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International Journal of Modern Physics B

Effects of superpositions of quantum states on quantum isoenergetic cycles: Efficiency and maximum power output

Xiangyu Niu, X. L. Huang, Y. F. Shang, X. Y. Wang

Published Apr 15, 2015• 7 citations
Publisher

Abstract

Superposition principle plays a crucial role in quantum mechanics, thus its effects on thermodynamics is an interesting topic. Here, the effects of superpositions of quantum states on isoenergetic cycle are studied. We find superposition can improve the heat engine efficiency and release the positive work condition in general case. In the finite time process, we find the efficiency at maximum power output in superposition case is lower than the nonsuperposition case. This efficiency depends on one index of the energy spectrum of the working substance. This result does not mean the superposition discourages the heat engine performance. For fixed efficiency or fixed power, the superposition improves the power or efficiency respectively. These results show how quantum mechanical properties affect the thermodynamical cycle.

DOI
10.1142/s0217979215500861

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Superposition principle plays a crucial role in quantum mechanics, thus its effects on thermodynamics is an interesting topic. Here, the effects of superpositions of quantum states on isoenergetic cycle are studied. We find superposition can improve the heat engine efficiency and release the positive work condition in general case. In the finite time process, we find the efficiency at maximum power output in superposition case is lower than the nonsuperposition case. This efficiency depends on one index of the energy spectrum of the working substance. This result does not mean the superposition discourages the heat engine performance. For fixed efficiency or fixed power, the superposition improves the power or efficiency respectively. These results show how quantum mechanical properties affect the thermodynamical cycle.
The difference between quantum isoenergetic process and quantum isothermal process comes from the violation of the law of equipartition of energy in the quantum regime. To reveal an important physical meaning of this fact, here we study a special type of quantum heat engine consisting of three processes: isoenergetic, isothermal and adiabatic processes. Therefore, this engine works between the energy and heat baths. Combining two engines of this kind, it is possible to realize the quantum Carnot engine. Furthermore, considering finite velocity of change of the potential shape, here an infinite square well with moving walls, the power output of the engine is discussed. It is found that the efficiency and power output are both closely dependent on the initial and final states of the quantum isothermal process. The performance of the engine cycle is shown to be optimized by control of the occupation probability of the ground state, which is determined by the temperature and the potential width. The relation between the efficiency and power output is also discussed.
We consider the finite-time operation of a quantum heat engine whose working substance is composed of a two-level atomic system. The engine cycle, consisting of two quantum adiabatic and two quantum isochoric (constant-frequency) processes and working between two heat reservoirs at temperatures Th{T}_{h}Th​ and Tc{T}_{c}Tc​(<(<(<Th){T}_{h})Th​), is a quantum version of the classical Otto cycle. By optimizing the power output with respect to two frequencies, we obtain the efficiency at maximum power output (EMP) and analyze numerically the effects of the times taken for two adiabatic and two isochoric processes on the EMP. In the absence of internally dissipative friction, we find that the EMP is bounded from the upper side by a function of the Carnot efficiency ${\ensuremath{\eta}}_{C}$, ${\ensuremath{\eta}}_{+}={\ensuremath{\eta}}_{C}^{2}/[{\ensuremath{\eta}}_{C}\ensuremath{-}(1\ensuremath{-}{\ensuremath{\eta}}_{C})\mathrm{ln}(1\ensuremath{-}{\ensuremath{\eta}}_{C})]$, with ${\ensuremath{\eta}}_{C}=1\ensuremath{-}{T}_{c}/{T}_{h}$. This analytic expression is confirmed by our exact numerical result and is identical to the one derived in an engine model based on a mesoscopic or macroscopic system. If the internal friction is included, we find that the EMP decreases as the friction coefficient increases.
Abstract Quantum cycles in established heat engines can be modeled with various quantum systems as working substances. As for example, heat engine can be modeled with an infinite potential well as the working substance to determine the efficiency and work done. However, in this method, the relationship between the quantum observables and the physically measurable parameters, i.e., the efficiency and work done is not well understood from the quantum mechanics approach. A detailed analysis is needed to link the thermodynamical variables (on which the efficiency and work done depends) with the uncertainty principle for better understanding. Here, we present the connection of sum uncertainty relation of position and momentum operators with thermodynamic variables in the quantum heat engine model. We are able to determine the upper and lower bounds on the efficiency of the heat engine through uncertainty relation.
A new model of a quantum heat engine cycle is established in which the working substance consists of many non-interacting spin-1/2 systems.Performance characteristics of the cycle and time evolution of the spin angular momentum are investigated,based on the quantum master equation and a semi-group approach.The general expressions of efficiency and power output are derived.Further,at high temperatures the optimal relations of efficiency and power output are studied in detail.Optimal performance of heat engine cycles is analyzed from a microscopic model.
Abstract We propose a quantum Otto engine operating through a cycle of two isochoric processes, where the working substance interacts with a single-mode radiation field, and two unitary strokes, during which the working substance is decoupled from the field. We investigate the influence of quantum superposition and quantum internal friction on the engine’s power output and efficiency, demonstrating that these quantum effects enhance both performance metrics. While these enhancements are accompanied by increased power fluctuations, we show that such fluctuations can be effectively mitigated through careful selection of control parameters. Our results reveal that the proposed quantum Otto engine can achieve performance regimes that are thermally inconceivable in classical systems, including surpassing the Otto efficiency limit and attaining 100% efficiency with nonzero power output.