In this case, the relevant issue to assess the functionality of these devices demands the optimisation of more practical figures of merit such as the efficiency at maximum cooling power ε *. in the reversible limit, the exchange of any finite amount of energy with the heat baths is performed in infinite time.įor practical purposes, however, one needs to operate at nonvanishing power. In particular, once their steady state builds up, quantum heat pumps are governed by formal analogues of the laws of thermodynamics and, as a consequence, their absolute efficiency ideally saturates to the corresponding Carnot limits ε C, albeit at vanishing ‘cooling power’ 14, i.e. Under the familiar conditions usually met in the quantum-optical regime, the dissipative processes may be assumed purely Markovian 17, 18, 19, which severely restricts the performance of any heat-driven device and confers a distinctive spectral structure to the environmental fluctuations 16. The different designs of quantum heat pumps share limitations that can be understood from the assumptions on their interactions with the environments. The consistent quantum-thermodynamic description of these elementary three-level prototypes was object of further study 1, 4 and, just recently, alternative finite-dimensional quantum systems realising autonomous heat pumps have been put forward in the literature 2, 3. The use of three-level solid-state masers as physical support for heat pumps was already discussed in the late 1950s 13, 14, when spin refrigeration was also experimentally demonstrated 15. A heat-driven quantum fridge is just one specific configuration of the more general quantum heat pump, that can function either as a heater, a chiller or even an engine. In spite of the increasing interest that quantum absorption cooling has attracted over the last few years 5, 7, 8, 9, 10, 11, 12, the field is far from new. In addition to their potential technological applications, these autonomous quantum-thermal devices are also appealing from the fundamental perspective, as they are naturally well suited for the study of thermodynamics at the level of individual open quantum systems 1, 4, 5, 6. In this picture, the cold bath would play the role of the macroscopic or mesoscopic object to be cooled. Such superefficient quantum-enhanced cooling realises a promising step towards the technological exploitation of autonomous quantum refrigerators.Īn absorption or heat-driven quantum refrigerator is a system capable of establishing a net steady-state transport of energy from a cold bath (c) to a hot bath (h), assisted only by the residual heat coming from an additional work reservoir (w) 1, 2, 3. We also show how those bounds may be pushed beyond what is classically achievable, by suitably tailoring the environmental fluctuations via quantum reservoir engineering techniques. We establish thermodynamic performance bounds for these machines and investigate their quantum origin. Here we study quantum absorption refrigerators, which are driven by heat rather than external work. Close analogues of those fundamental laws are now being established at the level of individual quantum systems, thus placing limits on the operation of quantum-mechanical devices. This triggered groundbreaking achievements in physics, chemistry and engineering over the last two centuries. Based on few natural assumptions together with the four laws, it sets the boundaries between possible and impossible in macroscopic aggregates of matter. Thermodynamics is a branch of science blessed by an unparalleled combination of generality of scope and formal simplicity.
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