Efficiency of a Quantum Otto Heat Engine Operating under a Reservoir at Effective Negative Temperatures.

We perform an experiment in which a quantum heat engine works under two reservoirs, one at a positive spin temperature and the other at an effective negative spin temperature, i.e., when the spin system presents population inversion. We show that the efficiency of this engine can be greater than that when both reservoirs are at positive temperatures. We also demonstrate the counterintuitive result that the Otto efficiency can be beaten only when the quantum engine is operating in the finite-time mode.

Cooling with fermionic thermal reservoirs.

The quantum reservoirs commonly considered in open-quantum systems theory are those modeled by quantum harmonic oscillators, which are called bosonic reservoirs. Recently, quantum reservoirs modeled by two-level systems, the so-called fermionic reservoirs, have received attention due to their features. Given that the components of these reservoirs have a finite number of energy levels, unlike bosonic reservoirs, some studies are being carried out to explore the advantages of using this type of reservoir, especially in the operation of heat machines. In this paper, we carry out a case study of a quantum refrigerator operating in the presence of bosonic or fermionic thermal reservoirs, and we show that fermionic baths have advantages over bosonic ones.

Quantum Otto-type heat engine with fixed frequency.

In this work we analyze an Otto-type cycle operating with a working substance composed of a quantum harmonic oscillator (QHO). Unlike other studies in which the work extraction is done by varying the frequency of the QHO and letting it thermalize with a squeezed reservoir, here we submit the QHO to a parametric pumping controlled by the squeezing parameter and let it thermalize with a thermal reservoir. We then investigate the role of the squeezing parameter in our Otto-type engine powered by parametric pumping and show that it is possible to reach the Carnot limit by arbitrarily increasing the squeezing parameter. Notably, for certain squeezing parameters r, e.g., r=0.4, the quasistatic Otto limit can be reached even at nonzero power. We also investigated the role of entropy production in the efficiency behavior during the unitary strokes, showing that positive (negative) changes in entropy production correspond to increases (decreases) in engine efficiency, as expected. Furthermore, we show that under thermal reservoirs a work extraction process that is more efficient than the Carnot engine is impossible, regardless of the quantum resource introduced via the Hamiltonian of the system.

Universal two-level quantum Otto machine under a squeezed reservoir.

We study an Otto heat machine whose working substance is a single two-level system interacting with a cold thermal reservoir and with a squeezed hot thermal reservoir. By adjusting the squeezing or the adiabaticity parameter (the probability of transition) we show that our two-level system can function as a universal heat machine, either producing net work by consuming heat or consuming work that is used to cool or heat environments. Using our model we study the performance of these machine in the finite-time regime of the isentropic strokes, which is a regime that contributes to make them useful from a practical point of view.