Collective effects and enhanced performances of quantum heat engines

Recent predictions for quantum-mechanical enhancements in the operation of small heat engines have 

raised renewed interest in their study from both a fundamental perspective and because of applications. One 

essential question is whether collective effects may help to carry enhancements over larger scales when increasing 

the number of constituents composing the working substance of the engine (with possibly the presence of spontaneous 

symmetry breaking). Such enhancement may consider not only its performance but also the stability of the engine 

concerning unavoidable environmental fluctuations. After a first analysis on the ultimate power in a simple model of a 

thermal machine (a two-level quantum system), I will discuss the properties of a quantum heat engine in which the 

working substance is composed by a system of interacting spins.