The properties of localised dipole emitters in the form of a quantum dot or a colour centre embedded in a crystal environment can be drastically modified by a change in the composition, size and shape of the environment in which the emitter is embedded. Thanks to recent advances in material deposition techniques and lithography, as well as the advances in detection techniques and optical manipulation, experimental work is now capable of revealing a new range of physical phenomena when the typical dimensions are of the order of an optical dipole transition wavelength and below. These advances have arisen at a time of a heightened research effort devoted to the important goal of identifying a qubit and a suitable environment that forms the basis for a scalable hardware architecture for the practical realisation of quantum information processing. A physical system that we have recently put forward as a candidate for such a purpose involves localised emitters in the form of quantum dots or colour centres embedded in a nanocrystal. This suggestion became more persuasive following the success of experiments which, for the first time, were able to demonstrate quantum cryptography using a nitrogen vacancy in a diamond nanocrystal as a single-photon source. It has, however, been realised that a more versatile scenario could be achieved by making use of the interplay between dielectric cavity confinement and dipole orientation. Besides position dependence the main properties exhibit strong dipole orientational dependece suggesting that the system is a possible candidate as a qubit for a scalable hardward architecture for quantum information processing. Cavity confinement can control processes since it can lead to the enhancement and the complete suppression of the de-excitation process, with further control provided by the manipulation of the dipole orientation by optical means. This article is concerned with the modelling of quantum processes for quantum systems localised in artificially fabricated structures made of high conductivity metals and dielectric cavities. The essential features of cavity field confinement in this context are presented and the effects on de-excitation rates are assessed.
The properties of localised dipole emitters in the form of a quantum dot or a colour centre embedded in a crystal environment can be drastically modified by a change in the composition, size and shape of the environment in which the emitter is embedded. Thanks to recent advances in material deposition techniques and lithography, as well as the advances in detection techniques and optical manipulation, experimental work is now capable of revealing a new range of physical phenomena when the typical dimensions are of the order of an optical dipole transition wavelength and below. These advances have arisen at a time of a heightened research effort devoted to the important goal of identifying a qubit and a suitable environment that forms the basis for a scalable hardware architecture for the practical realisation of quantum information processing. A physical system that we have recently put forward as a candidate for such a purpose involves localised emitters in the form of quantum dots or colour centres embedded in a nanocrystal. This suggestion became more persuasive following the success of experiments which, for the first time, were able to demonstrate quantum cryptography using a nitrogen vacancy in a diamond nanocrystal as a single-photon source. It has, however, been realised that a more versatile scenario could be achieved by making use of the interplay between dielectric cavity confinement and dipole orientation. Besides position dependence the main properties exhibit strong dipole orientational dependece suggesting that the system is a possible candidate as a qubit for a scalable hardward architecture for quantum information processing. Cavity confinement can control processes since it can lead to the enhancement and the complete suppression of the de-excitation process, with further control provided by the manipulation of the dipole orientation by optical means. This article is concerned with the modelling of quantum processes for quantum systems localised in artificially fabricated structures made of high conductivity metals and dielectric cavities. The essential features of cavity field confinement in this context are presented and the effects on de-excitation rates are assessed.
Very recently, the interest for quantum technologies by the scientific community and industry has strongly increased. Different types of implementations have been proposed as a practical implementation for a quantum bit. In particular, quantum photonics is a strong candidate for such applications. We are interested in using single photons and single spins in diamond as a solid state host matrix (nanodiamonds or membranes). Integration of nanosources of light is currently a major bottleneck preventing the realisation of all-photonic chips for quantum technologies and nanophotonics applications. Ideally, one needs optical circuitry, on-chip photodetection and on-chip generation of quantum states of light (single photons, entangled photons…). Our recent work on a new platform for quantum photonics using integrated optics can offer an easier and robust way to create compact quantum circuits that can be on chip and scalable. In this context, the coupling between waveguides and single photon emitters is critical. The goal of our research is to efficiently couple single photon emitters with a new platform made of optical glass waveguides. These waveguides are based on the so-called ion-exchange glass technology and is know in the photonics industry for quite some time but has never been used in the context of quantum technologies. Efficient light-matter interface is of primordial importance in this system. To achieve this goal, several paths are undertaken such as the use of dielectric and plasmonic structuration in order to increase the light interaction with the waveguide or to develop fabrication techniques to insert the emitters directly inside the guide (for nanodiamonds). We will show what is our current state of the art for placing single emitters at the right place on our optical waveguides made of ion-exchange in glass and in particular what can be done to improve our first promising results in order to get near unity coupling between the optical bus and single photon emitters. We will show first results with semiconductor nanocrystals (NCs) but also using nitrogen-vacancy and silicon-vacancy defects in nanodiamond. The very first step in of our approach consists in the design of the structured waveguide using electromagnetic FDTD. We demonstrated that it is possible to achieve more than 90% coupling. In practice, before using coloured centres in diamond, we started working with CdSe/ZnS semiconductor nanocrystals. So far, we use straight waveguides defined to be single modes at the nominal wavelength of the emission line of the nanoemitters. The positioning of nanoemitters is still a challenge to be achieved. We developed an original technique based on photopolymerisation of light where the nanocrystals are grafted into a light sensitive polymer and can be placed at adequate positions.
It is well established that the presence of interfaces separating regions of real space that are occupied by different materials, has given rise to a wealth of new phenomena and a number of significant applications. It is therefore evident that surfaces must feature prominently in the physics of structures at the small scale and the influence of such structures on the properties of quantum systems in their vicinity. This article is concerned with a structure created using two surfaces forming an open cavity, and we concentrate on the right-angle geometry. Although apparently simple, this structure adequately serves to illustrate the essential physics, which turns out to be surprisingly complex when one considers correlations. We discuss how excited quantum emitters localized within this open cavity, and which can be manipulated optically, would discharge the excitation, both when the emitters are in isolation from other similar emitters and when taken in pairs. Quantum correlations of this kind are essential in the context of implementing scalable architectures for quantum information processing and quantum computing. Optical manipulation near surfaces is one possible scenario that has been highlighted in that context.
This thesis is devoted to theoretically study the systems of single quantum dots (QDs) embedded in various photonic nanostructures, in close relation with experimental data. The word single is essential: due to their nanometer size, QDs have mainly been studied as ensembles. It is only recently that the experimental progress, in particular in the domain of QDs growth, has allowed to address single QDs. This helped to observe that the confined states in a QD resemble those of an atom, leading in turn to the macro-atom picture for QDs. It opened the path to the study of cavity quantum electrodynamics (CQED) in the solid state, as the analogous of a system of an atom in an electromagnetic cavity. However, already at the level of the QD (i.e. without a solid state cavity), the macro-atom picture has been proven to be oversimplified. Various effects related to the solid state environment of the QD have been experimentally and theoretically evidenced, like the influence of acoustic and optical phonons. These are channels of decoherence for the otherwise well isolated and confined QD levels. When adding the cavity, these effects are possibly enhanced, which can lead to very large deviations from the CQED ideal picture. Therefore the clear understanding of these effects is of essential importance, especially in view of the application of solid state systems to quantum information science. In Chapter 2, we address the radiation-matter coupling between two QDs in a nanocavity. This field of research combines the high quality of light resonators to the nonlinearities of the semiconductor material. It is promising in view of manipulating and transferring single long-lived excitations, an essential goal for quantum information science. Then, we study the deviations of a QD from the idealized macroatom picture. First, in Chapter 3, we include the influence of acoustic phonons on the light emitted by a QD-cavity system. We show that the QD modified susceptibility leads to the qualitative explanation of the cavity feeding effects, which is one of the most striking deviations of the QD-cavity system from the ideal CQED picture. In addition to phonons are extended two-dimensional states populated by the non resonant pumping laser in the QD's wetting layer (WL). In Chapters 4 and 5 we build a Monte-Carlo model of the excitation-decay mechanism in a QD-cavity system. We use it to describe the influence of WL states on the dynamics of the QD, leading in turn to a non-CQED behavior. This approach is particularly successful in the case of large QD-cavity detuning, where it predicts both time-dependent and spectral characteristics of the emitted light. Finally, in Chapter 6, we discuss our conclusions, emphasizing in particular that the knowledge of the actual solid state environment of the QD leads to a better control over the parameters allowing to reach the CQED regime. We also discuss the possible extensions of this work, in particular in relation to the analysis of the stimulated emission regime.
For future quantum technologies and in particular for quantum architecture systems, one needs to consider scalability and reproducibility in order to be able to handle and add easily as many qubits of information as possible on a given platform. Quantum coherence and quantum information are fragile and thus requires the need for many qubits to be created, to interact and quantum information to be preserved. Currently there are either very good qubits in terms of fidelity but poor scalability or very good scalability but poor fidelity thus requiring more qubits to compensate. Most of the current platforms are in solid-state physics such as superconducting, dopants in silicon, quantum dots in III-V semiconductors or defects in diamond. The main interest for condensed matter systems is the fact that it is potentially scalable as integration is possible in the future, already demonstrated in the ‘classical’ semiconductor industry. Nevertheless, there are some true challenges in controlling and understanding the mechanisms of decoherence, losses and unwanted effects in these ‘dirty’ systems. As quantum technologies require ultimate control of quantum effects such as maintaining quantum coherence, quantum superposition and entanglement, it pushes towards deeper knowledge of underlying material and condensed matter physics. Photons are of particular interest as they are good carriers of quantum information and one aim is to explore a fully integrated photonic quantum circuit which would be a hybrid system made of stationary solid-state qubits (we will call them quantum emitters) coupled together via single photons travelling within a common optical bus that is photonics-ready for quantum communications within a network of quantum nodes. In this presentation, we will present our latest developments and results towards the integration of quantum emitters with photonic structures with nanosize features. In the first part, our platform is described which consists of glass and thus directly compatible with optical fibres. It is based on the technique of exchanged ions within the glass in order to create locally a particular confinement of the light. The next part will concern our latest results on two different quantum emitters. The first one is based on perovskite nanocristals that can be synthetised chemically and giving rise to quantum optics-type of emitters. The second one is based on the so-called silicon-vacancy defect centre in nanodiamonds. These emitters are produced by the high-pressure/high-temperature method and lead to promising results for quantum technologies. Finally, the last part will treat the nanophotonics approaches in order to couple efficiently these emitters with our platform.
On-demand, high repetition rate sources of indistinguishable, polarized single photons are the key component for future photonic quantum technologies (O'Brien et al 2009 Nat. Photonics 3 687–95). Colour centres in diamond offer a promising solution, and the narrow linewidth of the recently identified nickel-based NE8 centre makes it particularly appealing for realizing the transform-limited sources necessary for quantum interference. Here, we report the characterization of dipole orientation and coherence properties of a single NE8 colour centre in a diamond nanocrystal at room temperature. We observe a single-photon coherence time of 0.21 ps and an emission lifetime of 1.5 ns. Combined with an emission wavelength that is ideally suited for applications in existing quantum optical systems, these results show that NE8 is a far more promising source than the more commonly studied nitrogen-vacancy centre and point the way to the realization of a practical diamond colour centre-based single-photon source.
With their ability to process and transfer quantum information, large-scale entanglement-based quantum networks could be at the heart of a new age of quantum information, enabling fundamentally new applications such as distributed quantum computation, quantum communication, and quantum enhanced sensing. Due to their long spin coherence, controllable local qubit registers and optically active spins, color centers in diamond are prime candidates for nodes of such a network, and have enabled some of the most advanced quantum network demonstrations to date. These demonstrations include the distribution of a 3-node GHZ state across a quantum network, entanglement swapping, entanglement distillation, and memory-enhanced quantum communication. To move beyond current proof-of-principle networks, a further increase of entanglement generation rates is crucial. This thesis presents theoretical and experimental work on enhancing the spin-photon interface of color centers in diamond to achieve this goal, making use of the Purcell effect. The discussed work follows two main directions: embedding of color centers in open, tuneable Fabry-Perot micro-cavities, and in all-diamond photonic crystal cavities. First, we describe theoretical and experimental progress towards cavity-enhanced quantum networks based on nitrogen-vacancy (NV) centers in diamond. Due to their first order sensitivity to electric fields, we choose to embed NV centers in microns-thin diamond membranes that can be integrated into open fiber-based micro-cavities. We develop analytical methods to optimize the design of such open cavity systems for maximum Purcell enhancement of embedded color centers. We demonstrate a method to fabricate optically coherent NV centers in microns-thin diamond membranes, and use such structures to demonstrate the resonant excitation and detection of coherent, Purcell enhanced NV emission. A theoretical model in excellent agreement with our results suggests our system can improve entanglement rates between distant NV centers by two orders of magnitude with near-term improvements to the setup. Second, we describe progress towards an efficient spin-photon interface of group-IV color centers in diamond by coupling them to photonic nanostructures, allowing for large-scale integration. Due to their first-order insensitivity to electric fields, group-IV color centers can be brought in close proximity to surfaces (~ 100 nm), allowing for sub-wavelength mode volumes and thus very high Purcell factors. We numerically optimize photonic crystal cavity designs to maximize the Purcell enhancement of embedded emitters, test the robustness of our designs to real-world fabrication imperfections, and devise a method to efficiently interface nanophotonic structures. We then proceed to fabricate all-diamond photonic crystal cavities, making use of a dry etching technique that is selective to different crystallographic directions, and characterize the resulting optical quality factors. Finally, we marry the developed fabrication methods to fabricate microns-sized diamond platelets that can be transferred from a holding frame in a controlled fashion. This capability could be crucial for the realization of hybrid photonic circuits that we expect to be at the heart of future large-scale quantum networks.