Virtual qubits, virtual temperatures, and the foundations of thermodynamics

TL;DR

This paper introduces the concept of virtual qubits and virtual temperatures to understand thermal machines, providing new insights into thermodynamics, efficiency limits, and the nature of work.

Contribution

It presents a novel framework using virtual qubits and temperatures to analyze thermal machines and thermodynamic phenomena, unifying various concepts under this perspective.

Findings

Thermal machines can be understood via virtual qubits at virtual temperatures.

Approaching Carnot efficiency involves machines becoming equivalent to the smallest thermal machines.

Entropy production and work can be analyzed through virtual temperature concepts.

Abstract

We argue that thermal machines can be understood from the perspective of `virtual qubits' at `virtual temperatures': The relevant way to view the two heat baths which drive a thermal machine is as a composite system. Virtual qubits are two-level subsystems of this composite, and their virtual temperatures can take on any value, positive or negative. Thermal machines act upon an external system by placing it in thermal contact with a well-selected range of virtual qubits and temperatures. We demonstrate these claims by studying the smallest thermal machines. We show further that this perspective provides a powerful way to view thermodynamics, by analysing a number of phenomena. This includes approaching Carnot efficiency (where we find that all machines do so essentially by becoming equivalent to the smallest thermal machines), entropy production in irreversible machines, and a way to view work in terms of negative temperature and population inversion. Moreover we introduce the idea of "genuine" thermal machines and are led to considering the concept of "strength" of work.