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ON THE EFFICIENCY OF QUANTUM HEAT ENGINE: AN EXAMPLE BASED ON COUPLED HARMONIC OSCILLATORS

Jian'an WU, Aiping Fang, Yuliang ZHAO, Chang GAO, C. L. Jiang, Jun FENG, Xiuxing ZHANG, Xiaoli WANG

Published Jun 1, 2024• 0 citations
Publisher

Abstract

This paper gives an introduction on the basic concepts of quantum heat engine and quantum thermodynamics while exploring relevant properties of the quantum heat engine system based on coupled harmonic oscillators. The authors drew references from classical thermodynamics and introduces Bogoliubov transformation so as to diagonalize the potential term in coupled quantum harmonic oscillators. The authors also based the quantum heat engine system on said oscillators and gives a detailed demonstration of acquiring the efficiency of Otto cycle utilized by this system, arriving at the conclusion that there is a unifying and cohesive factor that interconnects certain corresponding values between classical and quantum heat engines.

DOI
10.26599/phys.2024.9320318

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This paper establishes an entangled otto quantum heat engine(EQHE) working with the two-qubit anisotropic Heisenberg XXZ model.Based on the first law of quantum thermodynamics,we derive the expressions for the net work and the efficiency of EQHE.By numerical calculation we plot three dimensional graphics between the efficiency and the concurrence.It is found that the efficiency of the EQHE is smaller than the Carnot efficiency and the validity of the second law of thermodynamics is confirmed;The region of non-zero efficiency is mainly distributed in C1C2 and the value of efficiency and region of non-zero efficiency increase with the increase of the ratio of the temperature γ.The efficiency of the EQHE is discussed in two special cases Δ=0 and Δ=1.

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This paper gives an introduction on the basic concepts of quantum heat engine and quantum thermodynamics while exploring relevant properties of the quantum heat engine system based on coupled harmonic oscillators. The authors drew references from classical thermodynamics and introduces Bogoliubov transformation so as to diagonalize the potential term in coupled quantum harmonic oscillators. The authors also based the quantum heat engine system on said oscillators and gives a detailed demonstration of acquiring the efficiency of Otto cycle utilized by this system, arriving at the conclusion that there is a unifying and cohesive factor that interconnects certain corresponding values between classical and quantum heat engines.
Many models of heat engines have been studied with the tools of finite-time thermodynamics and an ensemble of independent quantum systems as the working fluid. Because of their convenient analytical properties, harmonic oscillators are the most frequently used example of a quantum system. We analyze different thermodynamical aspects with the final aim of the optimization of the performance of the engine in terms of the mechanical power provided during a finite-time Otto cycle. The heat exchange mechanism between the working fluid and the thermal reservoirs is provided by the Lindblad formalism. We describe an analytical method to find the limit cycle and give conditions for a stable limit cycle to exist. We explore the power production landscape as the duration of the four branches of the cycle are varied for short times, intermediate times, and special frictionless times. For short times we find a periodic structure with atolls of purely dissipative operation surrounding islands of divergent behavior where, rather than tending to a limit cycle, the working fluid accumulates more and more energy. For frictionless times the periodic structure is gone and we come very close to the global optimal operation. The global optimum is found and interestingly comes with a particular value of the cycle time.
Conventional heat engines convert thermal energy into mechanical work by using a working medium such as steam or gas that operates between a hot and a cold reservoir. The medium absorbs heat from the hot reservoir, produces work, then releases some heat to the cold reservoir and returns to the initial state. With the advancement of quantum technologies, researchers strive to understand whether quantum media, such as atoms, spins, superconducting circuits and semiconductor quantum dots, can be used in quantum counterparts of the classical heat engines. After a recap of the basics of classical heat engines, this review explores the theoretical foundations and advancements of quantum heat engines, focusing on the proposed two- and multilevel systems used as the working medium for the quantum Otto cycle, which is the ideal thermodynamic cycle that describes the operation of a spark-ignition internal combustion engine. The review compares the efficiency and the work extraction of these systems, discussing their performance in the high- and low-temperature limits. Alternative cycles and engines are also presented to establish the adaptability of the working media when operating under different processes and non-equilibrium conditions. In conclusion, the review offers possible applications and future developments of quantum heat engines.
The quantum Otto cycle serves as a bridge between the macroscopic world of heat engines and the quantum regime of thermal devices composed from a single element. We compile recent studies of the quantum Otto cycle with a harmonic oscillator as a working medium. This model has the advantage that it is analytically trackable. In addition, an experimental realization has been achieved, employing a single ion in a harmonic trap. The review is embedded in the field of quantum thermodynamics and quantum open systems. The basic principles of the theory are explained by a specific example illuminating the basic definitions of work and heat. The relation between quantum observables and the state of the system is emphasized. The dynamical description of the cycle is based on a completely positive map formulated as a propagator for each stroke of the engine. Explicit solutions for these propagators are described on a vector space of quantum thermodynamical observables. These solutions which employ different assumptions and techniques are compared. The tradeoff between power and efficiency is the focal point of finite-time-thermodynamics. The dynamical model enables the study of finite time cycles limiting time on the adiabatic and the thermalization times. Explicit finite time solutions are found which are frictionless (meaning that no coherence is generated), and are also known as shortcuts to adiabaticity.The transition from frictionless to sudden adiabats is characterized by a non-hermitian degeneracy in the propagator. In addition, the influence of noise on the control is illustrated. These results are used to close the cycles either as engines or as refrigerators. The properties of the limit cycle are described. Methods to optimize the power by controlling the thermalization time are also introduced. At high temperatures, the Novikov–Curzon–Ahlborn efficiency at maximum power is obtained. The sudden limit of the engine which allows finite power at zero cycle time is shown. The refrigerator cycle is described within the frictionless limit, with emphasis on the cooling rate when the cold bath temperature approaches zero.
Recent experimental breakthroughs produced the first nano heat engines that have the potential to harness quantum resources. An instrumental question is how their performance measures up against the efficiency of classical engines. For single ion engines undergoing quantum Otto cycles it has been found that the efficiency at maximal power is given by the Curzon-Ahlborn efficiency. This is rather remarkable as the Curzon-Alhbron efficiency was originally derived for endoreversible Carnot cycles. Here, we analyze two examples of endoreversible Otto engines within the same conceptual framework as Curzon and Ahlborn's original treatment. We find that for endoreversible Otto cycles in classical harmonic oscillators the efficiency at maximal power is, indeed, given by the Curzon-Ahlborn efficiency. However, we also find that the efficiency of Otto engines made of quantum harmonic oscillators is significantly larger.
The unavoidable irreversible loss of power in a heat engine is found to be of quantum origin. Following thermodynamic tradition, a model quantum heat engine operating in an Otto cycle is analysed, where the working medium is composed of an ensemble of harmonic oscillators and changes in volume correspond to changes in the curvature of the potential well. Equations of motion for quantum observables are derived for the complete cycle of operation. These observables are sufficient to determine the state of the system and with it all thermodynamical variables. Once the external controls are set, the engine settles to a limit cycle. Conditions for optimal work, power and entropy production are derived. At high temperatures and quasistatic operating conditions, the efficiency at maximum power coincides with the endoreversible result . The optimal compression ratio varies from in the quasistatic limit where the irreversibility is dominated by heat conductance to in the sudden limit when the irreversibility is dominated by friction. When the engine deviates from adiabatic conditions, the performance is subject to friction. The origin of this friction can be traced to the noncommutability of the kinetic and potential energy of the working medium.
This paper establishes an entangled otto quantum heat engine(EQHE) working with the two-qubit anisotropic Heisenberg XXZ model.Based on the first law of quantum thermodynamics,we derive the expressions for the net work and the efficiency of EQHE.By numerical calculation we plot three dimensional graphics between the efficiency and the concurrence.It is found that the efficiency of the EQHE is smaller than the Carnot efficiency and the validity of the second law of thermodynamics is confirmed;The region of non-zero efficiency is mainly distributed in C1C2 and the value of efficiency and region of non-zero efficiency increase with the increase of the ratio of the temperature γ.The efficiency of the EQHE is discussed in two special cases Δ=0 and Δ=1.