Ultrastrong coupling between light and matter has, in the past decade, transitioned from a theoretical idea to an experimental reality. It is a new regime of quantum light–matter interaction, which goes 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 to applications such as lasers, quantum sensing, and quantum information processing. Here we review the theory of quantum systems with ultrastrong coupling, discussing entangled ground states with virtual excitations, new avenues for nonlinear optics, and connections to several important physical models. We also overview the multitude of experimental setups, including superconducting circuits, organic molecules, semiconductor polaritons, and optomechanical systems, that have now achieved ultrastrong coupling. We conclude by discussing the many potential applications that these achievements enable in physics and chemistry.
Ultrastrong coupling between light and matter has, in the past decade, transitioned from a theoretical idea to an experimental reality. It is a new regime of quantum light–matter interaction, which goes 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 to applications such as lasers, quantum sensing, and quantum information processing. Here we review the theory of quantum systems with ultrastrong coupling, discussing entangled ground states with virtual excitations, new avenues for nonlinear optics, and connections to several important physical models. We also overview the multitude of experimental setups, including superconducting circuits, organic molecules, semiconductor polaritons, and optomechanical systems, that have now achieved ultrastrong coupling. We conclude by discussing the many potential applications that these achievements enable in physics and chemistry.
The interaction of light and matter at the single-photon level is of central importance in various fields of physics, including, e.g., condensed matter physics, astronomy, quantum optics, and quantum information. Amplification of such quantum light–matter interaction can be highly beneficial to, e.g., improve device performance, explore novel phenomena, and understand fundamental physics, and has therefore been a long-standing goal. Furthermore, simulation of light–matter interaction in the regime of ultrastrong coupling, where the interaction strength is comparable to the bare frequencies of the uncoupled systems, has also become a hot research topic, and considerable progress has been made both theoretically and experimentally in the past decade. In this review, we provide a detailed introduction of recent advances in amplification of quantum light–matter interaction and simulation of ultrastrong light–matter interaction, particularly in cavity and circuit quantum electrodynamics and in cavity optomechanics.
In the last decades, the experimental and theoretical study of light-matter interactions in confined quantum systems has allowed to considerably widen and deepen our understanding of the quantum world.The simplest fully-quantized light-matter interaction is the quantum Rabi model (QRM), where a two-level system is coupled to a single bosonic mode.The implementation of quantum-optical systems in which the interaction strength overcomes losses brought a revolution to this research area.Under this condition, dubbed strong coupling (SC) regime, quantum processes can be observed at the single-photon level, and even exploited to perform Quantum Information tasks.Recent developments in solid-state Quantum Technologies have made possible to push the line further, achieving the ultrastrong coupling (USC) regime of the QRM, in which the interaction strength is comparable with the bare frequencies of the interacting systems.In the USC regime of the QRM, light and matter degrees of freedom merge into collective bound states called polaritons, the system ground state is not the vacuum and excitations are not conserved.The growing interest in USC-related phenomena is mainly motivated by the fundamental counterintuitive modifications to light-matter interactions entailed by this regime.At the same time, the USC regime is also expected to provide computational benefits in terms of operational speed, coherence time, and noise protection.In this Thesis, we theoretically analyze novel quantum phenomena emerging in the USC regime, which can be observed using nowadays technology and can motivate foreseeable improvements.The results here presented have been derived using two conceptually different approaches.On one side, we have analyzed interesting models that can be implemented in superconducting circuits, a quantum platform where the USC regime can be naturally achieved.In particular, we focused on excitation transfer across USC impurities, entanglement generation via modulation of electrical boundary conditions and quantum state engineering in ultrastrongly-coupled systems.On the other hand, we have developed methods to reproduce the physics of light-matter interactions in the USC regime, by tailoring the Hamiltonian of atomic systems, like trapped ions and cold atoms.These proposals take profit of the specific features of such quantum platforms, in order to implement regimes and measurements that are not directly accessible.We believe that this Thesis will contribute to develop a thorough understanding of quantum phenomena related to the USC regime of light-matter interaction, and that it will foster the dialog between experimental and theoretical research in this area.I would like to thank first my PhD supervisor Prof. Enrique Solano, for his constant support and invaluable guidance.Countless thought-provoking discussions held with him during my PhD studies have contributed to develop my critical skills, and his advice has covered all facets of nowadays top-level research.
Strong light-matter coupling gives rise to polaritons - hybrid excitations whose mixed photonic and matter character enables control over optical, electronic and chemical properties. This Feature Article surveys the main architectures supporting polariton formation, including photonic microcavities, plasmonic nanostructures, open cavities and metasurfaces, and outlines how inorganic semiconductors, organic aggregates and hybrid systems access strong and ultrastrong coupling. Key phenomena such as coherent dynamics, vibronic interactions, dark-state reservoirs and polariton-mediated energy and electron transport are discussed, together with the experimental and theoretical tools used to study them. We highlight examples where strong coupling modifies charge transport, energy flow and chemical reactivity, and we summarize emerging regimes, including intermediate and dark-strong coupling, that broaden the landscape of hybrid light-matter physics.
In the ultrastrong coupling regime, light and matter can no longer be distinguished: their coupling strengths are as large as their own energy scales. Even for simple two-level systems, this represents a new regime to explore within the quantum Rabi model. There has been considerable theoretical and experimental progress on the physics of these systems in recent years, as presented in this review. Key experimental results are discussed in a set of current platforms arising in quantum information research, including superconducting and semiconducting devices, and other hybrid quantum systems.
Resonance interaction between a molecular transition and a confined electromagnetic field can reach the coupling regime where coherent exchange of energy between light and matter becomes reversible. In this case, two new hybrid states separated in energy are formed instead of independent eigenstates, which is known as Rabi splitting. This modification of the energy spectra of the system offers new possibilities for controlled impact on various fundamental properties of coupled matter (such as the rate of chemical reactions and the conductivity of organic semiconductors). To date, the strong coupling regime has been demonstrated in many configurations under different ambient conditions. However, there is still no comprehensive approach to determining parameters for achieving the strong coupling regime for a wide range of practical applications. In this review, a detailed analysis of various systems and corresponding conditions for reaching strong coupling is carried out and their advantages and disadvantages, as well as the prospects for application, are considered. The review also summarizes recent experiments in which the strong coupling regime has led to new interesting results, such as the possibility of collective strong coupling between X-rays and matter excitation in a periodic array of Fe isotopes, which extends the applications of quantum optics; a strong amplification of the Raman scattering signal from a coupled system, which can be used in surface-enhanced and tip-enhanced Raman spectroscopy; and more efficient second-harmonic generation from the low polaritonic state, which is promising for nonlinear optics. The results reviewed demonstrate great potential for further practical applications of strong coupling in the fields of photonics (low-threshold lasers), quantum communications (switches), and biophysics (molecular fingerprinting).
Abstract This article reviews theoretical methods developed in the last decade to understand cavity quantum electrodynamics in the ultrastrong‐coupling regime, where the strength of the light–matter interaction becomes comparable to the photon frequency. Along with profound modifications of fundamental quantum optical effects giving rise to a rich phenomenology, this regime introduces significant theoretical challenges. One of the most important is the break‐down of the rotating‐wave approximation which neglects all non‐resonant terms in light–matter interaction Hamiltonians. Consequently, a large part of the quantum optical theoretical framework has to be revisited in order to accurately account for all interaction terms in this regime. In this article, a broad overview of the recent progress is given, ranging from analytical estimates of ground‐state properties to proper derivations of master equations and computation of photodetection signals. For each aspect of the theory, the basic principles of the methods are illustrated on paradigmatic models such as quantum Rabi and spin‐boson models. The validity of effective Hamiltonians for the different experimental platforms is discussed in the last part of the article, addressing recent debates on fundamental issues related to gauge invariance in the ultrastrong‐coupling regime.