  There is currently significant interest in operating devices in the quantum
regime, where their behaviour cannot be explained through classical mechanics.
Quantum states, including entangled states, are fragile and easily disturbed by
excessive thermal noise. Here we address the question of whether it is possible
to create non-reciprocal devices that encourage the flow of thermal noise
towards or away from a particular quantum device in a network. Our work makes
use of the cascaded systems formalism to answer this question in the
affirmative, showing how a three-port device can be used as an effective
thermal transistor, and illustrates how this formalism maps onto an
experimentally-realisable optomechanical system. Our results pave the way to
more resilient quantum devices and to the use of thermal noise as a resource.
