  Quantum optics and classical optics have coexisted for nearly a century as
two distinct, self-consistent descriptions of light. What influences there were
between the two domains all tended to go in one direction, as concepts from
classical optics were incorporated into quantum theory's early development. But
it's becoming increasingly clear that a significant quantum presence exists in
classical territory-and, in particular, that the quintessential quantum
attribute, entanglement, can be seen, studied and exploited in classical
optics. This blurring of the classical-quantum boundary has opened up a
potential new direction for frontier work in optics.
