Negative temperature is cool for cooling

In this work, we study an autonomous refrigerator composed of three qubits [Phys. Rev. Lett. 105, 130401 (2010)] operating with one of the reservoirs at negative temperatures, which has the purpose of cooling one of the qubits. We find the values of the lowest possible temperature that the qubit of interest reaches when fixing the relevant parameters, and we also study the limit for cooling the qubit arbitrarily close to absolute zero. We thus proceed to a comparative study showing that reservoirs at effective negative temperatures are more powerful than those at positive temperatures for cooling the qubit of interest

Cooling with fermionic reservoir

Recently, much emphasis has been given to genuinely quantum reservoirs generically called fermionic reservoirs. These reservoirs are characterized by having finite levels, as opposed to bosonic reservoirs, which have infinite levels that can be populated via an increase in temperature. Given this, some studies are being carried out to explore the advantages of using quantum reservoirs, in particular in the operation of heat machines. In this work, we make a comparative study of a thermal refrigerator operating in the presence of either a bosonic or a fermionic reservoir, and we show that fermionic reservoirs have advantages over bosonic ones. We propose an explanation for the origin of these advantages by analyzing both the asymptotic behavior of the states of the qubits and the exchange rates between these qubits and their respective reservoirs.Comment: 5 pages, 2 figure