Browsing by Author "Rodrigues, J. D."
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- Collective processes in a large atomic laser cooling experimentPublication . Rodrigues, J. D.; Rodrigues, J. A.; Ferreira, A. V.; Mendonça, J. T.We introduce an hydrodynamic description of a laser cooled gas. In large traps, with as much as 10(10) atoms, multiple scattering of light leads to a collective interaction field which can be described by a Poisson-like equation, included in our formulation. A behaviour similar to a one-component trapped plasma should then be observed. By considering equilibrium conditions we extract the theoretical atomic density profiles and, in particular, its dependence on the effective plasma frequency of the system. The model is compared with experimentally measured profiles with an extraordinary agreement, thus corroborating both the plasma nature of the dynamics in the gas as well as the validity of the hydrodynamic formulation introduced here.
- Equation of state of a laser-cooled gasPublication . Rodrigues, J. D.; Rodrigues, J. A.; Moreira, O. L.; Tercas, H.; Mendonca, J. T.We experimentally determine the equation of state of a laser-cooled gas. By employing the Lane-Emden formalism, widely used in astrophysics, we derive the equilibrium atomic profiles in large magneto-optical traps where the thermodynamic effects are cast in a polytropic equation of state. The effects of multiple scattering of light are included, which results in a generalized Lane-Emden equation for the atomic profiles. A detailed experimental investigation reveals an excellent agreement with the model, with a twofold significance. On one hand, we can infer the details of the equation of state of the system, from an ideal gas to a correlated phase due to an effective electrical charge for the atoms, which is accurately described by a microscopical description of the effective electrostatic interaction. On the other hand, we are able map the effects of multiple scattering onto directly controllable experimental variables, which paves the way to subsequent experimental investigations of this collective interaction.
- Information compression at the turbulent phase transition in cold-atom gasesPublication . Giampaoli, R.; Figueiredo, J. L.; Rodrigues, J. D.; Rodrigues, José-António; Terças, H.; Mendonça, J. T.The statistical properties of physical systems in thermal equilibrium are blatantly different from their far-from -equilibrium counterparts. In the latter, fluctuations often dominate the dynamics and might cluster in ordered patterns in the form of dissipative coherent structures. Here, we study the transition of a cold atomic cloud, driven close to a sharp electronic resonance, from a stable to a turbulent phase. From the atomic density distribution- measured using a spatially resolved pump-probe technique-we have computed the Shannon entropy on two different basis sets. Information compression, corresponding to a minimum in the Shannon entropy, has been observed at criticality, where the system fluctuations organize into high-order (low-entropy) patterns. Being independent of the representation used, this feature is a property shared by a vast class of physical systems undergoing phase transitions.