Café com Física





Início

Agenda


20 de junho de 2018
16h30
Sala 18 (Cristalografia)

Marios Tsatsos
IFSC

Elaborating correlated states and quantum fluctuations: principles and applications of many-body theories in Bose-Einstein condensates

Bose-Einstein condensation, theoretically known to appear at ultralow temperatures since the 1920s, was achieved in the laboratory only less than 25 years ago, in dilute bosonic gases at temperatures close to absolute zero. The wide range of applications and controlabillity of the system parameters has made them unprecedented tools for exploring novel quantum phases and behavior. The most common theoretical method employed to tackle these complex systems, typically consisting of tens of thousands of interacting particles is the celebrated mean-field Gross-Pitaevskii model. Even though it has been proven successful in describing various types of nonlinear excitations it does not take into consideration fragmentation and correlations that can develop in time. Consider for instance the process of the quantum measurement: what result would a measurement yield for a coherent gas (ie Bose-Einstein condensed)? As long as no quantum correlations are there, an N-body measurent immediately reduces to N single-body measurement. In real situations however the processes are more complicated and, depending on the experimental settings, correlations might well affect the final observed state of the gas. In such situations mean fields fail to give reliable predictions. To go beyond that, I will briefly present a systematic theory, the MultiConfigurational Time-Dependet Hartree for Bosons (MCTDHB) [1] that has been developed in order to solve the many-body Schroedinger equation beyond the mean-field approach and can be in principle exact, and the latest numerical implementation (solver) that is freely distributed in the web [2]. I will then discuss some particular applications in Bose gase that are violently perturbed and out of equilibrium, such as turbulent and granulated gases. Joint theoretical and experimental work from Sao Carlos and also Rice, Texas corroborate our views. Last, promising applications from light-coupled BECs in optical cavities (such as Bloch oscillations and gravimeters) will be discussed.



[1] Alexej I. Streltsov, Ofir E. Alon, and Lorenz S. Cederbaum, Role of Excited States in the Splitting of a Trapped Interacting Bose-Einstein Condensate by a Time-Dependent Barrier, Phys. Rev. Lett. 99, 030402 (2007); Ofir E. Alon, Alexej I. Streltsov, and Lorenz S. Cederbaum, Multiconfigurational time-dependent Hartree method for bosons: Many-body dynamics of bosonic systems, Phys. Rev. A 77, 033613 (2008).

[2] A.U.J. Lode, M.C. Tsatsos and E. Fasshauer, The Multiconfigurational Time-Dependent Hartree for Indistinguishable particles software (2016), http://ultracold.org.