  Modern photonic and quantum technologies demand reciprocity breaking, e.g.,
isolators, full-duplex systems, noise isolation in quantum computers,
motivating searching for practical approaches beyond magnet-based devices. This
work overviews the up-and-coming advances in optical nonreciprocity, including
new materials (Weyl semimetals, topological insulators, metasurfaces), active
structures, time-modulation, PT-symmetry breaking, nonlinearity, quantum
nonlinearity, unidirectional gain and loss, chiral quantum states, and valley
polarization.
