Energy Transfer And Transition In Quantum Devices

Quantum devices are the development trend of future society.Similar to the digital-revolution driven by the industrialization of semiconductor devices,penetrating impacts to every aspects of human society are also expectable from the developments of the quantum devices.Comparing to the classical devices,the inherited properties from quantum mechanics bring quantum devices with richer and even astonishing characteristics.And it is those properties,which deviate from the usual understanding of the macroscopic world,that motivate the broadly applicational interests.Undoubtedly,the productivity enabled by the quantum devices and emergent technologies will surpass the one that has been produced from the previous,and even the sum of,several industrial revolutions.This embarking field of quantum technology is gigantic and highly inter-disciplinary,it can require hardly efforts from both the scientific and the engineering societies,offering great opportunities,challenging the most brilliant minds from our era.Quantum device means either a single elementary device with the quantum features,or archi-tecture that built-on individuals through various hybridizations.Because of quantum coherence,the combined systems which contain many or even infinite number of elementary devices offer rich physical phenomena and attractive applications.Many matured technical areas,such as semi-conductor lasering,scanning tunneling electron microscopy,and magnetic resonance imaging had all being inspired from principles that were rooted in quantum mechanics.On the other hand,in those quantum devices the collective excited modes and their quantum interferences lead to novel phenomena appearing in condensed matter systems.For example,the seminal macroscopic quan-tum phenomena of Bose-Einstein condensation as well as superconductivity can be understood as collective excitations under certain conditions,e.g.,temperature and magnetic field.Besides renovating the fundamental knowledge about nature,those macroscopic condensed matter effects also provide consolidated basis for theoretical as well as experimental explorations that devise technologies which benefit mankind.It is also evident that some microscopic quantum phenomena can exist in some life processes.In particular the discovery of quantum coherence in biological systems such as photosynthetic light harvesting antenna of plants,as well as the radical-pairs from cryptochrome for magnetic navigations in avian.Since then,scientists have been building various models to describe those processes,and with many interesting predictions confirmed.Studying those quantum effects in life-science will provide important heuristics,at both structural and dynamical levels,to design quantum devices.In this thesis,starting from the basic model of quantum devices,more realistic models featuring structural and functional complexities were constructed.Through discussions on topological boundary,energy transfer efficiency,transition selection rules,distributions of the absorption and the emissions spectra,a series of methods and paradigms were developed and es-tablished for some fundamental problems in quantum devices.Here,the optimization of energy transfer efficiency is of our mainly concern,which was explored from a geometric viewpoint.In order to be more persuasive for the purpose of researching and applications of quantum de-vices,realistic proposals were also devised and put-forward for structural optimization of particu-lar quantum system.In particular,the specific optimal coupling scheme might be realizable with quantum-dot-based technologies.Another central topic in quantum device is to reveal dynamical rules that governing transi-tions among energy levels,i.e.,the transition selection rules.The most efficient frequency chan-nels that fuelling energy harvest were uncovered through analyzing the absorption and the emis-sion spectra,using the concept of resonance fluorescence.The transition selection rules directly determine the shapes of the absorption and the emission spectra,they follows from the Schrodinger or the master equation which manifests differently depending on the systems.Among those,the collective spin system stands out because of its high-dimensional Hilbert space as well as spatial degrees of freedoms,which has natural advantages for application and manipulation of quantum devices.In this thesis,starting from the most basic collective spin system the explicit mathe-matical contraction process was shown,which enables a mapping from the Bloch sphere to the phase plane of a one-dimensional harmonic oscillator.This mapping provides important insights,from the well-established interpretations of the Franck-Condon principle in harmonic oscillator,for transition selection rules of the spin system.This enables a direct translation of relevant terms from the vibrational degrees of freedoms,such as the concept of vertical transition,to spin sys-tems.We hope those correspondences can provide useful insights for interpretation and design of manipulation or control protocols involving collective spin systems.This thesis is organized into five chapters.The first chapter is the introduction,where a survey of fundamental problems in quantum devices is given.The focus is on artificial light har-vesting and spin-state quantum controls.Background information as well as the established works in the respective fields are also given.In chapter two,the effect of topological boundary condition on energy transfer efficiency is illustrated with specific models in artificial light harvesting.The topological boundary condition and dimerization induce sub-band splitting for the donor ring,and this causes mixing between photon and acceptor to all other collective modes thus complicates the quantum dynamics of light harvesting.Supported by numerical simulation and analytical ana-lyze,dimerization with topological boundary condition can give rise to optimal energy transfer for a wide range of photon frequencies.Experimental realizations of the topological boundary condi-tion are also discussed.In chapter three,starting with a review on the coherent state in harmonic oscillator system as well as the closely related Franck-Condon principle,the principle is extended to collective spin systems by applying with the contraction relations.In chapter four,a system-atical investigation on transition selection rules for Bloch states in collective system is given.A remarkable correspondence between the favorable transitions and classical motion boundaries is revealed.Those results provide a novel viewpoint to interpret the distribution of Franck-Condon factors in collective spin system,and may also shine light on future studies in more complicated models.The summary and future perspectives are given in chapter five.