We investigate qubit-qubit entanglement mediated by plasmons supported by one-dimensional waveguides. We explore both the situation of spontaneous formation of entanglement from an unentangled state and the emergence of driven steady-state entanglement under continuous pumping. In both cases, we show that large values for the concurrence are attainable for qubit-qubit distances larger than the operating wavelength by using plasmonic waveguides that are currently available.

We propose the use of photonic crystal structures to design subwavelength optical lattices in two dimensions for ultracold atoms by using both Guided Modes and Casimir-Polder forces. We further show how to use Guided Modes for photon-induced large and strongly long-range interactions between trapped atoms. Finally, we analyze the prospects of this scheme to implement spin models for quantum simulation.

This Colloquium describes a new paradigm for creating strong quantum interactions of light and matter by way of single atoms and photons in nanoscopic lattices. Beyond the possibilities for quantitative improvements for familiar phenomena in atomic physics and quantum optics, there is a growing research community that is exploring novel quantum phases and phenomena that arise from atom-photon interactions in one-and two-dimensional nanophotonic lattices. Nanophotonic structures offer the intriguing possibility to control atom-photon interactions by engineering the medium properties through which they interact. An important aspect of these new research lines is that they have become possible only by pushing the state-of-the-art capabilities in nanophotonic device fabrication and by the integration of these capabilities into the realm of ultracold atoms. This Colloquium attempts to inform a broad physics community of the emerging opportunities in this new field on both theoretical and experimental fronts. The research is inherently multidisciplinary, spanning the fields of nanophotonics, atomic physics, quantum optics, and condensed matter physics.

Plasmon-enhanced Raman scattering can push single-molecule vibrational spectroscopy beyond a regime addressable by classical electrodynamics. We employ a quantum electrodynamics (QED) description of the coherent interaction of plasmons and molecular vibrations that reveal the emergence of nonlinearities in the inelastic response of the system. For realistic situations, we predict the onset of phonon-stimulated Raman scattering and a counterintuitive dependence of the anti-Stokes emission on the frequency of excitation. We further show that this QED framework opens a venue to analyze the correlations of photons emitted from a plasmonic cavity.Surface Enhanced Raman Scattering (SERS) is a spectroscopic technique in which the inelastic scattering from a molecule is increased by placing it in a hotspot of a plasmonic cavity, where the electric fields associated with the incident and the scattered photons are strongly enhanced (see the schematic in Fig. 1(a)). 1 The difference between the energy of those two photons provides a fingerprint of the molecule, i.e., detailed chemical information about its vibrational structure. Since the initial observation of Raman scattering from single molecules, 2,3 the use of a variety of plasmonic structures that act as effective optical nanoantennas over the last decades has allowed a tremendous advance of this molecular spectroscopy 4 . Metallic particles such as nanoshells 5,6 , nanorings 7 , nanorods 8 , nanowires 9 , or nano stars 10 , as well as plasmonic nanogap structures formed in particle dimers [11][12][13] , nanoparticle-on-a-mirror morphologies [14][15][16][17] , or nanoclusters 18 , are among the variety of structures that offer huge and controllable enhancements of the field intensity in their hotspots, boosting the inherently weak Raman scattering intensity, 1,19 and ultimately enabling the chemical identification and imaging of particular vibrational modes of a molecule with subnanometer resolution. 20 These results suggest that some experiments might have reached the regime where the quantum-mechanical nature of both the molecular vibrations and the plasmonic cavity emerges, 21 and call for an adequate theoretical description that goes beyond the classical treatment of the electric fields produced in plasmonic cavities. 1,22,23 In this work we address the underlying quantummechanical nature of Raman scattering processes by quantizing as bosonic excitations both the vibrations of the molecule and the electromagnetic field of a plasmonic cavity. The description of the vibrations through bosonic operators can be justified by considering the harmonic approximation to the energy landscape of the molecule along a generalized atomic coordinate ( Fig. 1(b)), such as the length of a molecular bond, e.g., C=O. 21,22 These vibrations interact with the cavity photons through a nonlinear Hamiltonian, reminiscent of that found in optomechanical systems. 24 In this description, the large enhancement of the Raman scattering from a molecule in the plasmonic cavity occurs thanks to ...

Here we present the theoretical foundation of the strong coupling phenomenon between quantum emitters and propagating surface plasmons observed in two-dimensional metal surfaces. For that purpose, we develop a quantum framework that accounts for the coherent coupling between emitters and surface plasmons and incorporates the presence of dissipation and dephasing. Our formalism is able to reveal the key physical mechanisms that explain the reported phenomenology and also determine the physical parameters that optimize the strong coupling. A discussion regarding the classical or quantum nature of this phenomenon is also presented. DOI: 10.1103/PhysRevLett.110.126801 PACS numbers: 73.20.Mf, 42.50.Nn, 71.36.+c Surface plasmon polaritons (SPPs), hybrid bound modes comprising both electromagnetic fields and charge currents, are well known to have both a subwavelength confinement and propagation lengths of tens or even hundreds of wavelengths [1,2]. For this reason, the interaction between quantum emitters (QEs) and SPPs has attracted great interest recently [3][4][5]. It has been shown that QE-SPP coupling can lead to single SPP generation [6][7][8] and that the interaction between two QEs can be mediated by SPPs, resulting in energy transfer, superradiance [9], and entanglement phenomena [10][11][12]. Recently, there have also been several experimental studies that show the emergence of strong coupling (SC), i.e., coherent energy exchange between propagating SPPs and excitons either in organic molecules [13][14][15][16][17][18] or in quantum dots [19][20][21]. However, to our knowledge, a first-principles explanation of these experimental results has not been presented yet.In this Letter, we analyze the phenomenon of SC between quantum emitters (or absorbers) and SPPs and present its theoretical foundation. We develop a complete quantum treatment that is not only able to calculate absorption spectra and reproduce the experimental phenomenology but also deal with more complex aspects such as photon statistics.In Fig. 1(a) we render a sketch of the general structure that mimics the experimental configuration: a collection of N QEs immersed into a layer of thickness W and placed on top of a thin metal film (thickness h). In this work, the acronym QE will refer to a quantum system with discrete electronic levels, like organic molecules or quantum dots. In some of the experimental setups and in order to avoid quenching of the QEs, a dielectric spacer of width s is located between the QEs and the metal substrate. We will take 2 ¼ 1 ¼ 1 in our calculations, and we will use the dielectric function of the metal (silver) m as tabulated in Ref. [22]. As a minor simplification, we will assume a semi-infinite metal substrate instead of the metal film considered in the experiments (these films are thick enough for the SPPs to be very similar to those of a single interface). Each QE is represented by a two-level system (2LS) fjgi; jeig and characterized by a transition frequency ! 0 (in this Letter we will use ! 0 ¼ 2 eV, @ ¼ 1...

The interaction of quantum emitters with structured baths modifies both their individual and collective dynamics. In Ref.[1] we show how exotic quantum dynamics emerge when QEs are spectrally tuned around the middle of the band of a two-dimensional structured reservoir, where we predict the failure of perturbative treatments, anisotropic non-markovian interactions and novel super and subradiant behaviour. In this work, we provide further analysis of that situation, together with a complete analysis for the quantum emitter dynamics in spectral regions different from the center of the band.

We study the generation of entanglement between two distant qubits mediated by the surface plasmons of a metallic waveguide. We show that a V-shaped channel milled in a flat metallic surface is much more efficient for this purpose than a metallic cylinder. The role of the misalignments of the dipole moments of the qubits, an aspect of great importance for experimental implementations, is also studied. A careful analysis of the quantum dynamics of the system by means of a master equation shows that two-qubit entanglement generation is essentially due to the dissipative part of the effective qubit-qubit coupling provided by the surface plasmons. The influence of a coherent external pumping, needed to achieve a steady-state entanglement, is discussed. Finally, we pay attention to the question of how to get information experimentally on the degree of entanglement achieved in the system.

The discovery of topological materials has motivated recent developments to export topological concepts into photonics to make light behave in exotic ways. Here, we predict several unconventional quantum optical phenomena that occur when quantum emitters interact with a topological waveguide quantum electrodynamics bath, namely, the photonic analog of the Su-Schrieffer-Heeger model. When the emitters’ frequency lies within the topological bandgap, a chiral bound state emerges, which is located on just one side (right or left) of the emitter. In the presence of several emitters, this bound state mediates topological, tunable interactions between them, which can give rise to exotic many-body phases such as double Néel ordered states. Furthermore, when the emitters’ optical transition is resonant with the bands, we find unconventional scattering properties and different super/subradiant states depending on the band topology. Last, we propose several implementations where these phenomena can be observed with state-of-the-art technology.