Theory of broadband dissipation in cavity quantum electrodynamics

Optical cavities are structures that confine light, allowing it to interact much more strongly with matter. Combining cavities with light-emitting materials like atoms, molecules, or other emitters is central to many proposed quantum technologies, ranging from quantum computation to enhanced sensing. However, as recent advancements in cavity design allow for light to be confined ever more tightly, the range of frequencies over which light and matter can interact is beginning to grow large enough that commonly used theoretical models begin to break down. In particular, the standard quantum models of how light leaks from the cavity can become insufficient to predict the outcome of experiments.

In a new paper published in Light: Science & Applications, a team of scientists at Stanford University, California, USA, Technische Universität Berlin, Germany, and Queen’s University, Ontario, Canada have developed an approach to accurately model the quantum dissipation in strongly-coupled cavity systems, while still retaining the simplicity and utility of previous models. Their approach is based on the concept of quasinormal modes, which are the fundamental resonances of open cavities. By using recent techniques to treat these modes quantum mechanically, they were able to derive an accurate dissipative quantum model for broadband light-matter interactions in a single-mode platform, which can be used to predict—for example—frequency-dependent dissipation rates of the coupled system.

One of the central findings of the researchers’ study is the identification of what they term the "broadband dissipative" regime of cavity quantum electrodynamics (QED). In this regime, the bandwidth of light-matter interaction is large enough that corrections described by the researchers’ theory are needed to accurately describe the system’s physics. This regime includes the so-called “ultrastrong coupling” regime of cavity-QED which has been the subject of much interest over the last two decades. Remarkably, the researchers have also shown that broadband dissipative behavior can occur with coupling strengths far weaker than those necessary for ultrastrong coupling.

In the broadband dissipative regime, the system dynamics begin to depend not only on the strength of the coupling between light and matter, but also on the spatial phase of the cavity field mode at the position of the emitter—a property typically absent in standard models. As this phase can be calculated by using Maxwell’s equations for a given cavity, the authors’ work allows for an “ab initio” approach, which, as they say, “significantly generalizes previous single-mode quantum models in the ultrastrong coupling regime for open cavity systems, in that it allows one to access to the coupling constants and cavity-environment spectral density from the quasinormal mode and the geometry properties of the scattering structure without any phenomenological fitting parameters.”

The researchers’ theory also allows for fundamental limits of single-mode models to be tested. As many simple models of quantum information processing elements relying on strong light-matter coupling assume single-mode operation, this work provides a practical benchmark on the validity of such assumptions.

By investigating realistic examples of cavities which support dominant quasinormal modes, including experimentally-motivated plasmonic and dielectric examples, the authors argue that many of the predicted effects could potentially be observed with current or near-future technology. As they write, "our results are relevant to emerging experiments and we predict they should in principle be observable in the near future."

This work provides a strong step towards a more complete first-principles description of how realistic quantum optical cavities leak light—an essential element towards greater control over quantum optical technologies and understanding the dissipation of light from engineered structures at a fundamental level.