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arXiv:1512.02165 · Physical review. B./Physical review. B

Quantum heat engine based on photon-assisted Cooper pair tunneling

Patrick P. Potts, J.-R. Souquet, Aashish A. Clerk

Published Jan 22, 2016• 109 citations
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Abstract

A voltage biased Josephson junction coupled to two microwave cavities which are in turn coupled to two ``hot'' and ``cold'' thermal environments, constitutes the proposal for the first thermoelectric heat engine where the heat current is completely separated from the electronic degrees of freedom.

DOI
10.1103/physrevb.93.041418

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A voltage biased Josephson junction coupled to two microwave cavities which are in turn coupled to two hot'' and cold'' thermal environments, constitutes the proposal for the first thermoelectric heat engine where the heat current is completely separated from the electronic degrees of freedom.
When the two electrons from a Cooper pair are separated into different reservoirs, the heat flow between the reservoirs is reverted, providing a quantum refrigerator. The process can be also used to define a nonlocal thermoelectric power generator. This is ultimately possible due to the quantum correlations of the pair. It, therefore, constitutes an experimentally accessible proposal of an entanglement-based quantum heat engine, able to optimally extract thermodynamic work from quantum many-body states.
Inelastic Cooper pair tunneling across a voltage-biased Josephson junction in series with one or more microwave cavities can generate photons via resonant processes in which the energy lost by the Cooper pair matches that of the photon(s) produced. We generalize previous theoretical treatments of such systems to analyze cases where two or more different photon generation processes are resonant simultaneously. We also explore in detail a specific case where the generation of a single photon in one cavity mode is simultaneously resonant with the generation of two photons in a second mode. We find that the coexistence of the two resonances leads to effective couplings between the modes which in turn generate entanglement.
The single-atom version of the quantum heat engine based on electromagnetically induced transparency is theoretically investigated in this paper. In contact with a hot and a cold bath, the atom trapped in an optical cavity is coherently coupled by a coherent field, thus generating photons as output through a process that mimics a heat engine. Using semiclassical theory, we obtain the conditions for the classical gain parameter being positive. Then by taking a full quantum approach, we discuss the statistical property of the generated photons depicted by the photon number distribution and Wigner function, and show that it can be effectively tuned by the coupling field. Investigations on the energy and entropy balances of the heat engine process lead to an interesting result that the power of the engine is simply the energy per time provided by the hot reservoir. The coupling field plays a critical role in supporting the heat-engine process. However, the energy that it provides is all dumped into the cold reservoir. The total energy exchange conforms to the first law of thermodynamics. The efficiency of the engine depends on the temperatures of the two reservoirs and the strength of the coupling field. The entropy change of the heat-engine process suggests that when working far above the threshold of lasing, the efficiency approaches that of its atomic-ensemble-based partner.
Quantum coherence can affect the thermodynamics of small quantum systems. Coherences have been shown to affect the power generated by a quantum heat engine (QHE) which is coupled to two thermal photon reservoirs and to an additional cavity mode. We show that the fluctuations of the heat exchanged between the QHE and the reservoirs strongly depend on quantum coherence, especially when the engine operates as a refrigerator, i.e., heat current flows from the cold bath to the hot bath. Intriguingly, we find that the ratio of positive and negative (with respect to the thermodynamic force) fluctuations in the heat current satisfies a universal coherence-independent fluctuation theorem.
Abstract When connecting a voltage-biased Josephson junction in series to several microwave cavities, a Cooper-pair current across the junction gives rise to a continuous emission of strongly correlated photons into the cavity modes. Tuning the bias voltage to the resonance where a single Cooper pair provides the energy to create an additional photon in each of the cavities, we demonstrate the entangling nature of these creation processes by simple witnesses in terms of experimentally accessible observables. To characterize the entanglement properties of the such created quantum states of light to the fullest possible extent, we then proceed to more elaborate entanglement criteria based on the knowledge of the full density matrix and provide a detailed study of bi- and multipartite entanglement. In particular, we illustrate how due to the relatively simple design of these circuits changes of experimental parameters allow one to access a wide variety of entangled states differing, e.g., in the number of entangled parties or the dimension of state space. Such devices, besides their promising potential to act as a highly versatile source of entangled quantum microwaves, may thus represent an excellent natural testbed for classification and quantification schemes developed in quantum information theory.