  Controlling the various degrees-of-freedom (DoFs) of structured light at both
quantum and classical levels is of paramount importance in optics. It is a
conventional paradigm to treat diverse DoFs separately in light shaping. While,
the more general case of nonseparable states of light, in which two or more
DoFs are coupled in a nonseparable way, has become topical recently.
Importantly, classical nonseparable states of light are mathematically analogue
to quantum entangled states. Such similarity has hatched attractive studies in
structured light, e.g. the spin-orbit coupling in vector beams. However,
nonseparable classical states of light are still treated in a fragmented
fashion, while its forms are not limited by vector beams and its potential is
certainly not fully exploited. For instance, exotic space-time coupled pulses
nontrivial light shaping towards ultrafast time scales, and ray-wave geometric
beams provide new dimensions in optical manipulations. Here we provide a bird's
eye view on the rapidly growing but incoherent body of work on general
nonseparable states involving various DoFs of light and introduce a unified
framework for their classification and tailoring, providing a perspective on
new opportunities for both fundamental science and applications.
