  We show that the novel one-way photonic modal conversion effect, as
demonstrated by Feng et al (Reports, 11 August, 2011, p. 729), is insufficient
in itself to enable optical isolation, since the underlying dielectric
structure possesses a symmetric scattering matrix. Moreover, one cannot
construct an optical isolator, by enclosing this structure with a system
containing any combination of components or signal processing elements, as long
as the overall system is linear, and is described by a static scalar dielectric
function. Instead, to achieve complete optical isolation, one would need to
augment the present design so that the final system is described by a
non-symmetric scattering matrix.
