In this review we look at the concepts and state-of-the-art concerning the strong coupling of surface plasmon-polariton modes to states associated with quantum emitters such as excitons in J-aggregates, dye molecules and quantum dots. We explore the phenomenon of strong coupling with reference to a number of examples involving electromagnetic fields and matter. We then provide a concise description of the relevant background physics of surface plasmon polaritons. An extensive overview of the historical background and a detailed discussion of more recent relevant experimental advances concerning strong coupling between surface plasmon polaritons and quantum emitters is then presented. Three conceptual frameworks are then discussed and compared in depth: classical, semi-classical and fully quantum mechanical; these theoretical frameworks will have relevance to strong coupling beyond that involving surface plasmon polaritons. We conclude our review with a perspective on the future of this rapidly emerging field, one we are sure will grow to encompass more intriguing physics and will develop in scope to be of relevance to other areas of science.

https://iopscience.iop.org/article/10.1088/0034-4885/78/1/013901/pdf

Strong coupling between surface plasmon polaritons and emitters: a review

Spinorbit coupling in electrons leads to many fascinating phenomena and important applications, from topological insulators to spintronics. Researchers have recently been exploring whether effects analogous to spinorbit coupling can arise in photons and, if so, what sort of perspectives this provides. Optical spinorbit coupling can lead to direction-dependent emissions and so may allow quantum optics to be chiral. This Review looks at experiments in the realm of chiral quantum optics and discusses how these demonstrations could add a new dimension of control to quantum networks and quantum many-body physics.

Chiral quantum optics

Microwave photonics with superconducting quantum circuits

Controlling and reversing the effects of loss are major challenges in optical systems. For lasers, losses need to be overcome by a sufficient amount of gain to reach the lasing threshold. In this work, we show how to turn losses into gain by steering the parameters of a system to the vicinity of an exceptional point (EP), which occurs when the eigenvalues and the corresponding eigenstates of a system coalesce. In our system of coupled microresonators, EPs are manifested as the loss-induced suppression and revival of lasing. Below a critical value, adding loss annihilates an existing Raman laser. Beyond this critical threshold, lasing recovers despite the increasing loss, in stark contrast to what would be expected from conventional laser theory. Our results exemplify the counterintuitive features of EPs and present an innovative method for reversing the effect of loss.

http://science.sciencemag.org/content/sci/346/6207/328.full.pdf

Loss-induced suppression and revival of lasing.

A vision has formed in recent years of the components necessary for a large-scale quantum network. Single trapped atoms can serve as the nodes of this network, with the links established by flying photons that are coupled to the atoms using optical resonators. This review describes progress towards the goal of multinode networks using the current generation of experiments, which have achieved unprecedented levels of atomic qubit control and light-matter coupling efficiencies.

Cavity-based quantum networks with single atoms and optical photons

Light is often described as a fully transverse-polarized wave, i.e., with an electric field vector that is orthogonal to the direction of propagation. However, light confined in dielectric structures such as optical waveguides or whispering-gallery-mode microresonators can have a strong longitudinal polarization component. Here, using single $^{85}\mathrm{Rb}$ atoms strongly coupled to a whispering-gallery-mode microresonator, we experimentally and theoretically demonstrate that the presence of this longitudinal polarization fundamentally alters the interaction between light and matter.

Strong Coupling between Single Atoms and Nontransversal Photons

We demonstrate highly efficient switching of optical signals between two optical fibers controlled by a single atom. The key element of our experiment is a whispering-gallery-mode bottle microresonator, which is coupled to a single atom and interfaced by two tapered fiber couplers. This system reaches the strong coupling regime of cavity quantum electrodynamics, leading to a vacuum Rabi splitting in the excitation spectrum. We systematically investigate the switching efficiency of our system, i.e., the probability that the fiber-optical switch redirects the light into the desired output. We obtain a large redirection efficiency reaching a raw fidelity of more than 60% without postselection. Moreover, by measuring the second-order correlation functions of the output fields, we show that our switch exhibits a photon-number-dependent routing capability.

https://physics.aps.org/featured-article-pdf/10.1103/PhysRevLett.111.193601

Fiber-Optical Switch Controlled by a Single Atom

The interaction between single quantum emitters and non-transversally polarized photons for which the electric field vector amplitude has a significant component in the direction of propagation is investigated. Even though this situation seems to be at odds with the description of light as a transverse wave, it regularly occurs when interfacing or manipulating emitters with non-paraxial, guided, or evanescent light. Here, this phenomenon for the case of single atoms that strongly interact with whispering-gallery-mode (WGM) microresonators is quantitatively investigated. These resonators confine light by continuous total internal reflection and offer the advantage of very long photon lifetimes in conjunction with near-lossless in- and out-coupling of light via tapered fiber couplers. In the setup, a novel type of high-Q WGM microresonator - a so-called bottle microresonator, which has the additional advantage of being fully tunable and provides a mode geometry that enables near-lossless simultaneous coupling of two independent tapered fiber couplers, is employed.

Strong coupling between single atoms and non-transversal photons

We demonstrate a fiber-integrated optical Kerr-nonlinearity where a single atom imprints the maximum nonlinear phase shift of pi between the one and two photon case. We employ this to generate entanglement between previously independent photons.

Juergen Volz

Papers

Strong Coupling between Single Atoms and Nontransversal Photons

Fiber-Optical Switch Controlled by a Single Atom

Strong coupling between single atoms and non-transversal photons

Nonlinear pi phase shift for single fiber-guided photons interacting with a resonator-enhanced atom