We study how the radiative properties of a dense ensemble of atoms can be modified when they are placed near or between metallic or dielectric surfaces. If the average separation between the atoms is comparable or smaller than the wavelength of the scattered photons, the coupling to the radiation field induces long-range coherent interactions based on the interatomic exchange of virtual photons. Moreover, the incoherent scattering of photons back to the electromagnetic field is known to be a many-body process, characterized by the appearance of superradiant and subradiant emission modes. By changing the radiation field properties, in this case by considering a layered medium where the atoms are near metallic or dielectric surfaces, these scattering properties can be dramatically modified. We perform a detailed study of these effects, with focus on experimentally relevant parameter regimes. We finish with a specific application in the context of quantum information storage, where the presence of a nearby surface is shown to increase the storage time of an atomic excitation that is transported across a one-dimensional chain.
We study how the radiative properties of a dense ensemble of atoms can be modified when they are placed near or between metallic or dielectric surfaces. If the average separation between the atoms is comparable or smaller than the wavelength of the scattered photons, the coupling to the radiation field induces long-range coherent interactions based on the interatomic exchange of virtual photons. Moreover, the incoherent scattering of photons back to the electromagnetic field is known to be a many-body process, characterized by the appearance of superradiant and subradiant emission modes. By changing the radiation field properties, in this case by considering a layered medium where the atoms are near metallic or dielectric surfaces, these scattering properties can be dramatically modified. We perform a detailed study of these effects, with focus on experimentally relevant parameter regimes. We finish with a specific application in the context of quantum information storage, where the presence of a nearby surface is shown to increase the storage time of an atomic excitation that is transported across a one-dimensional chain.
An ensemble of atoms coupled to the radiation field exhibits collective behaviours when the interatomic separation is comparable to the wavelength of photons emitted by the atoms. The collective coupling of the atoms with the electromagnetic field gives rise to coherent exchange interactions between the atoms. In addition, the spontaneous decay of photons from the ensemble becomes a collective process that can happen at a much faster (superradiant) or slower (subradiant) rate than that of a single atom. These processes affect the way that fluorescence light is scattered from the system when it is driven by a laser. However, little is known about the response of a dense atomic gas beyond the linear regime of very weak driving, due to the involved complexity of the problem. The first part of this thesis investigates how the presence of strong laser driving affects properties of the far-field fluorescence light emitted by the ensemble, such as the power spectrum and second order coherence. Depending on the strength of the driving laser and the particular arrangement of the atoms, the laser can modify considerably how the collective behaviours of the atomic ensemble are manifested in the fluorescence. The relative strengths of the coherent interaction and collective decay process are intrinsically linked to the way that the atoms couple to the radiation field; one cannot be changed without modifying the other. Altering the environment by, for example, placing the atoms near a surface or a waveguide, can change this relation significantly from that for atoms in free space. Therefore, modified environments represent a platform for the generation of otherwise inaccessible regimes of collective behaviour, with potential applications in quantum state engineering. The second part of this thesis provides a numerical investigation into how the collective properties of a dense atomic system can be manipulated by placing the atoms close to planar surfaces or an optical nanofiber. These structures modify the light-matter coupling by altering the electric field experienced by the atoms. It is shown that surfaces of metallic and dielectric materials can be used to either enhance or suppress the collective behaviours, depending on the particular system arrangement. In particular, the coupling of atoms with bound surface modes, such as the surface plasmon polaritons that are formed at an air-metal interface, is demonstrated to have a strong effect on the interactions. An optical nanofiber supports a set of fundamental guided modes, that propagate along the fiber. If the atoms are suitably polarised, then they will emit primarily into the guided modes travelling in one direction. By calculating and analysing the collective decay states of a chain of atoms placed near a nanofiber, it is demonstrated that the atom-fiber coupling leads to the formation of a superradiant mode that is preserved at large atomic separations. The presence of chiral emission is shown to introduce phase gradients to the collective modes.
Surface plasmon polaritons are charge density waves of electrons in a metal, which are coupled to an electromagnetic field at a metal-dielectric interface. The electro- magnetic field associated with surface plasmons is confined, resulting in increased field intensities near the interface. Optical emitters placed near a metal film interact strongly with the surface plasmon modes present, not only modifying the radiation rates, but also the radiation profile. These emitters may be used to store and process information, and understanding their behaviour in the presence of a lossy metal film is important in determining the advantages and limitations of using surface plasmons. In this thesis, we wish to understand how we can optically control atoms by exploring two physical phenomena: first, the influence of surface plasmons on collective effects in an ensemble of emitters. Secondly, the single photon excitation of an emitter near a metal film. In the first case, we analyze the collective radiative behaviour of multiple classical emitters near a metal film that radiatively couple to the far-field through surface plasmon modes. We demonstrate that the contributions of dipolar image charges within the metal film significantly affect the cooperative emission of nearby sources, generating sub- and super-radiant emission that can be controlled with a suitably detuned external driving field. In the second case, we determine the excitation probability of an atom near a metal film by a single photon pulse that is time reversed. Using a quantum electro- dynamic description of absorbing dielectrics, we show that non-radiative decay will limit the excitation probability to about half of the maximum that could be achieved in the absence of losses.
The realization and control of collective quantum effects so far have predominantly focused on cold atomic ensembles. Quantum photonic platforms, with their engineered Green's functions and integration capability of advanced solid-state quantum emitters, provide opportunities to explore regimes of light-matter interaction beyond the scope of atomic systems. In this work, we demonstrate that embedding quantum emitters within a thin dielectric layer fundamentally alters their collective radiative behavior. The optical modes in the dielectric layer mediate long-range dipole-dipole interactions between emitters, enabling both total and directional superradiance between emitters separated by several wavelengths. Crucially, this mechanism supports Dicke superradiance even in parameter regimes where standard settings fail to support an interaction, unveiling a dimensionality-driven enhancement of cooperative effects. By bridging many-body quantum optics and photonic engineering, our work reveals a distinct interplay between surrounding dimensionality and collective quantum dynamics. Experimental realization of these predictions, readily achievable in solid-state quantum optics platforms, paves the way for scalable, directional quantum light sources and frontiers in many-body quantum optics.
Over the last century, quantum mechanics has dramatically altered our understanding of light and matter. Impressively, exploring the relationship between the two continues to provide important insights into the physics of many-body systems. In this thesis, we add to this still growing field of study. Specifically, we discuss superradiant line-broadening and cooperative dipole-dipole interactions for cold atom clouds in the linear-optics regime. We then discuss how coherent radiation changes both the photon scattering properties and the excitation distribution of atomic arrays. After that, we explore the nature of superradiance in in intially inverted clouds of multi-level atoms. Finally, we explore the physics of clouds with degenerate Zeeman ground states, and show that this creates quantum effects that fundamentally change the photon scattering of atomic ensembles.
We study the collective radiation properties of cold, trapped ensembles of atoms. We consider the high-density regime with the mean interatomic distance being comparable to, or smaller than, the wavelength of the resonant optical radiation emitted by the atoms. We find that the emission rate of a photon from an excited atomic ensemble is strongly enhanced for an elongated cloud. We analyze collective single-excitation eigenstates of the atomic ensemble and find that the absorption-emission spectrum is broadened and shifted to lower frequencies as compared to the noninteracting (low-density) or single-atom spectrum. We also analyze the spatial and temporal profile of the emitted radiation. Finally, we explore how to efficiently excite the collective superradiant states of the atomic ensemble from a long-lived storage state in order to implement matter-light interfaces for quantum computation and communication applications.
The main objective of this research is to study a particular case of light-matter interaction: the scattering of light in dense atomic samples.The regime of high atomic densities is represented by samples where the distance between neighboring atoms is on the order of the wavelength associated with the atomic transition.In this regime, short-range atomic interactions cannot be neglected, and in these systems, we expect to observe significant modifications in collective effects such as subradiance or superradiance.Overall, the propagation of light through this type of system is not well-known to the scientific community, and the existing models work to describe very specific situations.To pave the way for achieving our overall goal, this work proposes an experimental setup to obtain a dense cloud of 88 Sr, which is the chemical species we work with in our laboratory.For this purpose, atoms previously trapped using a Magneto-Optical Trap are transferred to a Crossed Optical Dipole Trap.During the optimization and characterization of the optical trap, problems were encountered in producing an image with quantitative information of the atomic ensemble and in transferring a large number of atoms in a very small trapping volume.These problems were resolved with the implementation of a Phase Contrast Imaging system and the implementation of an Optical Molasses as an additional cooling step.Under these conditions, we obtained a cloud that can be suitable for conducting studies on collective effects, transitioning from the dilute to the dense regime.The initial experiments, once our atomic ensemble was prepared, involve measuring the coherent transmission of a low-intensity beam through the cloud.For this purpose, incident light near the resonance of a dipolar transition of the type J = 0 ↔ J = 1 of 88 Sr is used, interacting with a homogeneous region of the sample.Simulations of this type of process can be performed using the Coupled Dipole Model, which describes the overall response of light scattering by the sample considering dipole-dipole interactions mediated by light.With this work, we aim to advance the understanding of density effects by comparing the coherent optical response of the dense cloud with the theory based on the Coupled Dipole Model.