Bus bunching occurs when two or more buses travel head to tail. It is an annoying problem in public transportation because it increases passengers' average waiting time and traveling time, wastes bus capacity, reduces the frequency of bus service and increases the pressure on bus drivers. So eliminating bus bunching is important in public transportation. Eliminating bus bunching is highly challenging due to the complexity and variability of the bus dynamics. Bus bunching results from a positive feedback mechanism of headway evolution, which is a flaw born with the bus system. In this thesis, we quantify the intensity of the tendency to bus bunching and propose a headway control modeling framework to reverse tendency. Our framework subsumes many headway control schemes to coordinate buses and so enables batch analysis. Given different headway information, our framework produces different control schemes under which headways self-equalize. The stability of the bus system under control is characterized by a single measure and it can be optimized. Besides, the bus system under control is robust against traffic conditions and the level of ridership. The framework is based on a snapshot model capturing the bus dynamics including the tendency to bunch by taking traffic conditions and the level of ridership into account. It is linear and time-invariant, which makes the bus dynamics tractable. This model considers a single control point and constant bus velocity in a deterministic manner, but it can be extended to handle many control points, inhomogeneous velocity along the route, and randomness. Using our framework, we further study two simple control schemes---Threshold control and ``Prefol". Threshold control drives headways to self-equalize the fastest but the corresponding bus system needs large slack time for robustness. "Prefol" needs small slack time but headways self-equalize slower. We hybridize them and find the hybrid control scheme balances robustness and fast headway equalization. We also show that it outperforms several state-of-the-art control schemes in tests on a simulated bus route in Chicago.

This study investigates the main factors of production, their interaction and influence on cycle time efficiency for shovel-truck systems on surface mines. The main factors are truck payload, cycle time and operator proficiency. It is now routine that shoveltruck cycles are analysed using simulation methods. The Elbrond, FPC, Talpac, Arena and Machine Repair simulation models are discussed to explain how their model characteristics contribute to the differences in their reported cycle efficiency as indicated by productivity results. The Machine Repair Model based on Markov chains is adapted for shovel-truck systems and examined for calculating shovel-truck cycle times. The various probability distributions that can be use to model particular cycle time variables and some methods in selecting the “best” fit are examined. Truck cycle time variable sensitivity is examined by using the Excel® add-on program @Risk (Palisade Corp.) in determining their respective weighting or contribution within the total cycle time variability. The analysis of cycle efficiency leads ultimately to sizing of a shovel-truck system. When determining a fleet size for a particular surface operation the planning engineers will tend to use one and to a lesser extent perhaps two separate simulation models. This study calculates the productivity (tonnes per hour) for a “virtual mine” with a variable number of trucks, variable cycle distances and variable truck loading times. The study also includes a separate analysis of cycle time variables and their probability distributions for the Orapa diamond mine in Botswana, to show possible distributions for various cycle variables. The study concludes with a calculation of the truck fleet size using the Elbrond, FPC, Talpac and Arena and Machine Repair models for the Optimum Colliery coal mine and then compares the results and their correlation. The main findings are that the calculation of waiting time is different for the various models, each model yields a unique fleet sizing solution and any solution in effect represents a range of results.

The major difference between semiconductor quantum dots and bulk semiconductors is in the quantum confinement effect. It results the controllable exciton¡¦s absorption and emission spectra by tuning the size of the quantum dot. Moreover, multi-exciton states are reported to be observed in the highly symmetric quantum dot systems. In this dissertation, we use the single molecule fluorescence measurement to study the power dependence of multi-exciton state in single CdSe/ZnS semiconductor quantum dots. At low excitation fluence, anti-bunching behavior, and nearly single exponential relaxation dynamics are observed. By increasing the laser power, bi-exponential fluorescence decay dynamics as well as bunching behaviors from the same QD indicate the fast PL dynamics due to the relaxation from multi-exciton. The results indicate certain threshold energy level for multi-exciton generation. In addition, the multiple step cascade radiative relaxation processes are observed. Besides, we modulate linear polarization light to study the excitation orientation dependence. The results indicate the emission dipole of multi-exciton is similar to the single exciton, having a two dimensional transition dipole plane with c-axis symmetry. However, the absorption dipole of multi-exciton exhibits different orientation dependence from the single exciton.

We have constructed a single molecule detection system with the capability to simultaneously measure many parameters, including transient fluorescence intensity, fluorescence lifetime, and photon anti-bunching behavior via the Hanbury Brown-Twiss photon-correlation technique. In addition, we apply the system to study the single DiI (1, 1 '- dioctadecyl- 3, 3 , 3 ', 3 ' - tetramethylindocarbocyanine perchlorate) molecule, to characterize the photo-physical behaviors. Cyanine dyes are the molecules that constitute of two nitrogen centers, one of which is positive charged, and is linked by a conjugated chain with odd number of carbon atoms to the other nitrogen center. Cyanine dyes are interested in the photo sensitization, optical recording media, nonlinear optics, laser dyes, and many interesting photophysical and photochemical behaviors. Among them, DiI plays an important role in single molecule fluorescence investigations. The high photo-stability, good QE, and low inter-system crossing rates, make it a pioneer for the widely investigations in single molecule studies. Our experimental goal is to understand the characteristic of the monitored single molecule by the measuring photo-physical parameters. Our results include the typical behaviors in DiI molecules: clear on-off blinking, fluorescence anti-bunching, one-step photo-bleaching, and consistent fluorescence polarization orientation. In addition, we also observed some change during measurement, which indicates the corresponding change of structure. Few molecules also exhibit non-zero probability around the zero delay time, which indicates the simultaneous existence of more than one quantum emitters in the detected region. These results demonstrate that the parameters are essential for understanding and characterizing the observed molecules in single molecule level.

O aumento da mobilidade das pessoas é um desafio para a sociedade atual, o que leva a uma preocupação constante em busca de soluções adequadas. Um sistema de transporte público eficiente e confiável proporciona melhor mobilidade às pessoas, além de auxiliar na redução dos congestionamentos e nas emissões de poluentes nas áreas urbanas. O estudo do headway, ou o tempo de passagem entre dois veículos consecutivos de uma linha, é uma medida útil para avaliar o desempenho do sistema de ônibus, uma vez que perturbações durante a operação de uma linha podem gerar comboios (bunching), degradando rapidamente a operação. Este trabalho teve como objetivo apresentar a regulação operacional de linhas de transporte público urbano por meio do controle do headway, empregando ações corretivas como estratégia de controle a partir da identificação das anormalidades que perturbam a operação e atenuando seus impactos sobre o funcionamento da linha. A regulação operacional pelo controle do headway propiciou uma homogeneidade dos mesmos, redução de comboios, melhoria na distribuição de passageiros por viagem, assim como ganhos na dirigibilidade e segurança dos motoristas. / Increased mobility of people is a challenge for modern society, it leads to a constant concern in seeking appropriate solutions. An efficient and reliable public transport provides better mobility people and help in reducing congestion and emissions of pollutants in urban areas. Headway, or the transit time between two consecutive vehicles in a line, is a useful tool to evaluate the performance of the bus system because of disturbances during operation can generate bunching, quickly degrading the operation. This study aimed to present the operational control of public transport lines by controlling the headway using corrective actions as a control strategy starting the identification of abnormalities that disrupt the operation and mitigating its impact on the operation of the line. The operating regulation by the headway control provided a homogeneity of headways, reduction of bunching, improved distribution of passengers per trip, as well as gains in handling and safety of drivers.

O aumento da mobilidade das pessoas é um desafio para a sociedade atual, o que leva a uma preocupação constante em busca de soluções adequadas. Um sistema de transporte público eficiente e confiável proporciona melhor mobilidade às pessoas, além de auxiliar na redução dos congestionamentos e nas emissões de poluentes nas áreas urbanas. O estudo do headway, ou o tempo de passagem entre dois veículos consecutivos de uma linha, é uma medida útil para avaliar o desempenho do sistema de ônibus, uma vez que perturbações durante a operação de uma linha podem gerar comboios (bunching), degradando rapidamente a operação. Este trabalho teve como objetivo apresentar a regulação operacional de linhas de transporte público urbano por meio do controle do headway, empregando ações corretivas como estratégia de controle a partir da identificação das anormalidades que perturbam a operação e atenuando seus impactos sobre o funcionamento da linha. A regulação operacional pelo controle do headway propiciou uma homogeneidade dos mesmos, redução de comboios, melhoria na distribuição de passageiros por viagem, assim como ganhos na dirigibilidade e segurança dos motoristas. / Increased mobility of people is a challenge for modern society, it leads to a constant concern in seeking appropriate solutions. An efficient and reliable public transport provides better mobility people and help in reducing congestion and emissions of pollutants in urban areas. Headway, or the transit time between two consecutive vehicles in a line, is a useful tool to evaluate the performance of the bus system because of disturbances during operation can generate bunching, quickly degrading the operation. This study aimed to present the operational control of public transport lines by controlling the headway using corrective actions as a control strategy starting the identification of abnormalities that disrupt the operation and mitigating its impact on the operation of the line. The operating regulation by the headway control provided a homogeneity of headways, reduction of bunching, improved distribution of passengers per trip, as well as gains in handling and safety of drivers.

O aumento da mobilidade das pessoas é um desafio para a sociedade atual, o que leva a uma preocupação constante em busca de soluções adequadas. Um sistema de transporte público eficiente e confiável proporciona melhor mobilidade às pessoas, além de auxiliar na redução dos congestionamentos e nas emissões de poluentes nas áreas urbanas. O estudo do headway, ou o tempo de passagem entre dois veículos consecutivos de uma linha, é uma medida útil para avaliar o desempenho do sistema de ônibus, uma vez que perturbações durante a operação de uma linha podem gerar comboios (bunching), degradando rapidamente a operação. Este trabalho teve como objetivo apresentar a regulação operacional de linhas de transporte público urbano por meio do controle do headway, empregando ações corretivas como estratégia de controle a partir da identificação das anormalidades que perturbam a operação e atenuando seus impactos sobre o funcionamento da linha. A regulação operacional pelo controle do headway propiciou uma homogeneidade dos mesmos, redução de comboios, melhoria na distribuição de passageiros por viagem, assim como ganhos na dirigibilidade e segurança dos motoristas. / Increased mobility of people is a challenge for modern society, it leads to a constant concern in seeking appropriate solutions. An efficient and reliable public transport provides better mobility people and help in reducing congestion and emissions of pollutants in urban areas. Headway, or the transit time between two consecutive vehicles in a line, is a useful tool to evaluate the performance of the bus system because of disturbances during operation can generate bunching, quickly degrading the operation. This study aimed to present the operational control of public transport lines by controlling the headway using corrective actions as a control strategy starting the identification of abnormalities that disrupt the operation and mitigating its impact on the operation of the line. The operating regulation by the headway control provided a homogeneity of headways, reduction of bunching, improved distribution of passengers per trip, as well as gains in handling and safety of drivers.

Thesis advisor: Arthur Lewbel / This dissertation consists of two independent studies that seek to improve public policies in developing country context. I first study how anti-poverty programs in developing countries can improve their screening procedure so that they can better direct resources to the poor over time. Then, I investigate the impacts of trade openness on fertility decisions in countries known for son preference, thereby informing governments in those countries of the unintended consequences of such growth-focused trade policies. In the first chapter, “Bunching and Learning in Targeting Poverty: Evidence from Vietnam,” I examine how households manipulate eligibility criteria in order to appear eligible for an anti-poverty program. Despite ample evidence that households manipulate these criteria, little is known about how such behaviors evolve over time in a long-term program. Using data from Vietnam, I find that, early on in each phase of its National Anti-Poverty Program, about 1-2% of the population (or 8-18% relative to the program size) bunch at the official income cutoff in order to appear eligible. However, this fraction falls by 60-100% towards the end of the phase, only to increase yet again when a new phase starts with a new income cutoff. To explain this temporal pattern of bunching, I develop a model in which over time the program staff learn to rely on housing conditions, a less-manipulable criteria, to select households. This refined information, in turns, discourages households from manipulating their income. I find that an increase of 0.5 standard deviation in the housing quality index further reduces the chance of being accepted to the program by 25.11% after two years. Meanwhile, other criteria, including reported income and asset holdings, do not contribute any additional predictive power to the program status over the same period. Without this learning process, the program would have misallocated about 1.7%, or equivalently 32.3-36.4 million USD (PPP), of its budget to non-poor households during the first phase of the program. In the second chapter, “Why does the sex ratio at birth rise? Evidence from Vietnam,” joint with Nghiem Huynh, we investigate the causal link between a major trade agreement between the US and Vietnam and the rise in sex ratio at birth in Vietnam. We test three theories of the rise in sex ratio at birth and find evidence that the fertility mechanism explains the recent increase in Vietnam. Using the 2001 US-Vietnam Bilateral Trade Agreement as an exogenous shock, we show that mothers exposed to larger tariff cuts are more likely to have boys, work more hours and less likely to give birth. These results hold up when we account for other competing mechanisms, including changes in fathers' exposure to the policy and daughters’ economic returns in repeated cross-sectional and panel data. This chapter highlights the trade-off between work and children for mothers, and the potential role of trade policy in heightening this trade-off, leading to lower fertility and higher sex selection. Although both studies are situated in Vietnam, their results and implications are relevant to policy discussions in many developing countries. / Thesis (PhD) — Boston College, 2023. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Economics.

Dans le premier chapitre de la thèse, nous étudions la possibilité d’améliorer le couplage opto- mechanique photon-phonon entre le mode de résonance d’une cavité Fabry-Pérot de haute finesse et les vibrations mécaniques des éléments diélectriques (membranes) à l’intérieur de la cavité. En introduisant un défaut quadratique dans la disposition des membranes, nous montrons que le deux couplages (linéaire et quadratique) augmentent. Enfin, nous proposons un modèle très simple avec lequel on cherche à simuler un cristal photonique quasipériodique. Dans le deuxième chapitre de cette thèse, nous présentons nos résultats de recherche sur le transport d’excitons à travers une cavité visant à augmenter l’efficacité du transport. Le modèle que l’on étudie est une chaîne unidimensionnelle d’atomes froids comprenant chacun deux niveaux énergétiques. Grâce au couplage entre exciton et photon, ces deux quanta s’hybrident et forment deux branches de polariton à l’intérieur de la cavité. Nous avons observé qu’à résonance avec un des deux modes de polariton, on peut transmettre l’exciton via le mode polaritonique dans un temps très court. En outre, le désordre n’affecte la propagation excitonique que de façon algébrique. Dans le troisième chapitre de cette thèse, nous présentons nos résultats de recherche sur la réalisa- tion d’interactions entre photons grâce à la médiation d’atomes ultrafroids piégés dans un réseaux optique unidimensionnelle et placés à l’intérieur d’une fibre à cristaux photoniques. Nous avons détecté un régime dans lequel on peut réaliser le “bunching” photon-photon.Dans le quatrième et dernière chapitre de cette thèse, nous étendons les résultats du chapitre précédent aux atomes de Rydberg. / In the first chapter of this thesis, we study a quasiperiodic array of dielectric membranes inside a high-finesse Fabry-Pérot cavity. We work within the framework of the transfer matrix formal- ism. We show that, in a transmissive regime, the introduction of a quadratic spatial defect in the membrane positions enhances both the linear and quadratic optomechanical couplings between optical and mechanical degrees of freedom. Finally, we propose a theoretical model to simulate a one-dimensional quasiperiodic photonic crystal. In the second chapter of this thesis, we consider the problem of the transport of an exciton through a one-dimensional chain of two-level systems. We embed the chain of emitters in a transverse optical cavity and we show that, in the strong coupling regime, a ultrafast ballistic transport of the exciton is possible via the polaritonic modes rather than ordinary hopping. Due to the hybrid nature of polaritons, the transport efficiency is particularly robust against disorder and imperfections in the system. In the third chapter of this thesis, we consider an ordered array of cold atoms trapped in an optical lattice inside a hollow-core photonic crystal fiber. We study photon-photon interactions mediated by hard-core repulsion between excitons. We show that, in spite of underlying repulsive interac- tion, photons in the scattering states demonstrate bunching, which can be controlled by tuning the interatomic separation. We interpret this bunching as the result of scattering due to the mismatch of the quantization volumes for excitons and photons, and discuss the dependence of the effect on experimentally relevant parameters. In the fourth chapter of the thesis, we extend the results of the previous chapter to Rydberg atoms.

The context of this study is the increasing need for public transport as issues over high private vehicle usage are becoming increasingly obvious. Public transport services need to compete with private transport to improve patronage, and issues with reliability need to be addressed. Bus bunching affects reliability through disruptions to the scheduled headways. The purpose of this study was to collect and analyse data to compare how travel time and dwell time vary, to explore the variation of key variables, and to better understand the sources of these variations. The Orbiter bus service in Christchurch was used as a case study, as it is particularly vulnerable to bus bunching. The dwell time was found to be more variable than travel time. It appeared the Canterbury earthquake had significantly reduced the average speeds for the Orbiter service. In 1964, Newell and Potts described a basic bus bunching theory, which was used as the basis for an Excel bus bunching model. This model allows input variables to vary stochastically. Random values were generated from four specified distributions derived from manually collected data, allowing variance across all bus platforms and buses. However the complexity resulted in stability and difficulty in achieving convergence, so the model was run in single Monte Carlo simulations. The outputs were realistic and showed a higher degree of bunching behaviour than previous models. The model demonstrated bunching phenomena that had not been observed in previous models, including spontaneously un-pairing, overtaking of buses delayed at platforms, and odd-numbered bunches of three buses. Furthermore, the study identified areas of further research for data collection and model development.