprocity in this paper is incorrect and conflicts with the fundamental Lorentz reciprocity theorem. It is further pointed out that a previous publication already introduced the device concept.