  Two photon-pair creation processes can be arranged such that the paths of the
emitted photons are identical. Thereby the path information is not erased but
is never born in the first place. In addition to its implications for
fundamental physics, this concept has recently led to a series of discoveries
in the fields of imaging, spectroscopy, and quantum information science. Here
we present the idea of path identity and provide a comprehensive review of the
recent developments.
