Quantum heat engines

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Heat engines are one of the central tenets of thermodynamics. In cyclical heat engines, a working gas moves through a reversible cycle to transfer heat between a hot and a cold reservoir and perform useful work. The steam engine and the internal combustion engine are two well-known examples of cyclical heat engines. Particle-exchange heat engines also convert thermal energy into useful work, however heat is transferred from a hot to a cold reservoir via the exchange of particles, for example electrons, in a finite energy range. In the particular case of electron heat engines, electrons flow against an applied electric field to generate power. Typical embodiments of electron heat engines include thermoelectric and photovoltaic devices, where a semiconductor bandgap acts as an energy filter that results in a net charge flow from the hot to the cold reservoir. The thermodynamic efficiency of an electron heat engine depends on exact details of the energy filter and on the heat conductance mediated by nuclear vibrations, or phonons. In recent years the development of nanostructured thermoelectric materials has led to a significant increase in the efficiency of thermoelectric generators and coolers. Yet there is still plenty of room for improvement: conventional thermoelectric devices operate only at a fraction of the Carnot efficiency that is the upper limit that any classical engine can achieve in converting heat to work.Quantum dot heat engines have the potential to operate close to the thermodynamic limit. Their discrete spectrum of provides an ideal energy filter, and their nuclear vibrations could be controlled through clever molecular design. To date there is however sparse experimental evidence of heat- to-electricity conversion in graphene quantum dots. Moreover, quantum effects that are known to play a role in charge transport through graphene quantum dots, and could enhance the efficiency of quantum dot heat engines. Yet, the link between quantum mechanics and thermodynamics on the level of an individual molecule has remained largely unexplored. This project will develop a comprehensive experimental toolkit for investigating quantum dot thermoelectric devices and explore the ultimate efficiency limits of nano-scale thermoelectric energy conversion to establish the paradigm of quantum dot heat engines in nanoscale thermodynamics.