Ultrastrong coupling between light and matter

Abstract: Ultrastrong coupling between light and matter has, in the past decade, transitioned from theoretical idea to experimental reality. It is a new regime of quantum light-matter interaction, going beyond weak and strong coupling to make the coupling strength comparable to the transition frequencies in the system. The achievement of weak and strong coupling has led to increased control of quantum systems and applications like lasers, quantum sensing, and quantum information processing. Here we review the theory of quantum systems with ultrastrong coupling, which includes entangled ground states with virtual excitations, new avenues for nonlinear optics, and connections to several important physical models. We also review the multitude of experimental setups, including superconducting circuits, organic molecules, semiconductor polaritons, and optomechanics, that now have achieved ultrastrong coupling. We then discuss the many potential applications that these achievements enable in physics and chemistry.

• The paper demonstrates that ultrastrong coupling, with normalized strengths above 0.1 and reaching values near 1.43, fundamentally alters light-matter interactions.
• It employs numerical insights and a systematic review of experimental realizations across superconducting circuits, organic molecules, semiconductor polaritons, and optomechanical systems.
• The study revisits foundational quantum models by highlighting the essential role of typically neglected counter-rotating terms, impacting decoherence and quantum state dynamics.

Ultrastrong Coupling between Light and Matter

The research paper titled "Ultrastrong coupling between light and matter" presents an in-depth exploration of a novel and emerging regime within quantum light-matter interaction, known as ultrastrong coupling (USC). This regime is characterized by coupling strengths that are on par with the transition frequencies in the system, transcending the more explored weak and strong coupling regimes within cavity quantum electrodynamics (CQED).

Numerical Insights and Experimental Realizations

The paper provides numerical insights into the distinctive ground-state properties observed in the USC regime. For instance, conventional perturbation theories cease to be applicable as coupling strengths can reach above 0.1 in normalized terms ($\eta$), indicating the USC regime, and can extend to deep-strong coupling (DSC) when $\eta > 1$. The theoretical foundation is supported by notable experimental realizations across various platforms, including superconducting circuits, organic molecules, semiconductor polaritons, and optomechanical systems. These experimental setups have showcased the practical attainability of USC, and in some cases, DSC, with values of $\eta$ approaching 1.43 within Landau polariton contexts.

Theoretical Foundations and Model Implications

The paper systematically reviews the theoretical underpinnings of formal models such as the quantum Rabi model, Dicke model, and Hopfield model, crucial for understanding USC phenomena. A key theoretical assertion made is that USC necessitates a re-evaluation of standard approximations; for example, counter-rotating terms generally neglected in the Jaynes-Cummings model become non-negligible and significantly impact the dynamics and properties of the system, demanding new approaches to decoherence and input-output theories.

Practical Implications and Future Prospects

The implications of USC span several fields. Within quantum computing and information processing, USC can enable faster quantum operations and more stable quantum states. Additionally, the novel coherent phenomena facilitated by USC, such as multi-photon Rabi oscillations, open pathways into scalable quantum technologies.

Another profound implication is the USC regime's potential impact on chemical phenomena. In molecular polariton setups, ultrastrong coupling might enable modifying and controlling chemical reactions through tailored coupling of light and molecular states. Future chemical QED applications might involve leveraging USC for novel reaction pathways or even synthesizing new materials with desirable electronic properties.

Conclusion and Outlook

With advances in experimental techniques, the USC regime stands at the frontier of expanding the known capabilities in light-matter interactions. It necessitates developing robust theoretical frameworks and revisiting foundational models of quantum mechanics. The research highlights that USC not only enriches quantum optics but also promises practical advances across multiple disciplines, potentially laying the groundwork for transformative applications in quantum technologies and materials science. As experimental realizations continue to improve in fidelity and stability, the scope for exploring and harnessing USC remains vast and promising. Future developments will likely focus on systematically understanding new phenomena emergent in USC and realizing complex quantum systems exploiting these interactions.