Nonequilibrium heat transport and work with a single artificial atom coupled to a waveguide: emission without external driving

TL;DR

This paper demonstrates photon emission from a superconducting qubit without external drive due to nonequilibrium heat transport between two baths, and explores its implications for quantum thermodynamics and noise spectroscopy.

Contribution

It introduces a two-bath model explaining emission without external drive and proposes using waveguide spectroscopy for studying quantum heat engines and noise calibration.

Findings

Photon emission occurs without external drive due to hot and cold bath coupling.

Thermal occupation of the hot bath is significantly higher than the cold waveguide.

Waveguide spectroscopy can measure thermodynamic work and heat in quantum systems.

Abstract

We observe the continuous emission of photons into a waveguide from a superconducting qubit without the application of an external drive. To explain this observation, we build a two-bath model where the qubit couples simultaneously to a cold bath (the waveguide) and a hot bath (a secondary environment). Our results show that the thermal-photon occupation of the hot bath is up to 0.14 photons, 35 times larger than the cold waveguide, leading to nonequilibrium heat transport with a power of up to 132 zW, as estimated from the qubit emission spectrum. By adding more isolation between the sample output and the first cold amplifier in the output line, the heat transport is strongly suppressed. Our interpretation is that the hot bath may arise from active two-level systems being excited by noise from the output line. We also apply a coherent drive, and use the waveguide to measure thermodynamic work and heat, suggesting waveguide spectroscopy is a useful means to study quantum heat engines and refrigerators. Finally, based on the theoretical model, we propose how a similar setup can be used as a noise spectrometer which provides a new solution for calibrating the background noise of hybrid quantum systems.