  Engines are systems and devices that convert one form of energy into another,
typically into a more useful form that can perform work. In the classical
setup, physical, chemical, and biological engines largely involve the
conversion of heat into work. This energy conversion is at the core of
thermodynamic laws and principles and is codified in textbook material. In the
quantum regime, however, the principles of energy conversion become ambiguous,
since quantum phenomena come into play. As with classical thermodynamics,
fundamental principles can be explored through engines and refrigerators, but,
in the quantum case, these devices are miniaturized and their operations
involve uniquely quantum effects. Our work provides a broad overview of this
active field of quantum engines and refrigerators, reviewing the latest
theoretical proposals and experimental realizations. We cover myriad aspects of
these devices, starting with the basic concepts of quantum analogs to the
classical thermodynamic cycle and continuing with different quantum features of
energy conversion that span many branches of quantum mechanics. These features
include quantum fluctuations that become dominant in the microscale,
non-thermal resources that fuel the engines, and the possibility of scaling up
the working medium's size, to account for collective phenomena in many-body
heat engines. Furthermore, we review studies of quantum engines operating in
the strong system-bath coupling regime and those that include non-Markovian
phenomena. Recent advances in thermoelectric devices and quantum information
perspectives, including quantum measurement and feedback in quantum engines,
are also presented.

Quantum Physics