Etienne Huens

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 million 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 as 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.

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 100,000 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.

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.

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.