  In a recent work [A. Metelmann and A. A. Clerk, Phys. Rev. X 5, 021025
(2015)], a general reservoir-engineering approach for generating non-reciprocal
quantum interactions and devices was described. We show here how in many cases
this general recipe can be viewed as an example of autonomous feed-forward: the
full dissipative evolution is identical to the unconditional evolution in a
setup where an observer performs an ideal quantum measurement of one system,
and then uses the results to drive a second system. We also extend the
application of this approach to non-reciprocal quantum amplifiers, showing the
added functionality possible when using two engineered reservoirs. In
particular, we demonstrate how to construct an ideal phase-preserving
cavity-based amplifier which is full non-reciprocal, quantum-limited and free
of any fundamental gain-bandwidth constraint.
