Hall 123

Coupled systems as quantum thermodynamic machines

George Thomas

IMSc

Extension of thermodynamics in quantum regime attracted wide range of attention in recent years. Different models of thermodynamic machines are useful tools to study along such directions. Here, we make a comparative study between the performances of coupled quantum harmonic oscillators and coupled spin-1/2 systems, when they constitute as the working media of thermodynamic machines. Spins are coupled via Heisenberg XX or XY interactions and analogous interactions are considered in the case of harmonic oscillators. With suitable co-ordinate transformation, the coupled system appears to be uncoupled in the new frame of reference. The subsystems can work as engine or refrigerator depending upon the parameters chosen. We show that the figures of merit, that is, the efficiency for engine and the coefficient of performance for refrigerator, are bounded (both from above and from below) by the figures of merit of the independent subsystems. We also discuss the characteristics of the optimal work extractable with the coupled systems. Interestingly, for particular kind of interactions, the efficiency of the coupled oscillators outperforms that of the coupled spin-1/2 systems when they work as heat engines. However, for the same interaction, the coefficient of performance behaves in a reverse manner, while the system operates as refrigerator. Therefore same coupling can cause opposite effects in the figures of merit of heat engine and refrigerator.