  Combating the effects of disorder on light transport in micro- and
nano-integrated photonic devices is of major importance from both fundamental
and applied viewpoints. In ordinary waveguides, imperfections and disorder
cause unwanted back-reflections, which hinder large-scale optical integration.
Topological photonic structures, a new class of optical systems inspired by
quantum Hall effect and topological insulators, can realize robust transport
via topologically-protected unidirectional edge modes. Such waveguides are
realized by the introduction of synthetic gauge fields for photons in a
two-dimensional structure, which break time reversal symmetry and enable
one-way guiding at the edge of the medium. Here we suggest a different route
toward robust transport of light in lower-dimensional (1D) photonic lattices,
in which time reversal symmetry is broken because of the {\it non-Hermitian}
nature of transport. While a forward propagating mode in the lattice is
amplified, the corresponding backward propagating mode is damped, thus
resulting in an asymmetric transport that is rather insensitive to disorder or
imperfections in the structure. Non-Hermitian transport in two lattice models
is considered: a tight-binding lattice with an imaginary gauge field
(Hatano-Nelson model), and a non-Hermitian driven binary lattice. In the former
case transport in spite of disorder is ensured by a mobility edge that arises
because of a non-Hermitian delocalization transition. The possibility to
observe non-Hermitian delocalization induced by a synthetic 'imaginary' gauge
field is suggested using an engineered coupled-resonator optical waveguide
(CROW) structure.
