We put forth a quantum simulation of a stroboscopic two-stroke thermal engine in the IBM quantum (IBMQ) processor. The system consists of a quantum spin chain connected to two baths at their boundaries, prepared at different temperatures using the variational quantum thermalizer algorithm. The dynamics alternates between heat and work strokes, which can be separately designed using independent quantum circuits. The results show good agreement with theoretical predictions, showcasing IBMQ as a powerful tool to study thermodynamics in the quantum regime, as well as the implementation of variational quantum algorithms in real-world quantum computers. It also opens the possibility of simulating quantum heat transport across a broad range of chain geometries and interactions.
We put forth a quantum simulation of a stroboscopic two-stroke thermal engine in the IBM quantum (IBMQ) processor. The system consists of a quantum spin chain connected to two baths at their boundaries, prepared at different temperatures using the variational quantum thermalizer algorithm. The dynamics alternates between heat and work strokes, which can be separately designed using independent quantum circuits. The results show good agreement with theoretical predictions, showcasing IBMQ as a powerful tool to study thermodynamics in the quantum regime, as well as the implementation of variational quantum algorithms in real-world quantum computers. It also opens the possibility of simulating quantum heat transport across a broad range of chain geometries and interactions.
The formulation of models describing quantum versions of heat engines plays an important role in the quest toward establishing the laws of thermodynamics in the quantum regime. Of particular importance is the description of stroke-based engines, which can operate at finite time. In this paper we put forth a framework for describing stroboscopic, two-stroke engines, in generic quantum chains. The framework is a generalization of the so-called SWAP engine and is based on a collisional model, which alternates between pure heat and pure work strokes. The transient evolution towards a limit cycle is also fully accounted for. Moreover, we show that once the limit cycle has been reached, the energy of the internal sites of the chain no longer changes, with the heat currents being associated exclusively to the boundary sites. Using a combination of analytical and numerical methods, we show that this type of engine offers multiple ways of optimizing the output power, without affecting the efficiency. Finally, we also show that there exists an entire class of models, characterized by a specific type of interchain interaction, which always operate at Otto efficiency, irrespective of the operating conditions of the reservoirs.
In the last decade, the study of thermodynamic phenomena in ultra-small scales, where quantum mechanics becomes imperative, has gained a lot of attention. The possibility of controlling single quantum states in nowadays experimental setups has encouraged a more intense inquiry over the intersection between thermodynamics and quantum mechanics, which is known as quantum thermodynamics. Particularly relevant in this framework is the study of quantum heat engines, that is, quantum systems undergoing thermodynamic cycles. Thermodynamic cycles contain all the aspects of thermodynamics, thus its a good testbed for a better comprehension of the thermodynamics of quantum systems. Moreover, modelling quantum heat engines is crucial for the design of future ultra-small engines. Nonetheless, another aspect must be taken into account, finite-time operation. Its very important for the optimization of the output power of the engine. In this dissertation, we present a new model of finite-time quantum heat engines. By making use of collisional models, we construct a model in which a generic quantum chain experiences sequential pure heat and pure work strokes. Dictated by stroboscopic evolution, the engines state goes through a transient regime until the limit-cycle is reached. After the achievement of the limit-cycle, our results indicate that only the boundary sites of the quantum chain are relevant for the heat currents exchanged with the baths. By means of analytical and numerical methods, we present how the model is useful for optimizing the output power of stroke-based quantum heat engines, without decreasing their respective efficiencies. Lastly, we prove that there is a universal efficiency value, the Otto efficiency, for a whole family of models containing a specific kind of internal interactions. For completeness, other methods from the literature which deal with finite-time quantum heat engines are also presented and discussed.
We suggest alternative quantum Otto engines, using heat bath algorithmic cooling with a partner pairing algorithm instead of isochoric cooling and using quantum swap operations instead of quantum adiabatic processes. Liquid state nuclear magnetic resonance systems in a single entropy sink are treated as working fluids. The extractable work and thermal efficiency are analyzed in detail for four-stroke and two-stroke types of alternative quantum Otto engines. The role of the heat bath algorithmic cooling in these cycles is to use a single entropy sink instead of two so that a single incoherent energy resource can be harvested and processed using an algorithmic quantum heat engine. Our results indicate a path to programmable quantum heat engines as analogs of quantum computers beyond traditional heat engine cycles. We find that for our NMR system example implementation of quantum algorithmic heat engine stages yields more power due to increased cycle speeds.
Quantum heat engines, compared to their classical counterparts, offer many advantages due to quantum effects. The study of different cycle modes contributes to the improvement of heat engine efficiency. In this paper, based on a quantum hybrid heat engine, we compare three cycle modes involving Bell-type quantum weak measurement (without observing the results) and two heat baths at different temperatures, where quantum weak measurements play the role of heat source. Thus, there are two types of heat sources and three processes involving heat absorption or release in our model. In other words, we study a multi-stroke quantum hybrid heat engine. Moreover, we further investigate three types of cycle modes using a working substance composed of a three-particle XXZ spin chain. In the adiabatic stroke, we select three Hamiltonian parameters and vary them adiabatically from their initial to final values, comparing their effects on engine efficiency. In particular, we examine how the ordering of the weak-measurement stroke and the adiabatic stroke influences heat absorption, heat release, and engine efficiency. Additionally, we analyze the impact of the high-temperature bath’s temperature and the weak-measurement strength on the engine efficiency.
The minimal-coupling quantum heat engine is a thermal machine consisting of an explicit energy storage system, heat baths, and a working body, which alternatively couples to subsystems through discrete strokes --- energy-conserving two-body quantum operations. Within this paradigm, we present a general framework of quantum thermodynamics, where a work extraction process is fundamentally limited by a flow of non-passive energy (ergotropy), while energy dissipation is expressed through a flow of passive energy. It turns out that small dimensionality of the working body and a restriction only to two-body operations make the engine fundamentally irreversible. Our main result is finding the optimal efficiency and work production per cycle within the whole class of irreversible minimal-coupling engines composed of three strokes and with the two-level working body, where we take into account all possible quantum correlations between the working body and the battery. One of the key new tools is the introduced ``control-marginal state" --- one which acts only on a working body Hilbert space, but encapsulates all features regarding work extraction of the total working body-battery system. In addition, we propose a generalization of the many-stroke engine, and we analyze efficiency vs extracted work trade-offs, as well as work fluctuations after many cycles of the running of the engine.
Abstract Real quantum heat engines lack the separation of time and length scales that is characteristic for classical engines. They must be understood as open quantum systems in non-equilibrium with time-controlled coupling to thermal reservoirs as integral part. Here, we present a systematic approach to describe a broad class of engines and protocols beyond conventional weak coupling treatments starting from a microscopic modeling. For the four stroke Otto engine the full dynamical range down to low temperatures is explored and the crucial role of the work associated with the coupling/de-coupling to/from reservoirs as an integral part in the energy balance is revealed. Quantum correlations turn out to be instrumental to enhance the efficiency which opens new ways for optimal control techniques.