Single-ion quantum Otto engine with always-on bath interaction

We demonstrate how a quantum Otto engine (QOE) can be implemented using a single ion and an always-on thermal environment. The internal degree of freedom of the ion is chosen as the working fluid, while the motional degree of freedom can be used as the cold bath. We show, that by adiabatically changing the local magnetic field, the work efficiency can be asymptotically made unity. We propose a projective measurement of the internal state of the ion that mimics the release of heat into the cold bath during the engine cycle. In our proposal, the coupling to the hot and the cold baths need not be switched off and on in an alternative fashion during the engine cycle, unlike other existing proposals of QOE. This renders the proposal experimentally feasible using the available tapped-ion engineering technology.Comment: 8 pages, 5 figure

Anisotropy-assisted thermodynamic advantage of a local-spin thermal machine

We study quantum Otto thermal machines with a two-spin working system coupled by anisotropic interaction. Depending on the choice of different parameters, the quantum Otto cycle can function as different thermal machines, including a heat engine, refrigerator, accelerator and heater. We aim to investigate how the anisotropy plays a fundamental role in the performance of the quantum Otto engine operating in different time scales. We find that while the efficiency of the engine efficiency increases with the increase in anisotropy for the quasistatic operation, quantum internal friction and incomplete thermalization degrade the performance in a finite time cycle. Further, we study the QOE with one of the spins, the local spin, as the working system. We show that the efficiency of such an engine can surpass the standard quantum Otto limit, along with maximum power, thanks to the anisotropy. This can be attributed to quantum interference effects. We demonstrate that the enhanced performance of a local-spin QOE originates from the same interference effects, as in a measurement-based QOE for their finite time operation.Comment: total 14 pages, 10 figure