  In this survey, we first introduce quantum fields and open quantum systems,
then we present continuous-mode single-photon states and discuss discrete
measurements of a single-photon field. After that, we introduce linear quantum
systems and show how a linear quantum system responds to a single-photon input.
Then we investigate how a coherent feedback network can be used to manipulate
the temporal pulse shape of a single-photon state. Afterwards, we present
single-photon filter and master equations. Finally, we discuss the generation
of Schr\"odinger cat states by means of photon addition and subtraction
