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We investigate the thermodynamics and fluctuations of a finite-time quantum Otto engine alter- natively driven by a hot-squeezed and a cold thermal reservoir. We show that reservoir squeezing significantly enhances the performance by increasing the thermodynamic efficiency and the power and enables higher stability by decreasing the relative power fluctuations and speeding up the convergence of quantum efficiency to its most probable value. We also demonstrate the counterintuitive result that the efficiency can be larger than the Otto limit in the finite-time operation. Experimental demonstration of this quantum heat engine can be available, based on a single-electron spin pertaining to a trapped ^{40}Ca^{+} ion [D. von Lindenfels et al., Phys. Rev. Lett. 123, 080602 (2019)0031-900710.1103/PhysRevLett.123.080602]. We provide a general framework for reliably studying the finite-time nanoengine in finite-time operation which accounts for quantum friction and coherence, deriving important insights into the thermodynamic behaviors beyond the classical thermal machines.

We investigate the efficiency at the maximum power output (EMP) of an irreversible Carnot engine performing finite-time cycles between two reservoirs at constant temperatures Th, and Tc (

We study the efficiency at maximum power, &eegr;m, of irreversible quantum Camot engines (QCEs) that perform finite-time cycles between a hot and a cold reservoir at temperatures Th and Tc., respectively. For QCEs in the reversible limit (long cycle period, zero dissipation), &eegr;m becomes identical to the Carnot efficiency &eegr;c = 1 -- Tc/Th- For QCE cycles in which nonadiabatic dissipation and the time spent on two adiabais are included, the efficiency &eegr;m at maximum power output is bounded from above by &eegr;c/(2 -- &eegr;c) and from below by &eegr;c/2. In the case of symmetric dissipation, the Curzon-Ahlbom efficiency &eegr;CA = 1 - √Tc/Th is recovered under the condition that the time allocation between the adiabais and the contact time with the reservoir satisfy a certain relation. [ABSTRACT FROM AUTHOR]

We present a performance analysis of a two-state heat engine model working with a single-mode radiation field in a cavity. The heat engine cycle consists of two adiabatic and two isoenergetic processes. Assuming the wall of the potential moves at a very slow speed, we determine the optimization region and the positive work condition of the heat engine model. Furthermore, we generalize the results to the performance optimization for a two-state heat engine with a one-dimensional power-law potential. Based on the generalized model with an arbitrary one-dimensional potential, we obtain the expression of efficiency as η = 1 - EC/EH, with EH (EC) denoting the expectation value of the system Hamiltonian along the isoenergetic process at high (low) energy. This expression is an analog of the classical thermodynamical result of Carnot, ηc = 1 - TC/TH, with TH (TC) being the temperature along the isothermal process at high (low) temperature. We prove that under the same conditions, the efficiency η = 1 - EC/EH is bounded from above the Carnot efficiency, ηc = 1 - TC/TH, and even quantum dynamics is reversible. [ABSTRACT FROM AUTHOR]

Based on counting statistics and Bogoliubov theory, we present a recurrence relation for the microcanonical partition function for a wealdy interacting Bose gas with a finite number of particles in a cubic box. According to this microcanonical partition function, we calculate numerically the distribution function, condensate fraction, and condensate fluctuations for a finite and isolated Bose-Einstein condensate. For ideal and weakly interacting Bose gases, we compare the condensate fluctuations with those in the canonical ensemble. The present approach yields an accurate account of the condensate fluctuations for temperatures close to the critical region. We emphasize that the interactions between excited atoms turn out to be important for moderate temperatures. [ABSTRACT FROM AUTHOR]

We consider the finite-time performance of a quantum Otto engine working between a hot squeezed and a cold thermal bath at inverse temperatures βh and βc(>βh) with (kB≡1)β=1/T. We derive the analytical expressions for work, efficiency, power, and power fluctuations, in which the squeezing parameter is involved. By optimizing the power output with respect to two frequencies, we derive the efficiency at maximum power as ηmp=(ηgenC)2/[ηgenC-(1-ηgenC)ln(1-ηgenC)], where the generalized Carnot efficiency ηgenC in the high-temperature or small squeezing limit simplifies to an analytic function of squeezing parameter γ: ηgenC=1-βh/[βccosh(2γ)]. Within the context of irreversible thermodynamics, we demonstrate that the expression of efficiency at maximum power satisfies a general form derived from nonlinear steady state heat engines. We show that, the power fluctuations are considerably increased, although the engine efficiency is enhanced by squeezing. [ABSTRACT FROM AUTHOR]

We investigate the finite-power performance of quantum heat engines working in the linear response regime where the temperature gradient is small. The engine cycles with working substances of ideal harmonic systems consist of two heat transfer and two adiabatic processes, such as the Carnot cycle, Otto cycle, and Brayton cycle. By analyzing the optimal protocol under maximum power we derive the explicitly analytic expression for the irreversible entropy production, which becomes the low dissipation form in the long duration limit. Assuming the engine to be endoreversible, we derive the universal expression for the efficiency at maximum power, which agrees well with that obtained from the phenomenological heat transfer laws holding in the classical thermodynamics. Through appropriate identification of the thermodynamic fluxes and forces that a linear relation connects, we find that the quantum engines under consideration are tightly coupled, and the universality of efficiency at maximum power is confirmed at the linear order in the temperature gradient. [ABSTRACT FROM AUTHOR]

We analyze general models of quantum heat engines operating a cycle of two adiabatic and two isothermal processes. We use the quantum master equation for a system to describe heat transfer current during a thermodynamic process in contact with a heat reservoir, with no use of phenomenological thermal conduction. We apply the endoreversibility description to such engine models working in the linear response regime and derive expressions of the efficiency and the power. By analyzing the entropy production rate along a single cycle, we identify the thermodynamic flux and force that a linear relation connects. From maximizing the power output, we find that such heat engines satisfy the tight-coupling condition and the efficiency at maximum power agrees with the Curzon-Ahlborn efficiency known as the upper bound in the linear response regime. [ABSTRACT FROM AUTHOR]

We consider the efficiency at maximum power of a quantum Otto engine, which uses a spin or a harmonic system as its working substance and works between two heat reservoirs at constant temperatures Th and Tc (

A two-level atomic system as a working substance is used to set up a refrigerator consisting of two quantum adiabatic and two isochoric processes (two constant-frequency processes ωa and ωb with ωa < ωb), during which the two-level system is in contact with two heat reservoirs at temperatures Th, and Tc(

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 and Tc(

We generalize the quantum heat engine (QHE) model which was first proposed by Bender et al. [J. Phys. A 33, 4427 (2000)] to the case in which an ideal Fermi gas with an arbitrary number N of particles in a box trap is used as the working substance. Besides two quantum adiabatic processes, the engine model contains two isoenergetic processes, during which the particles are coupled to energy baths at a high constant energy Eh and a low constant energy Ec, respectively. Directly employing the finite-time thermodynamics, we find that the power output is enhanced by increasing particle number N (or decreasing minimum trap size LA) for given LA (or N), without reduction in the efficiency. By use of global optimization, the efficiency at possible maximum power output (EPMP) is found to be universal and independent of any parameter contained in the engine model. For an engine model with any particle-number N, the efficiency at maximum power output (EMP) can be determined under the condition that it should be closest to the EPMP. Moreover, we extend the heat engine to a more general multilevel engine model with an arbitrary ID power-law potential. Comparison between our engine model and the Carnot cycle shows that, under the same conditions, the efficiency η = 1 -- Ec&frac;Eh of the engine cycle is bounded from above the Carnot value ηc = 1 -- Tc&frac;Th. [ABSTRACT FROM AUTHOR]

We establish a quantum Otto engine (QOE) of a two-level atom, which is confined in a one-dimensional (ID) harmonic trap and is coupled to single-mode radiation fields. Besides two adiabatic processes, the QOE cycle consists of two isochoric processes, along one of which the two-level atom as the working substance interacts with a single-mode radiation field. Based on the semigroup approach, we derive the time for completing any adiabatic process and then present a performance analysis of the heat engine model. Furthermore, we generalize the results to the performance optimization for a QOE of a single two-level atom trapped in a 1D power-law potential. Our result shows that the efficiency at maximum power output is dependent on the trap exponent 9 but is independent of the energy spectrum index &sgr;. [ABSTRACT FROM AUTHOR]

We propose and theoretically investigate a system of two coupled harmonic oscillators as a heat engine. We show how these two coupled oscillators within undamped regime can be controlled to realize an Otto cycle that consists of two adiabatic and two isochoric processes. During the two isochores the harmonic system is embedded in two heat reservoirs at constant temperatures Th and Tc(< Th). respectively, and it is tuned slowly along a protocol to realize an adiabatic process. To illustrate the performance in finite time of the quantum heat engine, we adopt the semigroup approach to model the thermal relaxation dynamics along the two isochoric processes, and we find the upper bound of efficiency at maximum power (EMP) η* to be a function of the Carnot efficiency ηc(=1 - Tc/Th): η* ≲ η+ ≡ ηc²/[ηc - (1 - ηc)ln(l - ηc)], identical to those previously derived from ideal (noninteracting) microscopic, mesoscopic, and macroscopic systems. [ABSTRACT FROM AUTHOR]

We propose a quantum absorption refrigerator driven by photons. The model uses a four-level system as its working substance and couples simultaneously to hot, cold, and solar heat reservoirs. Explicit expressions for the cooling power Qc, and coefficient of performance (COP)ηcop are derived, with the purpose of revealing and optimizing the performance of the device. Our model runs most efficiently under the tight coupling condition, and it is consistent with the third law of thermodynamics in the limit T → 0. [ABSTRACT FROM AUTHOR]

We study the thermodynamic performance of a nanosized photoelectric refrigerator consisting of three coupled single-level quantum dots embedded between two reservoirs at different temperatures. Based on the quantum master equation, we derive expressions for the cooling power and coefficient of performance (COP) of the refrigerator and plot the characteristic curves between the cooling power and the COP. We analyze the optimal performance parameters under conditions of maximum cooling power and maximum COP, and we discuss the influence of the energy level difference and the temperature ratio on the optimal performance parameters in detail. [ABSTRACT FROM AUTHOR]

We study the coefficient of performance (COP) and its bounds for a Carnot-like refrigerator working between two heat reservoirs at constant temperatures Th, and Tc, under two optimization criteria X and Ω. In view of the fact that an "adiabatic" process usually takes finite time and is nonisentropic, the nonadiabatic dissipation and the finite time required for the adiabatic processes are taken into account by assuming low dissipation. For given optimization criteria, we find that the lower and upper bounds of the COP are the same as the corresponding ones obtained from the previous idealized models where any adiabatic process is undergone instantaneously with constant entropy. To describe some particular models with very fast adiabatic transitions, we also consider the influence of the nonadiabatic dissipation on the bounds of the COP, under the assumption that the irreversible entropy production in the adiabatic process is constant and independent of time. Our theoretical predictions match the observed COPs of real refrigerators more closely than the ones derived in the previous models, providing a strong argument in favor of our approach. [ABSTRACT FROM AUTHOR]