A central goal within quantum optics is to realize efficient, controlled interactions between photons and atomic media. A fundamental limit in nearly all applications based on such systems arises from spontaneous emission, in which photons are absorbed by atoms and then re-scattered into undesired channels. In typical theoretical treatments of atomic ensembles, it is assumed that this re-scattering occurs independently, and at a rate given by a single isolated atom, which in turn gives rise to standard limits of fidelity in applications such as quantum memories for light or photonic quantum gates. However, this assumption can in fact be dramatically violated. In particular, it has long been known that spontaneous emission of a collective atomic excitation can be significantly suppressed through strong interference in emission between atoms. While this concept of "subradiance" is not new, thus far the techniques to exploit the effect have not been well-understood. In this work, we provide a comprehensive treatment of this problem. First, we show that in ordered atomic arrays in free space, subradiant states acquire an elegant interpretation in terms of optical modes that are guided by the array, which only emit due to scattering from the ends of the finite system. We also go beyond the typically studied regime of a single atomic excitation, and elucidate the properties of subradiant states in the many-excitation limit. Finally, we introduce the new concept of "selective radiance." Whereas subradiant states experience a reduced coupling to all optical modes, selectively radiant states are tailored to simultaneously radiate efficiently into a desired channel while scattering into undesired channels is suppressed, thus enabling an enhanced atom-light interface. We show that these states naturally appear in chains of atoms coupled to nanophotonic structures, and we analyze the performance of photon storage exploiting such states. We find numerically that selectively radiant states allow for a photon storage error that scales exponentially better with number of atoms than previously known bounds.PACS numbers: 42.50.Ct, 42.50.Nn || kz and J ⊥ kz , respectively) are given by:

Molecular chemistry offers a unique toolkit to draw inspiration for the design of artificial metamolecules. For a long time, optical circular dichroism has been exclusively the terrain of natural chiral molecules, which exhibit optical activity mainly in the UV spectral range, thus greatly hindering their significance for a broad range of applications. Here we demonstrate that circular dichroism can be generated with artificial plasmonic chiral nanostructures composed of the minimum number of spherical gold nanoparticles required for three-dimensional (3D) chirality. We utilize a rigid addressable DNA origami template to precisely organize four nominally identical gold nanoparticles into a three-dimensional asymmetric tetramer. Because of the chiral structural symmetry and the strong plasmonic resonant coupling between the gold nanoparticles, the 3D plasmonic assemblies undergo different interactions with left and right circularly polarized light, leading to pronounced circular dichroism. Our experimental results agree well with theoretical predictions. The simplicity of our structure geometry and, most importantly, the concept of resorting on biology to produce artificial photonic functionalities open a new pathway to designing smart artificial plasmonic nanostructures for large-scale production of optically active metamaterials.

We introduce a theory to describe the interaction of swift electrons with strong evanescent light fields. This allows us to explain recent experimental results of multiple energy losses and gains for electrons passing near illuminated nanostructures. A complex evolution of the electron state over attosecond time scales is unveiled, giving rise to non-Poissonian distributions of multiphoton features in the electron spectra. Prospects for application to nanoscale-resolved transmission electron microscopy and spectroscopy are discussed.

Tailoring the interactions between quantum emitters and single photons constitutes one of the cornerstones of quantum optics. Coupling a quantum emitter to the band edge of a photonic crystal waveguide (PCW) provides a unique platform for tuning these interactions. In particular, the cross-over from propagating fields E(x) ∝ e ±ikx x outside the bandgap to localized fields E(x) ∝ e −κx jxj within the bandgap should be accompanied by a transition from largely dissipative atom-atom interactions to a regime where dispersive atom-atom interactions are dominant. Here, we experimentally observe this transition by shifting the band edge frequency of the PCW relative to the D 1 line of atomic cesium for N = 3.0 ± 0.5 atoms trapped along the PCW. Our results are the initial demonstration of this paradigm for coherent atomatom interactions with low dissipation into the guided mode.quantum optics | nanophotonics | atomic physics R ecent years have witnessed a spark of interest in combining atoms and other quantum emitters with photonic nanostructures (1). Many efforts have focused on enhancing emission into preferred electromagnetic modes relative to vacuum emission, thereby establishing efficient quantum matter-light interfaces and enabling diverse protocols in quantum information processing (2). Photonic structures developed for this purpose include high-quality cavities (3-7), dielectric fibers (8-13), metallic waveguides (14-16), and superconducting circuits (17-19). Photonic crystal waveguides (PCWs) are of particular interest, because the periodicity of the dielectric structure drastically modifies the field propagation, yielding a set of Bloch bands for the guided modes (GMs) (20). For example, recent experiments have shown superradiant atomic emission because of a reduction in group velocity for an atomic frequency near a band edge of a PCW (21).A quite different paradigm for atom-light interactions in photonic crystals was proposed in the works in refs. 22-25 but has yet to be experimentally explored. In particular, when an atomic transition frequency is situated within a bandgap of a PCW, an atom can no longer emit propagating waves into GMs of the structure. However, an evanescent wave surrounding the atoms can still form, resulting in the formation of atom-photon-bound states (26,27). This phenomenon has attracted new interest recently as a means to realize dispersive interactions between atoms without dissipative decay into GMs. The spatial range of atomatom interactions is tunable for 1D and 2D PCWs and set by the size of the photonic component of the bound state (28, 29). Manybody physics with large spin exchange energies and low dissipation can thereby be realized in a generalization of cavity quantum electrodynamics (CQED) arrays (30,31). Fueled by such perspectives, there have been recent experimental observations with atoms (21, 32, 33) and quantum dots (34, 35) interacting through the GMs of PCWs, albeit in frequency regions outside the bandgap, where GMs are propagating fields.In this manuscript, we re...

The important role played by hot electrons in photocatalysis and light harvesting has attracted great interest in their dynamics and mechanisms of generation. Here, we theoretically study the temporal evolution of optically excited conduction electrons in small plasmon-supporting gold and silver nanoparticles. We describe the electron dynamics through a master equation incorporating transition rates for optical excitations and electron–electron collisions that are calculated using the screened interaction within an independent-electron picture. Upon optical excitation of the particle by a light pulse, a nonthermal electron distribution is produced, which takes 10s fs to thermalize at an elevated electron temperature due to electron–electron collisions and eventually relaxes back to ambient temperature via coupling to phonons and thermal diffusion. Phonons and diffusion are introduced through a phenomenological inelastic attenuation rate. We find the temporal evolution of the electron energy distribution to strongly depend on the total absorbed energy, which is in turn determined by particle size, pulse fluence, and photon energy. Our results provide detailed insight into hot-electron dynamics that can be beneficial for the design of improved photocatalysis and photodetection devices.

Based on a formalism that describes atom-light interactions in terms of the classical electromagnetic Green's function, we study the optical response of atoms and other quantum emitters coupled to one-dimensional photonic structures, such as cavities, waveguides, and photonic crystals. We demonstrate a clear mapping between the transmission spectra and the local Green's function, identifying signatures of dispersive and dissipative interactions between atoms. We also demonstrate the applicability of our analysis to problems involving three-level atoms, such as electromagnetically induced transparency. Finally we examine recent experiments, and anticipate future observations of atom-atom interactions in photonic bandgaps.

No abstract

The properties of coupled emitters can differ dramatically from those of their individual constituents. Canonical examples include sub-and super-radiance, wherein the decay rate of a collective excitation is reduced or enhanced due to correlated interactions with the environment. Here, we systematically study the properties of collective excitations for regularly spaced arrays of quantum emitters coupled to a one-dimensional waveguide. We find that, for low excitation numbers, the modal properties are well-characterized by spin waves with a definite wavevector. Moreover, the decay rate of the most subradiant modes obeys a universal scaling with a cubic suppression in the number of emitters. Multiexcitation subradiant eigenstates can be built from fermionic combinations of single excitation eigenstates; such 'fermionization' results in multiple excitations that spatially repel one another. We put forward a method to efficiently create and measure such subradiant states, which can be realized with superconducting qubits. These measurement protocols probe both real-space correlations (using on-site dispersive readout) and temporal correlations in the emitted field (using photon correlation techniques).