A cavity quantum electrodynamics (cQED) system consisting of a confined single photon and a single quantum emitter serves as a fundamental block for quantum optics and photonic quantum technologies. The canonical optical mode employed in the conventional cavity quantum electrodynamics features a uniform polarization distribution, leading to the scalar light-matter interaction in most existing experiments. Despite the rapid progress in the generation of structured light with spatially varied polarizations, the structured light-matter interaction, especially at the single quanta level, is highly intriguing yet largely unexplored. Here, we present the structured light-matter interaction at the single-photon level in a semiconductor cavity quantum electrodynamics system. Four distinct structured cavity modes that are spectrally close to each other are constructed in a micropillar cavity. By spatially locating a single epitaxial quantum dot (QD) at the periphery of a semiconductor micropillar cavity and spectrally tuning the QD emission wavelength into the resonances of the structured cavity modes, cavity-enhanced single-photon emissions with spin-locked chiral orbital angular momentum (OAM) and engineerable spin-orbit entanglements are achieved within a single wavelength-scale device. Our work opens an unexplored paradigm of structured quantum light-matter interactions and may further advance chiral quantum optics and high-dimensional photonic quantum technology.
A cavity quantum electrodynamics (cQED) system consisting of a confined single photon and a single quantum emitter serves as a fundamental block for quantum optics and photonic quantum technologies. The canonical optical mode employed in the conventional cavity quantum electrodynamics features a uniform polarization distribution, leading to the scalar light-matter interaction in most existing experiments. Despite the rapid progress in the generation of structured light with spatially varied polarizations, the structured light-matter interaction, especially at the single quanta level, is highly intriguing yet largely unexplored. Here, we present the structured light-matter interaction at the single-photon level in a semiconductor cavity quantum electrodynamics system. Four distinct structured cavity modes that are spectrally close to each other are constructed in a micropillar cavity. By spatially locating a single epitaxial quantum dot (QD) at the periphery of a semiconductor micropillar cavity and spectrally tuning the QD emission wavelength into the resonances of the structured cavity modes, cavity-enhanced single-photon emissions with spin-locked chiral orbital angular momentum (OAM) and engineerable spin-orbit entanglements are achieved within a single wavelength-scale device. Our work opens an unexplored paradigm of structured quantum light-matter interactions and may further advance chiral quantum optics and high-dimensional photonic quantum technology.
In this paper, exciting progress of quantum optics in solid state is reviewed. The focus is on semiconductor microcavities with self-assembled quantum dots embedded in the active layer. Due to enormous progress in semiconductor nanotechnology, such photonic structures have become a model system for the study of quantum optics on a scalable and integrable technology platform with high potential for future applications in quantum information technology. Quantum optical phenomena have become accessible due to 3-D confinement of light and matter on the length scale of their wavelength in state-of-the-art semiconductor micro- and nanostructures. This confinement leads to a quantization of the associated photonic and electronic energy levels and requires a quantum mechanical description of the system in the framework of cavity quantum electrodynamics (cQED). This approach considers the dipole interaction between two-level quantum emitters and discrete photonic states of a microcavity. Within the well-known Jaynes-Cummings model, the dipole interaction is described in terms of a coherent exchange of energy between the emitter and the resonator mode. This coherent interaction in the so-called strong coupling regime of cQED is reflected in a normal mode splitting, the vacuum Rabi splitting, of the involved modes and represents a central feature of quantum optics in solid state. Another important example of quantum optics in semiconductor nanostructures is the generation of nonclassical light in specific quantum devices. Of particular interest is the realization of Fock states which represent states containing a well-defined number of photons. Single-photon sources, for instance, allow for the generation of single photons on demand, which is highly desirable for quantum communication systems. In this context, this review paper will present recent experimental studies of quantum optics in solid state. This paper is meant for readers who would like to become familiar with this topic and for experts being interested in the progress in this field. It will cover a broad range of studies ranging from examples of fundamental light-matter interaction in the quantum limit to devices capable of emitting single photons and entangled photon pairs on demand.
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
Controlling the position of individual quantum dots (QDs) by means of overgrowing a prepatterned substrate is a prospering research topic. In combination with an accurate alignment procedure, single site-controlled quantum dots (SCQDs) can be precisely retrieved after overgrowth and even integrated in optical resonator devices. This is an essential step towards the parallel fabrication of spatial resonant QD-resonator systems that can be used as efficient single photon sources or to study cavity quantum electrodynamics (cQED) effects. In this contribution, cQED-effects of SCQDs into micropillar cavities are demonstrated. In order to demonstrate cQED effects we performed temperature tuning of a single SCQD spectrally located on the high energy side of the fundamental resonator mode with a quality factor of ~1700 and a pillar diameter of 1 mum. It is concluded that the results are promising with respect to the application of site-controlled QDs in quantum light sources.
Cavity quantum electrodynamic (cQED) systems have attracted extensive research interest in the past decades as the main driving force of quantum optics. First experimental demonstrations focused on the enhancement of spontaneous emission and optical nonlinearities related to a single quantum emitter confined in a microcavity. Major technological effort was needed to enter the strong coupling regime of single emitter cQED to study for instance the non-harmonicity of resulting Jaynes-Cummings ladder and to perform quantum non-demolition measurements at the heart of quantum optics. Corresponding milestone experiments paved the way towards the realization of a wide variety of non-classical light sources bringing the concept of photonic quantum networks to a practical level. cQED systems can be realized using atomic, superconducting and semiconductor platforms. In view of future applications in the quantum information technology, the solid state platform of microcavities with embedded quantum emitters is the most attractive one as it promises low-cost mass production as well as interoperability with the integrated electronic circuit technology of today. Therefore, quantum dot (QD)-microcavity systems are very attractive for further investigations in this field. Indeed, even though cQED effects have been studied for many years now, there are still fundamental aspects remaining to be explored. One example is the exploration of a strongly coupled cQED system under coherent optical excitation which is one of the primary goals of this work. The work presented in this thesis aims at a deeper understanding of cQED effects in semiconductor based implementations of this central topic of quantum optics. It includes comprehensive spatially-resolved studies of the QD-micropillar system in the both weak and strong coupling regime. For this purpose an advanced 90 degree excitation and detection scheme, suitable for efficient wavelength-independent driving of the coupled exciton (X) and cavity mode (C) system, was implemented. This flexible and powerful technique enables access to the three-dimensional emission characteristics of QD-micropillars providing important new insight into cQED effects which is the central aspect of the first part of this thesis. The performed studies show for instance a pronounced anticorrelation (correlation) of the directional emission characteristics down to the single emitter level in the weak (strong) coupling regime demonstrating directly their fundamentally different nature. Related investigations of the lasing regime present a straightforward characterization method to demonstrate lasing action in high-Beta microlasers without the need for time consuming studies employing photon statistics. The second part of the thesis addresses another very important yet unexplored fundamental aspect of cQED. The main focus is on coherently-driven strongly-coupled QD-microcavity systems. Of particular interest is the regime where the coherent excitation dresses the X-C polariton at high excitation powers. This regime is found at the crossover from a quantum (Jaynes Cummings) to a semi-classical (Mollow Triplet-like) system and can be observed between the limiting cases of an anharmonic and a harmonic ladder solely by varying the excitation strength. An indispensable condition for the first observation of this transitory regime is driving the system through the matter (X) component, which distinguishes this work from previous studies. Counterintuitively, significant cavity losses with respect to the coupling strength are required to create the highly coherent state of the laser-dressed polariton. Moreover, this work pioneers in resonance fluorescence (RF) studies of strongly-coupled QD-microcavity systems and reveals that strong X-C coupling suppresses the RF substantially. Additionally, injection pulling of a single polariton is observed for the first time in the QD-microcavity system, which links this classical effect of injection locking, which is also found in macroscopic lasers, to the quantum regime. Complementary studies on long range off-resonant X-C coupling, mutual coupling of two QD Xs as well as temperature stability of the coherent coupling regime deepen the fundamental understanding of the strong light-matter interaction in semiconductor systems.
Cavity quantum electrodynamics studies light-matter interactions at single quanta level. Chiral photon-emitter coupling in photonic structures is characterized as unidirectional propagation locked by the local polarization of light. However, to realize the strong interaction of photon-emitter with direction-locked propagation, which will bring novel applications in nonreciprocal quantum devices, has not been reported yet. Here, we propose the coupled photonic crystal and metallic nanoparticle structure, where through strong local field with high helicity, the rate of circularly polarized photons emitting into photonic crystal waveguide is one order larger than that without the nanoparticle and the linewidth of Rabi splitting spectra is about one-tenth of that with only the nanoparticle, both with ∼ 95% photons propagating unidirectionally, which can be utilized in directional quantum light sources.
A single atom in a cavity is the model system of cavity quantum electrodynamics (CQED). The strong coupling regime between the atom and cavity-confined photon corresponds to the reversible exchange of energy between the two modes, and underpins a wide range of CQED phenomena with applications in quantum information science, including for example as quantum logic gates and as sources of entangled states. An important advance was achieved recently when strong coupling between excitons and cavity photons was reported for the first time for localized quantum dots (QDs) in micron-size solid state cavities. This has significance in terms of scalability and integration with other optical devices, and could lead to the emergence of "quantum optics on a chip" technology. However the results presented so far for quantum dots are in the linear regime, corresponding to coupling to the vacuum field (vacuum Rabi splitting); they are not a true QED effect and can equally well be described by classical physics as the coupling between two oscillators. In this paper, we present evidence for a purely quantum phenomenon for the QD/cavity photon system, namely the increase in splitting of the levels when the mean number of photons in the cavity is increased. This corresponds to non-linearities on the single-photon scale: the presence of a single excitation in the cavity changes the level structure, affecting the emission energies for a second photon. Such results are a first step in demonstrating the promise of quantum dots for CQED applications.