  All existing heat machines are dissipative open systems. Hence, they cannot
operate fully coherently. We propose to replace this conventional thermodynamic
paradigm by a completely different one, whereby heat machines are nonlinear
coherent closed systems comprised of few field modes. Their thermal-state input
is transformed by nonlinear interactions into non-thermal output with
controlled quantum fluctuations and the capacity to deliver work in a chosen
mode. This new paradigm allows the bridging of quantum coherent and
thermodynamic descriptions.
