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arXiv:1504.02183 · Physical Review E

Quantum statistics and the performance of engine cycles

Yuanjian Zheng, Dario Poletti

Published Jul 7, 2015• 48 citations
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Abstract

We study the role of quantum statistics in the performance of Otto cycles. First, we show analytically that the work distributions for bosonic and fermionic working fluids are identical for cycles driven by harmonic trapping potentials. Subsequently, in the case of nonharmonic potentials, we find that the interplay between different energy level spacings and particle statistics strongly affects the performances of the engine cycle. To demonstrate this, we examine three trapping potentials which induce different (single-particle) energy level spacings: monotonically decreasing with the level number, monotonically increasing, and the case in which the level spacing does not vary monotonically.

DOI
10.1103/physreve.92.012110

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We study the role of quantum statistics in the performance of Otto cycles. First, we show analytically that the work distributions for bosonic and fermionic working fluids are identical for cycles driven by harmonic trapping potentials. Subsequently, in the case of nonharmonic potentials, we find that the interplay between different energy level spacings and particle statistics strongly affects the performances of the engine cycle. To demonstrate this, we examine three trapping potentials which induce different (single-particle) energy level spacings: monotonically decreasing with the level number, monotonically increasing, and the case in which the level spacing does not vary monotonically.
We study the performance of a quantum Otto cycle operating in trapping potentials of different shapes. We show that, while both the mean work output and the efficiency of two Otto cycles in different trapping potentials can be made equal, the work probability distribution will still be strongly affected by the difference in structure of the energy levels. To exemplify our results, we study the family of potentials of the form V(t)(x) ∼ x(2q). This family of potentials possesses a simple scaling property that allows for analytical insights into the efficiency and work output of the cycle. We perform a comparison of quantum Otto cycles in various physically relevant scenarios and find that in certain instances, the efficiency of the cycle is greater when using potentials with larger values of q, while in other cases, the efficiency is greater with harmonic traps.
In this thesis we investigate the work output from an Otto-engine operating simple, fewbody quantum systems under quasi-static driving.We consider 1D systems using the infinite square well as the trapping potential.The aim is to more deeply understand how the working medium of the engine affects the work output, and under which conditions (such as temperatures of the heat reservoirs, particle number etc) a particular working medium is favourable over another.We will first look at how the work output using non-interacting, spin-polarised fermions and bosons differ in the Otto engine.Then, we continue by examining what effect particle number has and how the work output of the non-interacting quantum particles corresponds to that of using a classical, ideal gas as the working medium.After that, a two-body interaction, with a tunable interaction strength is introduced.We will take the interaction to be of the same form as an effective 1D Coulomb interaction for a system under strong cylindrical confinement.We examine how the interacting system compares to the non-interacting one, and what happens when the interaction strength is varied.For the non-interacting case, we find that fermions and bosons can have significantly different work output.The difference will be due to a combination of their respective particle statistics and the monotonically increasing energy level spacings of the infinite square well.Additionally, fermions are seen to be able to have a larger work output than a classical ideal gas, while for bosons, only a system of one or two particles can.Including the two-body interaction, we find that it allows for both fermions and bosons to have a larger work output compared to the non-interacting case.We also see that increasing the interaction strength high enough, fermions and bosons tend to the same work output.
By harnessing quantum phenomena, quantum devices have the potential to outperform their classical counterparts. Here, we examine using wave function symmetry as a resource to enhance the performance of a quantum Otto engine. Previous work has shown that a bosonic working medium can yield better performance than a fermionic medium. We expand upon this work by incorporating a singular interaction that allows the effective symmetry to be tuned between the bosonic and fermionic limits. In this framework, the particles can be treated as anyons subject to Haldane’s generalized exclusion statistics. Solving the dynamics analytically using the framework of “statistical anyons”, we explore the interplay between interparticle interactions and wave function symmetry on engine performance.
Abstract A key focus of designing quantum thermal devices is the potential advantage that can be gleaned from genuine quantum effects when compared to classical devices. The recent experimental realization of the Pauli engine (Koch et al 2023 Nature 621 723)—where energy is extracted via changes in particle statistics as an alternative to conventional heat sources—has opened new avenues of research where quantum statistics can be considered as a thermodynamic resource. In this work we propose hybrid quantum heat engines which can utilize additional strokes that change the single particle statistics between bosonic and fermionic descriptions during the cycle. To accomplish this we consider the 1D Lieb–Liniger gas, in which the s-wave interactions can be tuned between the non-interacting and the hard-core limit, which are described by bosonic and fermionic statistics respectively. We show that by suitably choosing where to implement these statistical strokes during an Otto-like cycle in the quasi-static limit, the efficiency and work output can be significantly enhanced when compared to fully bosonic or fully fermionic engines. Furthermore, in the degenerate regime our engine can operate at the Carnot efficiency, due to the interplay between the different contributions of heat and work induced by the statistical strokes. Finally, we highlight how our thermodynamic cycles can realize other thermal operations, such as refrigerators, promising similar statistical enhancements for a wide range of temperatures.
In this work we study the effect of quantum and thermal fluctuations on the statistics of quantum heat engine performance parameters, like efficiency and power. We begin by deriving an explicit solution for the characteristic function of the heat distribution of a thermal quantum harmonic oscillator. We then derive a general framework based on the standard two-point-measurement scheme to compute the efficiency distribution of a quantum Otto cycle. We analyze the generic properties of this distribution for scale-invariant driving Hamiltonians which describe a large class of single-particle, many-body, and nonlinear systems. We find that the efficiency is deterministic and that its mean is equal to the macroscopic efficiency for adiabatic driving. We continue our research by studying the efficiency large deviation function of two exemplary quantum heat engines, the harmonic oscillator and the two-level Otto cycles. While the efficiency statistics follow the ’universal’ theory of Verley et al. [Nature Commun. 5, 4721 (2014)] for nonadiabatic driving, we find that the latter framework does not apply in the adiabatic regime. We can relate this unusual property to the perfect anticorrelation between work output and heat input that suppresses thermal as well as quantum fluctuations. We then probe our findings in an experimental NMR setup using spin-1/2 systems and find them to agree rather well with our theoretical predictions. Afterward, we move on to the finite-time quantum Carnot cycle and investigate its power fluctuations. In particular, we consider how level degeneracy and level number, two commonly found properties in quantum systems, influence the relative work fluctuations. We find that their optimal performance may surpass those of nondegenerate two-level engines or harmonic oscillator motors. Our results highlight that these parameters can be employed to realize high-performance, high-stability cyclic quantum heat engines.
At low-temperatures a gas of bosons will undergo a phase transition into a quantum state of matter known as a Bose-Einstein condensate (BEC), in which a large fraction of the particles will occupy the ground state simultaneously. Here we explore the performance of an endoreversible Otto cycle operating with a harmonically confined Bose gas as the working medium. We analyze the engine operation in three regimes, with the working medium in the BEC phase, in the gas phase, and driven across the BEC transition during each cycle. We find that the unique properties of the BEC phase allow for enhanced engine performance, including increased power output and higher efficiency at maximum power.