  Photonic quantum computation refers to quantum computation that uses photons
as the physical system for doing the quantum computation. Photons are ideal
quantum systems because they operate at room temperature, and photonic
technologies are relatively mature. The field is largely divided between
discrete- and continuous-variable photonic quantum computation. In
discrete-variable (DV) photonic quantum computation, quantum information is
represented by one or more modal properties (e.g. polarization) that take on
distinct values from a finite set. Quantum information is processed via
operations on these modal properties and eventually measured using single
photon detectors. In continuous-variable (CV) photonic quantum computation,
quantum information is represented by properties of the electromagnetic field
that take on any value in an interval (e.g. position). The electromagnetic
field is transformed via Gaussian and non-Gaussian operations, and then
detected via homodyne detection. Both CV and DV photonic quantum computation
have been realized experimentally and they each have a unique set of challenges
that need to be overcome to achieve scalable photonic universal quantum
computation. This article is an introduction to photonic quantum computing,
charting its development from the early days of linear optical quantum
computing to recent developments in quantum machine learning.
