The cavity quantum electrodynamics framework, in which an atom in a cavity is coupled to the cavity's optical modes, is being explored as a potential building block for quantum information-processing architectures. However, atom mobility within the cavity can hinder control of the process. The use of artificial atoms, or quantum dots, confined to a high-quality optical cavity is one possible solid-state solution to the dynamics issue. Despite being designed to be identical, though, the dots can differ slightly from one another in size and atomic composition, thereby giving rise to distinct excitation spectra for each dot. This spectral variability is expected to cause a problem for communication between dots in an ensemble. By heating individual quantum dots through the laser excitation of a nearby connected heat pad, Faraon et al. show that the spectra of individual dots can be tweaked by up to 1.8 nm, thus providing the possibility of tuning the excitation spectra of an ensemble of quantum dots. — ISO
The cavity quantum electrodynamics framework, in which an atom in a cavity is coupled to the cavity's optical modes, is being explored as a potential building block for quantum information-processing architectures. However, atom mobility within the cavity can hinder control of the process. The use of artificial atoms, or quantum dots, confined to a high-quality optical cavity is one possible solid-state solution to the dynamics issue. Despite being designed to be identical, though, the dots can differ slightly from one another in size and atomic composition, thereby giving rise to distinct excitation spectra for each dot. This spectral variability is expected to cause a problem for communication between dots in an ensemble. By heating individual quantum dots through the laser excitation of a nearby connected heat pad, Faraon et al. show that the spectra of individual dots can be tweaked by up to 1.8 nm, thus providing the possibility of tuning the excitation spectra of an ensemble of quantum dots. — ISO
The interaction of light with matter is at the heart of quantum optics, which itself enables insight into the fundamental aspects of quantum mechanics. First experimental access to this research field has been realized by coupling atoms to light. Here, the transition between discrete energy states of the atom is associated with the absorption and emission of a photon, a single quantum of the electromagnetic field. As a central aspect of quantum optics, the light-emitter interaction can be significantly enhanced by placing the emitter in optical cavity that is on resonance with the emitter. In recent years, this has led to a rapidly evolving research field known as cavity quantum electrodynamics (CQED). In CQED two different regimes are distinguished: the strong and the weak coupling regimes. In the strong coupling regime, the emitted photon is reflected from the cavity mirrors and eventually reabsorbed by the emitter. In contrast, the weak coupling regime describes the irreversible emission, where the photon leaks out of the cavity before it can be reabsorbed. Both the weak and strong coupling regimes enabled fundamental experiments for a better understanding of quantum optical phenomena. \n \n \n \nCQED grants access to the quantum world and hence offers potentially revolutionizing applications, particularly in the field of quantum information processing. A central aspect for the successful implementation of quantum applications is the system's scalability. Unfortunately, placing atoms deterministically inside a cavity remains technologically elaborate and hence minimizes the prospect of scaling a atom-CQED system. \n \nA possibility to address this issue is to implement CQED in the solid state, where sophisticated fabrication strategies allow miniaturization and scalability of the system. Particularly the development of self-assembled quantum dots (QD) in semiconductors represent a promising route. QDs can be considered as artificial atoms that mimic the atomic two-level system. These structures interact strongly with light and therefore have the potential for replacing atoms in CQED. As a central advantage, QDs are naturally trapped, which greatly simplifies the deterministic incorporation into the cavity. \n \nIn recent years, many efforts have been made to couple self-assembled QDs to microcavities. Generally, the successful implementation of CQED requires a cavity with a high quality factor Q and a low mode volume. In a majority of the approaches, the high Q/small mode volume cavities were monolithically defined around the QD, embedding the QD at a fixed position inside the cavity. Both the weak and strong coupling regimes have been reached with these systems. However, for future applications they suffer from several disadvantages. The fixed position of the QD inside the cavity minimizes the prospects for spectral tunability and spatial positioning the QD inside the cavity. Furthermore, prospects for further increasing of the cavity Q-factor and minimization of the mode volume remain limited in these systems. \n \nIn this thesis the mentioned disadvantages are addressed by developing a fully tunable miniaturized Fabry-Pérot microcavity with low mode volume. The design enables both spatial positioning of the emitter inside the cavity and spectral tunability. Successful coupling of a single QD to the microcavity is demonstrated including the strong coupling regime. Further a new approach to decrease the cavity mode volume is presented, where we demonstrate weak coupling is achieved
Though QD arrays have initially been mostly studied in an attempt to improve semiconductor lasers, much attention has been recently devoted to the revolutionary prospects opened by the unique properties of single QDs. These artificial atoms allow to mimic in solid-state systems some of the quantum optics experiments performed on atoms since the early 80’s. Recent demonstrations of coherent control experiments on QDs illustrate this fruitful analogy [10]. We will focus here our discussion on the impact of QDs on two other fields, solid-state cavity quantum electrodynamics (CQED) and the generation of quantum states of light. A single QD can be used to emit triggered single photon pulses and photon pairs, whereas its narrow emission spectrum permits to implement microcavities very efficiently to tailor its emission properties. By combining both kind of effects, it is possible to control simultaneously the number of emitted photons as well as their quantum state and to build a solid-state singlemode source of single photons (singlemode S4P) as proposed [11] and demonstrated recently by our group [12]. This novel device represents the ultimate step in a field which has been extremely active in the last 15 years, spontaneous emission (SE) control in optoelectronic devices using confined electronic and/or photonic systems [13]. Before reviewing the development of this single mode S4P, based on a single QD in a pillar microcavity, we will briefly discuss its potential applications in the field of quantum information processing and quantum communications (QIPC).
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.
Semiconductor quantum dots have emerged as promising candidates for studying quantum optical phenomena. Observation of photon antibunching and spontaneous emission limited coherence decay rates have demonstrated that quantum dots behave as artificial atoms. Cavity-quantum electrodynamics (QED) phenomena can be investigated using a single quantum dot embedded inside a photonic nanostructure, where both carriers and photons are confined within sub-micron length scales in all three dimensions. Since quantum dot location inside the cavity is fixed by the growth, this system is free of the stringent trapping requirements that limit its atomic counterpart. Possibility of fabricating photonic nanostructures with ultra-small optical-mode volumes and long photon lifetimes enhances the prospects for applications in quantum information processing. 1
Next-generation information technology is expected to rely on discrete two-state quantum systems that can deterministically emit single photons. Quantum dots are mesoscopic (~10,000 atoms large) semiconductor islands grown in a host crystal of larger band-gap that make well-defined two-level quantum systems and are very attractive due to stability, record coherence times, and the possibility of integrating them into larger structures, such as optical microcavities. This work presents experimental progress towards understanding the coherent optical processes that occur in single quantum dots, particularly such phenomena that might be one day utilized for quantum communication applications. High resolution low temperature optical spectroscopy is used in conjunction with first order (amplitude) and second-order (intensity) correlation measurements of the emitted field. A novel technique is introduced that is capable of harvesting the fluorescence of single dots at the same frequency as the laser, previously impossible due to insurmountable scattering. This technique enables the observation, for the first time, of single quantum dot resonance fluorescence, in both the weak and strong excitation regimes, which forms the basis for deterministic generation of single photons. Guided by the rich theoretical description available from quantum optics with atoms we obtain insight into the complex dynamics of this driven system. Quantum dots confined to novel optical microcavities were further investigated using micro photoluminescence. An optical microcavity properly coupled to a two-level system can profoundly modify its emission characteristics via quantum electrodynamical effects, which are highly attractive for single photon sources. The all-epitaxial structures we probe are distinguished by a bulk morphology that overcomes the fragility problems of existing approaches, and provides high quality factors as well as small mode volumes. Lasing is obtained with larger strucutres. Additionally, isolation of individual dots is further realized in smaller cavities and the Purcell effect observed in time-resolved photon counting experiments.
Semiconductor quantum dots, also referred to as artificial atoms, provide a new paradigm for studying quantum optical phenomena. The effects of cavity-quantum electrodynamics (QED) can be investigated by use of a single quantum dot embedded in a photonic nanostructure. Since quantum dot location inside the cavity is fixed by growth, this system is free of the stringent trapping requirements that limit its atomic counterpart. Fabricating photonic nanostructures with ultrasmall cavity-mode volumes enhances the prospects for applications in quantum information processing.