Exploring the mobility of mobile phone users

Mobile phone datasets allow for the analysis of human behavior on an unprecedented scale. The social network, temporal dynamics and mobile behavior of mobile phone users have often been analyzed independently from each other using mobile phone datasets. In this article, we explore the connections between various features of human behavior extracted from a large mobile phone dataset. Our observations are based on the analysis of communication data of 100000 anonymized and randomly chosen individuals in a dataset of communications in Portugal. We show that clustering and principal component analysis allow for a significant dimension reduction with limited loss of information. The most important features are related to geographical location. In particular, we observe that most people spend most of their time at only a few locations. With the help of clustering methods, we then robustly identify home and office locations and compare the results with official census data. Finally, we analyze the geographic spread of users' frequent locations and show that commuting distances can be reasonably well explained by a gravity model.Comment: 16 pages, 12 figure

Geographical dispersal of mobile communication networks

In this paper, we analyze statistical properties of a communication network constructed from the records of a mobile phone company. The network consists of 2.5 million customers that have placed 810 millions of communications (phone calls and text messages) over a period of 6 months and for whom we have geographical home localization information. It is shown that the degree distribution in this network has a power-law degree distribution $k^{-5}$ and that the probability that two customers are connected by a link follows a gravity model, i.e. decreases like $d^{-2}$, where $d$ is the distance between the customers. We also consider the geographical extension of communication triangles and we show that communication triangles are not only composed of geographically adjacent nodes but that they may extend over large distances. This last property is not captured by the existing models of geographical networks and in a last section we propose a new model that reproduces the observed property. Our model, which is based on the migration and on the local adaptation of agents, is then studied analytically and the resulting predictions are confirmed by computer simulations.Comment: 17 pages, 8 figure

Interaction de l'hydrogène atomique avec une impulsion laser courte et intense: développement d'une méthode numérique d'analyse et application à l'étude de la stabilisation

Doctorat en sciences physiques -- UCL, 199

Inhibition of Atomic Ionization in Strong Laser Fields

When an atom interacts with an intense laser pulse which generates electric fields comparable to the atomic (Coulombic) field, high order processes become dominant as for example multiphoton ionization(1) and above threshold ionization(2). In some particular circumstances however, coherence phenomena may be at the origin of a substantial inhibition of ionization and lead to the stabilization of the atom. In this contribution, we analyze in detail a physical mechanism which is responsible for strong suppression of ionization(3). We show that the intense field excitation of the atom may generate a new kind of spatially extended wave packet through virtual transitions from the initial state via high-lying Rydberg and continuum states to a coherent superposition of Rydberg states with a small initial overlap with the nucleus. Conventional atomic wave packets(4) are created by direct short pulse excitation of overlapping Rydberg states from a compact initial source which ensures a large initial overlap with the nucleus and a substantial ionization. By contrast, our wave packet stems from extended high-lying states which are accessed virtually through Raman coupling rather than directly through short pulse excitation from compact low-lying states

Effective Dynamic Stabilization of Atomic Hydrogen in Strong Laser Fields

Two-photon ionization of atomic hydrogen is studied initially in the 2s-state using an intense ultrashort laser pulse. A physical mechanism that leads to effective dynamic stabilization of the atom is also described. This stabilization is the result of two distinct processes. In the tail of the pulse for low or moderate laser intensities, hydrogen is left in a coherent superposition of the initial state and in a set of p-Rydberg states, which leads to a substantial inhibition of ionization. At higher laser intensities, a second process becomes important, namely, transferring the population towards Rydberg states of higher angular momentum. This population migration occurs through degenerate nonresonant Raman coupling. This kind of stabilization occurs for pulse durations presently available for experimentation. These results are obtained using a spectral method and numerically solving the time-dependent Schrodinger equation

Probabilistic Occupancy Counts and Flight Criticality Measures in Air Traffic Management

Airspace congestion is a major challenge for future European air traffic management. When air traffic control believes that a sector will exceed its maximal capacity, a regulation is applied to it, which limits the number of aircraft entering the sector. These actions have a large cost because they affect all the flights that cross the sector. Moreover, they are based on the partial data available to the controller and do not take into account the network situation. First, a probabilistic framework for modeling air traffic occupancy count and sector congestion is proposed. This allows more precise information on the probability of sector overload to be provide to air traffic control. Second, based on this framework, metrics for individual flights are defined that measure their impact on the congestion of the whole network. These metrics are intended to be used in demand and capacity balancing tools, allowing for optimized choices for the whole network. Numerical experiments are presented for one day of European data, which include 33,219 flights in 1991 sectors. The simulations advocate the metrics and show how actions taken on selected flights have a positive impact on the network congestion

Dynamical Stabilization of Atoms at Current Laser Performances

Recent theories indicate that a one-electron system exposed to a very strong laser pulse may become stable against ionization. By stable, we mean that there is a regime of high field intensity in which the ionization probability decreases with increasing field intensity. One usually defines two kinds of stabilization: dynamical stabilization and adiabatic stabilization. Dynamical stabilization1 results from a coherent process: following the rapid turn-on of the laser field, the system is pumped into a coherent superposition of states which is stable against ionization. This process is therefore expected to be rather sensitive to the pulse duration and to the laser frequency. By contrast, adiabatic stabilization2 results from the adiabatic evolution of a well defined unperturbed eigenstate towards individual dressed states of the system. For photon energies higher than the unperturbed ionization potential, it turns out that the energy width of these dressed states tends to zero for very high fields, exceeding the binding field of the system

Probabilistic Traffic Models for Occupancy Counting

Air Traffic Management is subject to many uncertainties. These uncertainties drastically reduce overall predictability and force the introduction of margins having a negative impact on the capacity of the system. Moreover, as of today, uncertainties are not explicitly accounted for and human operator judgement and experience is relied upon to assess the “quality” of the estimations provided by the support tools. One way to make it explicit is to convey uncertainty through probability distributions. Building on this approach, the paper describes how to derive probabilistic traffic models from historical data. These models are used as input to the algorithm developed by Gonze et al. [1] in order to compute occupancy count distributions. The application of the approach to one sector of EUROCONTROL’s MUAC airspace is presented to show how uncertainty is captured by the proposed models