Power maximization of two-stroke quantum thermal machines

Abstract

We present a detailed study of quantum thermal machines employing quantum systems as working substances. In particular, we study two different types of two-stroke cycles where two collections of identical quantum systems with evenly spaced energy levels are initially prepared at thermal equilibrium by putting them in contact with a cold and a hot thermal bath, respectively. The two cycles differ in the absence or the presence of a mediator system, while, in both cases, non-resonant exchange Hamiltonians are exploited as particle interactions. We show that the efficiencies of these machines depend only on the energy gaps of the systems composing the collections and are equal to the efficiency of "equivalent" Otto cycles. Focusing on the cases of qubits or harmonic oscillators for both models, we maximize the engine power and analyze, in the model without the mediator, the role of the waiting time between subsequent interactions. It turns out that the case with the mediator can bring performance advantages when the interaction times are comparable with the waiting time of the correspondent cycle without the mediator. We find that in both cycles, the power peaks of qubit systems can surpass the Curzon-Ahlborn efficiency. Finally, we compare our cycle without the mediator with previous schemes of the quantum Otto cycle showing that high coupling is not required to achieve the same maximum power.

Multilevel quantum thermodynamic swap engines

“…The present model shares some similarities with those studied in Refs. [70,71], where the same Otto efficiency is achieved. In particular, in Ref.…”

“…One of the most studied quantum thermodynamic engines is based on the Otto cycle [60][61][62][63][64][65][66][67][68][69][70][71], with possible implementations by different physical systems as working fluid, e.g. ion traps, Cooper-pair boxes, or quantized modes of the radiation field.…”

Unified trade-off optimization of quantum harmonic Otto engine and refrigerator

“…In order to compare the performance of the engine for the adiabatic driving and sudden switch case, we plot Eqs. (11) and (20) along with the expressions for Eq. ( 29) in Fig.…”

“…The goal of finite-time thermodynamics is finding the optimal performance of thermal machines when these limitations are taken into account as well as devising ways to improve on it [8][9][10][11]. One is usually interested in optimizing the power output of a heat engine and its corresponding efficiency [12][13][14][15][16][17][18][19][20][21][22][23], whereas, for a refrigerator, the most desirable figure of merit is cooling * varinder@ibs.re.kr power [11,[24][25][26]. A well-known observation that the thermal engines operating at maximum power also dissipate a large amount of power due to entropy production, which ultimately pollutes the environment [7,27,28].…”

Driven quantum harmonic oscillators: A working medium for thermal machines

“…The working substance itself may consist of quantum systems of different nature, including quantum harmonic oscillators (QHO's) [8][9][10][11][12], qubits [13][14][15] or multilevel systems [16][17][18][19]. Thermal machines can be normally split into three categories depending on how the working substance is operated upon: autonomous (where the Hamiltonian of the working substance is time-independent), discrete-stroke (e.g.…”