[Paper Review] Efficiency bounds for quantum engines powered by non-thermal baths

Wolfgang Niedenzu, David Gelbwaser-Klimovsky+2 more|arXiv (Cornell University)|Aug 26, 2015

Advanced Thermodynamics and Statistical Mechanics3 citations

TL;DR

This paper investigates quantum engines driven by non-thermal baths—such as squeezed or coherently displaced thermal baths—that can induce non-passive states in the working fluid, enabling it to store and deliver work. Unlike conventional heat engines bounded by Carnot efficiency, these machines operate as thermo-mechanical engines powered by both heat and mechanical work from the bath, achieving efficiencies beyond the Carnot limit under certain conditions.

ABSTRACT

We analyse the operation principles and performance bounds of quantum engines whose working fluid (WF) is energised by a non-thermal bath. We show that such a bath (e.g., a squeezed or coherently displaced thermal bath) can render the WF state non-passive, i.e., capable of storing and delivering work. This non-passivity converts the heat engine into a thermo-mechanical machine that is powered by mechanical work, as well as heat, from the non-thermal bath. Its efficiency is unrestricted by the Carnot bound, which only applies to heat engines. By contrast, for certain WF--bath interactions and non-thermal bath states the WF thermalises. The machine then operates as a heat engine, but its Carnot bound may correspond to a higher temperature than its thermal-bath counterpart.

Motivation & Objective

• To understand how non-thermal baths can enable quantum engines to exceed Carnot efficiency bounds.
• To analyze the conditions under which a working fluid becomes non-passive when coupled to a non-thermal bath.
• To distinguish between thermo-mechanical operation (with mechanical work input) and traditional heat engine behavior in such systems.
• To determine when the working fluid thermalizes and reverts to a heat engine with potentially higher effective Carnot bounds.

Proposed method

• Model the working fluid as a quantum system interacting with a non-thermal bath, such as a squeezed or coherently displaced thermal bath.
• Use quantum thermodynamics to analyze the passivity of the working fluid’s steady state under different bath conditions.
• Assess the thermodynamic work extraction capacity of the working fluid based on its non-passive character.
• Compare efficiency bounds under non-thermal bath operation to the standard Carnot bound for thermal baths.
• Identify parameter regimes where the system behaves as a thermo-mechanical engine versus a heat engine.

Experimental results

Research questions

• RQ1 Under what conditions does a non-thermal bath render the working fluid non-passive?
• RQ2 How does non-passivity enable the working fluid to store and deliver work, altering the engine’s operational mode?
• RQ3 Can the efficiency of such engines exceed the Carnot bound, and if so, under what physical conditions?
• RQ4 When does the working fluid thermalize, and how does this affect the effective temperature and efficiency bound?

Key findings

• Non-thermal baths such as squeezed or coherently displaced thermal baths can induce non-passive states in the working fluid, enabling it to store and deliver work.
• The resulting machine operates as a thermo-mechanical engine powered by both heat and mechanical work from the bath, thus escaping the Carnot efficiency bound.
• Efficiency in such systems is unrestricted by the Carnot limit, which only applies to heat engines operating between thermal reservoirs.
• For certain interaction types and bath states, the working fluid thermalizes, reverting the system to a heat engine with a potentially higher effective Carnot bound than its thermal-bath counterpart.