The Quantum Otto Engine With Four-qubit Small Spin Cluster As Working Substance

Quantum heat engine is a heat engine that takes"quantum system"as its working substance.As the working substance has quantum properties,quantum heat engines has many unusual properties compared with traditional heat engine.In recent years,many researches have been carried out to explore the various possibilities of quantum heat engines.Among them,it is an interesting problem to study quantum heat engines using coupling systems as working substance.In the previous studies,two-qubit and three-qubit systems are considered as working matter to study the quantum thermodynamics cycle.In recent years,four-qubit coupling system with more complex energy spectrum has gradually become a research hotspot.In this thesis,we construct a quantum Otto heat engine with four-qubit small spin cluster as the working substance and calculate its output work and efficiency.Based on these results,consider the effects of coupling constants on the heat engine performance.In chapter 2,we briefly introduce the basic quantum thermodynamics cycle process,quantum Otto heat machine,quantum entanglement and other thermodynamics and quantum mechanics concept.In chapter 3,we study the relationship between the output work,efficiency and coupling constant of quantum Otto heat machine with four-qubit small spin cluster as working matter.We first calculate the output work W and efficiency?of the quantum Otto engine.For convenience,we set the nearest neighbor four-spin coupling constantK₁ and the second nearest neighbor four-spin coupling constantK₂ equal,i.e.K₁(28)K₂(28)Kand three different adiabatic changes are assumed,that is only the next neighbor coupling constantJ₂ changes,only the neighbor coupling constantJ₁ changes,and only the four spin coupling constant K changes.The influences of the three conditions on the thermal engine performance are compared.It is found that the variation of output work W and efficiency?of the engine with respect to each coupling constant is related to the magnitude,positive and negative of the coupling constant.When only the neighbor coupling constantJ₁ changes and in the region thatJ₂ is small enough,the system forms an efficient three-level engine.These three levels are independent ofJ₂,and the output work remains constant,and the engine may not work in a certain range.In chapter 4,we consider the case of the nearest neighbor four-spin coupling constant and the second nearest neighbor four-spin coupling constantK₂ are not equal,i.e.K₁?K ₂.When the two coupling constants change in the same proportion,the influences of the other two coupling constants on the performance of the heat engine are explored,which are divided into four cases for discussion.We find that the influences of the nearest neighbor coupling constant are more prominent than that of the second-nearest neighbor coupling constant.Compared with the four-spin coupling constant,the coupling constant between two particles has a more obvious effect on the quantum Otto thermal engine performance.In addition,the influences of adiabatic compression ratio and temperature on the performance are discussed briefly.We find that when the four coupling constants change in equal proportion simultaneously,the peak output work increases and then decreases with the adiabatic compression ratio.And as the temperature ratio decreases,the range of adiabatic compression ratio that enables the engine to operate normally increases.Finally,the influences of thermal entanglement on engine performance are discussed.We find that when different coupling constants change,the heat engine can work in different regions under the influence of thermal entanglement.