Gauge freedom, quantum measurements, and time-dependent interactions in cavity and circuit QED

TL;DR

This paper explores the complexities and ambiguities in defining subsystems, measurements, and interactions in ultrastrong and deep-strong cavity QED regimes, offering solutions to advance quantum technology understanding.

Contribution

It provides a detailed analysis of gauge freedom and measurement issues in high-coupling cavity and circuit QED, proposing methods to resolve fundamental ambiguities.

Findings

Clarified the role of gauge choices in ultrastrong coupling regimes

Proposed frameworks for consistent quantum measurements in high-interaction regimes

Identified key differences in ground state structures at strong coupling

Abstract

The interaction between the electromagnetic field inside a cavity and natural or artificial atoms has played a crucial role in developing our understanding of light-matter interaction, and is central to various quantum technologies. Recently, new regimes beyond the weak and strong light-matter coupling have been explored in several settings. These regimes, where the interaction strength is comparable (ultrastrong) or even higher (deep-strong) than the transition frequencies in the system, can give rise to new physical effects and applications. At the same time, they challenge our understanding of cavity QED. When the interaction strength is so high, fundamental issues like the proper definition of subsystems and of their quantum measurements, the structure of light-matter ground states, or the analysis of time-dependent interactions are subject to ambiguities leading to even qualitatively distinct predictions. The resolution of these ambiguities is also important for understanding and designing next-generation quantum devices that will exploit the ultrastrong coupling regime. Here we discuss and provide solutions to these issues.