Quantum Thermal Machines
Abstract: Quantum thermodynamics is an emerging field of research that aims to explore the thermodynamics of non-equilibrium processes in quantum systems using mainly the tools of quantum information theory. In this talk, we will focus on our two recent contributions to the field which involves quantum thermal machines.
Firstly, we examine the issue of thermalization and heat exchange for a composite system consisting of a target qubit (the system) that randomly, repeatedly, interacts with a cluster of N identical spin-1/2 (qubit) particles (the bath). In such a composite system asymptotic thermalization or its absence turns out to be strongly dependent on the initial state of the N-qubit cluster: We allow for multi-qubit coherences in the cluster bath and show that depending on the type of coherences the bath may drive the target qubit into either a thermal or a coherently displaced state. Even in cases where the target qubit thermalizes, it can exhibit a quantum advantage, whereby the target-qubit temperature can be scaled up with N2, and the thermalization time can be shortened by a similar factor, provided the appropriate coherence in the cluster is prepared by non-thermal means. We dub these effects quantum super-thermalization because of their similarity (but also differences) with superradiance.
On the other hand, following the developments in the field of quantum thermodynamics there are different proposals on making of an Otto engine with its working medium constituted by quantum systems. A standard Otto cycle consists of four stages: isochoric heating and cooling stages, together with isentropic expansion and compression stages. The efficiency of the engine attains its maximum when the cycle is performed adiabatically such that the compression and expansion of the working medium is made quasi-statically to ensure there are no unwanted transitions between the energy levels. This, however, implies that the engine will have a vanishing power output due to infinitely long time it takes to complete the cycle. It is possible to improve the performance of these engines by employing techniques of shortcut-to-adiabaticity (STA) which allows one to mimic an adiabatic transformation at a finite-time by externally driving the system. Nevertheless, such protocols come with their own energetic costs that also needs to be accounted in evaluating the performance of a cycle. We investigate the trade-offs between these STA costs and the thermodynamic figures of merit of a finite-time quantum Otto engine which have qubits as its working medium.