  Entanglement is a quintessential quantum mechanical phenomenon with no
classical equivalent. First discussed by Einstein, Podolsky, and Rosen and
formally introduced by Schr\"odinger in 1935, entanglement has grown from a
scientific debate to a radically new resource that sparks a technological
revolution. This review focuses on the fundamentals and recent advances in
entanglement-based quantum information technology (QIT), specifically in
photonic systems. Photons are unique quantum information carriers with several
advantages, such as their ability to operate at room temperature, their
compatibility with existing communication and sensing infrastructures, and the
availability of readily accessible optical components. Photons also interface
well with other solid-state quantum platforms. We will first provide an
overview on entanglement, starting with an introduction to its development from
a historical perspective followed by the theory for entanglement generation and
the associated representative experiments. We will then dive into the
applications of entanglement-based QIT for sensing, imaging, spectroscopy, data
processing, and communication. Before closing, we will present an outlook for
the architecture of the next-generation entanglement-based QIT and its
prospective applications.
