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.
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.
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.
Semiconductor nanostructures such as quantum dots (QDs) have offered unique opportunities to investigate quantum optical effects in solid-state systems. These include quantum interference, Rabi oscillations, as well as photon antibunching, and were previously observable only in isolated atoms or ions. In addition, QDs can be integrated into optical microcavities, making them attractive for applications in quantum information processing and high efficiency quantum light sources. Despite much progress towards these goals, one area that was little explored is coherent control of such solid-state quantum emitters in cavities. The main technical hurdle lies in overcoming the laser background scattering. By using a sample structure in which QDs are embedded in a planar Fabry-Perot cavity and by using an orthogonal excitation geometry, we have achieved a nearly complete elimination of laser background scattering. This in turn allows us to show resonantly controlled light emission of quantum dots in the cavity including (a) Rabi flopping using pulse control, (b) direct observation of Mollow triplets in the frequency domain, and (c) simultaneously measured first-order and second order photon-photon correlations.
Quantum optics, the study of light at the level of its building blocks, \nhas been one of the frontiers of science. The arena of quantum optics \nhas been fruitful for investigations of a wide range of interesting questions, \nfrom those on the fundamental structures in driven dissipative \nsystems to those on applications of quantum information processing \nand quantum communication. \nThe first part of this thesis is concerned with one of the holy grails \nof quantum optics: an efficient transfer of energy (coupling) between \nsingle photons and matter degrees of freedom. From a certain point \nof view, the problem of efficient coupling is purely geometrical. The \nefficiency of coupling between an atom and light is difficult because \nit is difficult to shape the wavefunction of a single photon such \nthat it overlaps perfectly with the matter degree of freedom (emitter). \nThe first contribution of this thesis is addressing this issue by considering \nan emitter which perfectly matches the single photon plane-wave. \nMoreover, luckily, such an emitter is already realized in nature in the \nform of a two-dimensional crystal. We show that a new generation \nof two-dimensional semiconductors, transition metal dichalcogenide \n(TMD) monolayers, have especially favorable characteristics for allowing \nhighly efficient coupling between photons and matter degrees \nof freedom. \nOne of the most important goals in realizing an efficient coupling \nbetween photons and matter degrees of freedom is that unlike photons, \nmatter excitations interact with each other. Hence, if we would \nlike to have photons which act as if they interact, one possible way \nis to first convert them into matter excitations, and then transfer the \ncorrelations due matter interactions back to photons. As the second \ncontribution of this thesis, we demonstrate that TMD monolayers placed \nwithin a cavity can help us create photons which avoid each other \nas if they interact. The effect can be simply understood as the TMD \nmonolayer acting as a filter for photons, such that it only allows the \npassage of one photon at a time. \nBesides being the fundamental building block of light, photons also \nserve as mediators of interactions between charged particles. In a \nsense, two positive charges repel each other because there is always \na photon which lets them know that they are close to one another. \nOn the other hand, systems of many interacting particles behave in \na completely different way then when they are alone. It may even be \nargued that all physical phenomena at some level emerge from the \nbehavior of many interacting constituents. Given this point of view, \nit is crucial to find ways to control photons in order to control the \ninteractions that they mediate between matter degrees of freedom. \nAs the third contribution of this thesis, we analyze an experimental \nscheme where the control of photonic degrees of freedom can allow \nan experimentalist to modify the strength and range of interactions \nbetween matter. In particular, we propose squeezed photon states as \na resource to control such interactions. \nThe second part of this thesis is concerned with the emergent phenomena \narising in a system of many interacting particles. In particular, \nthe fourth contribution of this thesis considers the combination \nof two fascinating phenomena which emerges such condensed matter \nsystems: superconductivity and geometric effects in lattice band \nstructures. Separately, both superconductivity and the non-trivial geometrical \neffects in lattice systems are well-established subfields of condensed \nmatter physics. Yet only more recently, the combination of \nthese two phenomena were investigated in the context of topological \nsuperconductors, as well as other superconducting states which emerge \non top of band structures with non-trivial geometry. In our study, \nwe attempt at clarifying one of the ways that the non-trivial geometry \nassociated with the lattice band structure may affect the dynamics of \nan emergent superconducting state. Surprisingly, our analysis shows \nthat one of the most robust characteristics of the superconductor, vortices \nenclosing a quantum of magnetic flux, can be modified due to \ngeometrical effects.
Nanotechnologically fabricated structures out of compound semiconductor materials make up the functional parts in LEDs, in semiconductor lasers and in electronics components. The small size of the structures evidently leads to requiring the modeling to look at phenomena at the quantum level. Especially for optoelectronical devices, one needs to treat the interaction between light and semiconductor material. This research adds to the understanding of inter-pretations of light-matter interaction phenomena and makes it possible to find totally new and undiscovered functionalities in semiconductor nanostructures. \nThis thesis presents work on modeling and creates new theoretical foundations for phenom-ena appearing in semiconductor quantum structures. Of special interest has been to generate a theoretical description to such phenomena that have been caused by an optical field and in which one sees changes, e.g., in angular momentum or spatial distribution of excited states. Basically, two kinds of structures are examined, quantum rings and wells. A quantum ring is a toroidal object of which volume is very small, but its diameter can be significant when com-pared to the wavelength of light. Another class of systems under study is a double quantum well structure, where there are two quantum wells on both sides of a tunnel barrier. In this system, the type-I and type-II semiconductor band-structure properties become combined in spatial coordinates, since the electrons and holes can occupy either the same or the separate quantum wells. \nThe two most interesting results are the angle-of-emission dependent photoluminescence from the quantum rings and the coherent control of vertical transport of desired quasiparticles through material interfaces in the quantum-well system in a selective manner. Since the ob-tained results on quantum rings are closely connected to the orbital-angular-momentum coupling between light and matter, they can prove to be important in different quantum infor-mation schemes where this angular-momentum aspect of light has been found to be highly beneficial. In quantum optics, there already are visible trends towards this direction. The predicted coherent control in a double-quantum-well system may mean a large technological progress, since by utilizing this effect one can study transport properties between materials over an interface, one of the most interesting examples of such phenomena being the transfer of quantum correlations through the interface without changing the local densities of electrons.
After they have interacted, quantum particles generally behave as a single nonseparable entangled system. The concept of entanglement plays an essential role in quantum physics. We have performed entanglement experiments with Rydberg atoms and microwave photons in a cavity and tested quantum mechanics in situations of increasing complexity. Entanglement resulted either from a resonant exchange of energy between atoms and the cavity field or from dispersive energy shifts affecting atoms and photons when they were not resonant. With two entangled particles (two atoms or one atom and a photon), we have realized new versions of the Einstein-Podolsky-Rosen situation. The detection of one particle projected the other, at a distance, in a correlated state. This process could be viewed as an elementary measurement, one particle being a ‘‘meter’’ measuring the other. We have performed a ‘‘quantum nondemolition’’ measurement of a single photon, which we detected repeatedly without destroying it. Entanglement is also essential to understand decoherence, the process accounting for the classical appearance of the macroscopic world. A mesoscopic superposition of states (‘‘Schrodinger cat’’) gets rapidly entangled with its environment, losing its quantum coherence. We have prepared a Schrodinger cat made of a few photons and studied the dynamics of its decoherence, in an experiment which constitutes a glimpse at the quantum/classical boundary. We have also investigated entanglement as a resource for the processing of quantum information. By using quantum two-state systems (qubits) instead of classical bits of information, one can perform logical operations exploiting quantum interferences and taking advantage of the properties of entanglement. Manipulating as qubits atoms and photons in a cavity, we have operated a quantum gate and applied it to the generation of a complex three-particle entangled state. We finally discuss the perspectives opened by these experiments for further fundamental studies.