Multiphoton correlations and emitters

A thesis submitted in partial fulfilment of the requirements of the University of Wolverhampton for the degree of Doctor of Philosophy.The characterisation of the quantum states of light and their subsequent realisation is thought to be an indispensable step to bring in quantum technologies to the real world. The emergence of quantum cryptography, quantum security protocols or quantum computers, among others, demand implicitly or explicitly trustworthy tools and components to carry through the research in its first stages. A deterministic or on-demand single-photon source and, more recently, an N-photon emitter, seem to play a crucial role. Nevertheless, even the correct characterisation of the former is still a source of discussion and there exist several criteria to do so. The identification of the latter is, as expected, a challenging task. With the emergence of multiphoton physics, the horizon of quantum light sources is wider. The tools to identify and classify multiphoton emission are still in development. We present the methods to study the dynamics and correlations of some candidate systems that have been proposed, focusing on the analytical solutions through perturbative methods, valid, for instance, for weakly driven or weakly coupled systems. In particular, the frequency-resolved correlations can be exactly obtained in this way. We also consider the effect of detection on the correlations. The noisy apparatus and their finite time resolution can modify the photon statistics. Some photons may be left undetected or misplaced (in time), additional counts may be recorded as well. We revisit the photon counting formula, that was popular in the birth of Quantum Optics, to obtain the counting probabilities in continuously driven (CW) systems and we focus then on the spontaneous emission of N photons. We observe, for probability distributions of CW systems, a clear deviation from Poissonian statistics in both the short and long time regimes. We find how such a behaviour is inherited from the photon correlations. A good starting point to study the bundler—the N-photon emitter—is the spontaneous emission of N photons. The counting probabilities are computed without and with spectral filtering, making emphasis on how the kind of filter affects the detection. Then, the full structure of the bundle is completely captured by the probability functions of the emission time of the individual photons. The results are ultimately compared with the actual bundler, showing qualitative and quantitative agreement. A brief introduction is given to spatial correlation induced by the ensemble statistics. Some clarifying examples reveal how the statistics are manifested depending on the kind of states. On the other hand, a dynamical model introducing a space dependent sensor method is provided for the scattering and how the spatial distribution is modified by the time resolution limitation. Interestingly, the wave packet before and after the scattering get effectively admixed and interfere with itself displaying characteristic fringes. The main objective of this Thesis is to make an exhaustive characterisation of multiphoton emission, starting with the usual treatment in terms of the luminescence spectrum and the second-order photon correlation function g(2), considering mechanisms that can take place in the detection process such as spectral filtering or contamination of the signal due to time jitter and noise. We develop tools to facilitate and speed up the computation of these quantities, either analytically or numerically, within the range of validity of the Born–Markov approximation and highlighting situations in which perturbation theory is applicable. Finally, we go beyond and take into account other statistical quantities such as the waiting time distribution or higher order correlators and eventually compute counting statistics, which results in a good and promising procedure to characterise and subsequently classify multiphoton emission

Two-photon emission in detuned resonance fluorescence

We discuss two-photon correlations from the side peaks that are formed when a two-level system emitter is driven coherently, with a detuning between the driving source and the emitter (quasi-resonance fluorescence). We do so in the context of the theories of frequency-resolved photon correlations and homodyning, showing that their combination leads to a neat picture compatible with perturbative two-photon scattering that was popular in the early days of quantum electrodynamics. This should help to control, enhance and open new regimes of multiphoton emission. We also propose a way to evidence the quantum coherent nature of the process from photoluminescence only, through the observation of a collapse of the symmetry of the lineshape accompanied by a surge of its intensity of emission. We discuss several of our results in the light of recent experimental works.Comment: 17 pages, 4 figures -- added ideal two-photon cascade formula (Eq. 3), additional references, corrected some typos & other minor changes. Submitted to Quantum Sci. Techno

Loss of antibunching

We describe some of the main external mechanisms that lead to a loss of antibunching, i.e., that spoil the character of a given quantum light to deliver its photons separated from each other. Namely, we consider contamination by noise, a time jitter in the photon detection, and the effect of frequency filtering (or detection with finite bandwidth). The formalism to describe time jitter is derived and connected to the already existing one for frequency filtering. The emission from a two-level system under both incoherent and coherent driving is taken as a particular case of special interest. The coherent case is further separated into its vanishing- (Heitler) and high- (Mollow) driving regimes. We provide analytical solutions which, in the case of filtering, reveal an unsuspected structure in the transitions from perfect antibunching to thermal (incoherent case) or uncorrelated (coherent case) emission. The experimental observations of these basic and fundamental transitions would provide additional compelling evidence of the correctness and importance of the theory of frequency-resolved photon correlation

Accuracy requirements and uncertainties in radiotherapy: a report of the International Atomic Energy Agency.

BACKGROUND: Radiotherapy technology continues to advance and the expectation of improved outcomes requires greater accuracy in various radiotherapy steps. Different factors affect the overall accuracy of dose delivery. Institutional comprehensive quality assurance (QA) programs should ensure that uncertainties are maintained at acceptable levels. The International Atomic Energy Agency has recently developed a report summarizing the accuracy achievable and the suggested action levels, for each step in the radiotherapy process. Overview of the report: The report seeks to promote awareness and encourage quantification of uncertainties in order to promote safer and more effective patient treatments. The radiotherapy process and the radiobiological and clinical frameworks that define the need for accuracy are depicted. Factors that influence uncertainty are described for a range of techniques, technologies and systems. Methodologies for determining and combining uncertainties are presented, and strategies for reducing uncertainties through QA programs are suggested. The role of quality audits in providing international benchmarking of achievable accuracy and realistic action levels is also discussed. RECOMMENDATIONS: The report concludes with nine general recommendations: (1) Radiotherapy should be applied as accurately as reasonably achievable, technical and biological factors being taken into account. (2) For consistency in prescribing, reporting and recording, recommendations of the International Commission on Radiation Units and Measurements should be implemented. (3) Each institution should determine uncertainties for their treatment procedures. Sample data are tabulated for typical clinical scenarios with estimates of the levels of accuracy that are practically achievable and suggested action levels. (4) Independent dosimetry audits should be performed regularly. (5) Comprehensive quality assurance programs should be in place. (6) Professional staff should be appropriately educated and adequate staffing levels should be maintained. (7) For reporting purposes, uncertainties should be presented. (8) Manufacturers should provide training on all equipment. (9) Research should aid in improving the accuracy of radiotherapy. Some example research projects are suggested

Cáncer localizado de próstata

Se analizaron 560 pacientes consecutivos con cáncer localizado de próstata tratados con radioterapia conformal tridimensional entre 1993 y 2001. La sobrevida global actuarial a los nueve años fue de 71%, y la sobrevida causa específica de 88%. La mortalidad cruda por cáncer de próstata fue de 4,8% mientras que la mortalidad por otras causas fue de 8,3%. La sobrevida libre de recaída bioquímica en función del grupo de riesgo fue de 83%, 68% y 41% a los nueve años, respectivamente para los grupos de riesgo bajo, intermedio y alto en 504 pacientes (p<0,05). La dosis mayor o igual a 72 Gy mejora el control bioquímico a los siete años en todos los grupos de riesgo aunque sólo es estadísticamente significativa en el grupo de riesgo alto (p<0,006). El modelo de Cox reveló que solamente el grupo de riesgo, la dosis total como variable continua y el antígeno prostático específico inicial como variable categórica fueron significativos. Cuando el modelo de Cox fue aplicado a los 188 pacientes de riesgo alto, el uso de la hormonoterapia de inducción o concomitante, o ambas, y la edad, resultaron además significativos. La tasa actuarial de complicaciones severas a diez años grados 3 y 4 (no hubo complicaciones grado 5) fue de 1,2% para las urinarias y de 1,4% para las digestivas. La radioterapia conformal tridimensional a altas dosis es un tratamiento efectivo y de baja morbilidad para el tratamiento del cáncer localizado de próstata