The role of dissipation in the emergence of coherent phenomena is ambiguous. On the one hand, the dissipative coupling to an environment is the biggest adversary of coherence, corrupting the quantum properties of a physical state. On the other hand, recent theoretical and experimental results revealed that dissipation itself can be the driving force behind the occurrence of coherent behavior. This Thesis theoretically investigates both aspects of this interplay in mesoscopic solid-state systems.
The role of dissipation in the emergence of coherent phenomena is ambiguous. On the one hand, the dissipative coupling to an environment is the biggest adversary of coherence, corrupting the quantum properties of a physical state. On the other hand, recent theoretical and experimental results revealed that dissipation itself can be the driving force behind the occurrence of coherent behavior. This Thesis theoretically investigates both aspects of this interplay in mesoscopic solid-state systems.
Abstract Recent experimental results point to the existence of coherent quantum phenomena in systems made of a large number of particles, despite the fact that for many-body systems the presence of decoherence is hardly negligible and emerging classicality is expected. This behaviour hinges on collective observables, named quantum fluctuations , that retain a quantum character even in the thermodynamic limit: they provide useful tools for studying properties of many-body systems at the mesoscopic level, in-between the quantum microscopic scale and the classical macroscopic one. We herein present the general theory of quantum fluctuations in mesoscopic systems, and study their dynamics in a quantum open system setting, taking into account the unavoidable effects of dissipation and noise induced by the external environment. As in the case of microscopic systems, decoherence is not always the only dominating effect at the mesoscopic scale: certain types of environment can provide means for entangling collective fluctuations through a purely noisy mechanism.
Coherence is a familiar concept in physics. It is the driving force behind wavelike phenomena such as the diffraction of light. Moreover, wave-particle duality implies that all quantum objects can exhibit coherence, and this quantum coherence is crucial to understanding the behaviour of a plethora of systems. In this paper, which is written at an undergraduate level, we shall briefly introduce what is meant by coherence in a well-known classical setting, before going on to describe its quantum version. We will show that coherence is important in describing the properties of solid-state nanosystems and especially quantum dots. Simple experiments that reveal the coherent nature of matter-and how this leads to some very powerful applications-will be described. Finally, we shall discuss the fragility of coherence and shall introduce a method for describing decoherence in open quantum systems.
Coherence is a familiar concept in physics. It is the driving force behind wavelike phenomena such as the diffraction of light. Moreover, wave–particle duality implies that all quantum objects can exhibit coherence, and this quantum coherence is crucial to understanding the behaviour of a plethora of systems. In this paper, which is written at an undergraduate level, we shall briefly introduce what is meant by coherence in a well-known classical setting, before going on to describe its quantum version. We will show that coherence is important in describing the properties of solid-state nanosystems and especially quantum dots. Simple experiments that reveal the coherent nature of matter—and how this leads to some very powerful applications—will be described. Finally, we shall discuss the fragility of coherence and shall introduce a method for describing decoherence in open quantum systems.
The controlled, coherent manipulation of quantum-mechanical systems is an important challenge in modern science and engineering, with significant applications in quantum information science. Solid-state quantum systems such as electronic spins, nuclear spins, and superconducting islands are among the most promising candidates for realization of quantum bits (qubits). However, in contrast to isolated atomic systems, these solid-state qubits couple to a complex environment which often results in rapid loss of coherence, and, in general, is difficult to understand. Additionally, the strong interactions which make solid-state quantum systems attractive can typically only occur between neighboring systems, leading to difficulties in coupling arbitrary pairs of quantum bits. This thesis presents experimental progress in understanding and controlling the complex environment of a solid-state quantum bit, and theoretical techniques for extending the distance over which certain quantum bits can interact coherently. Coherent manipulation of an individual electron spin associated with a nitrogen-vacancy center in diamond is used to gain insight into its mesoscopic environment. Furthermore, techniques for exploiting coherent interactions between the electron spin and a subset of the environment are developed and demonstrated, leading to controlled
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.
A mesoscopic Josephson junction in a superconducting ring enclosing a magnetostatic flux with dissipation is considered. The dissipation in this system is described as a consequence of its coupling to a reservoir. By solving the master equation it is shown that the state of the junction can evolve into a quantum superposition of two coherent states even when the dissipation is present. It is also shown that squeezing effects can exist in such a system, and the dynamic behavior of the supercurrent shows collapse and revival phenomena. The quantum decoherence is also discussed.