  These lecture notes from the 2019 Les Houches Summer School on Quantum
Information Machines provides a pedagogical introduction to the theory of
non-reciprocal quantum interactions and devices. The goal is connect various
approaches and concepts, including Hamiltonians encoding synthetic gauge
fields, scattering descriptions, quantum master equations, and non-Hermitian
Hamiltonians. The importance of having both non-trivial synthetic gauge fields
and dissipation for obtaining non-reciprocal interactions is stressed.
Connections to broader topics such as quantum reservoir engineering and the
quantum theory of continuous-measurement based feedforward are also discussed.
