  Nonreciprocal photonic devices enable "one-way" light flows and are essential
building blocks of optical systems. Here, we investigate an alternative
paradigm to break reciprocity and achieve unidirectional subwavelength light
propagation fully compatible with modern all-photonic highly-integrated
systems. In agreement with a few recent studies, our theoretical model predicts
that a graphene sheet biased with a drift electric current has a strong
nonreciprocal tunable response. Strikingly, we find that the propagation of the
surface plasmon polaritons can be effectively "one-way" and may be largely
immune to the backscattering from defects and obstacles. Furthermore, the
drift-current biasing may boost the propagation length of the graphene plasmons
by more than 100%. Our findings open new inroads in nonreciprocal photonics and
offer a new opportunity to control the flow of light with one-atom thick
nonreciprocal devices.
