Squeezing as the source of inefficiency in the quantum Otto cycle

TL;DR

This paper investigates how quantum squeezing, caused by finite-speed compression and expansion, limits the efficiency of a quantum Otto cycle implemented in a tunable quantum circuit.

Contribution

It identifies quantum squeezing during finite-time processes as a key source of inefficiency in quantum heat engines, expanding understanding of quantum thermodynamics.

Findings

Quantum squeezing reduces efficiency of the Otto cycle.

Finite-speed operations cause thermal state squeezing.

Efficiency is bounded by the Carnot limit despite quantum effects.

Abstract

The availability of controllable macroscopic devices, which maintain quantum coherence over relatively long time intervals, for the first time allows an experimental realization of many effects previously considered only as Gedankenexperiments, such as the operation of quantum heat engines. The theoretical efficiency \eta of quantum heat engines is restricted by the same Carnot boundary \eta_C as for the classical ones: any deviations from quasistatic evolution suppressing \eta below \eta_C. Here we investigate an implementation of an analog of the Otto cycle in a tunable quantum coherent circuit and show that the specific source of inefficiency is the quantum squeezing of the thermal state due to the finite speed of compression/expansion of the system.