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Q: Fachverband Quantenoptik und Photonik

Q 10: Quantum Gases (Bosons) I

Q 10.4: Talk

Monday, March 5, 2018, 11:15–11:30, K 2.020

Damping of BEC Josephson oscillations by dynamical fluctuation excitation — •Tim Lappe¹, Anna Posazhennikova², and Johann Kroha¹ — ¹Physikalisches Institut and Bethe Center for Theoretical Physics, Universität Bonn, Nussallee 12, 53115 Bonn, Germany — ²Department of Physics, Royal Holloway, University of London, Egham, Surrey TW20 0EX, UK

The nonequilibrium dynamics of Bose-Josephson junctions can be investigated with Bose-Einstein condensates (BEC) of cold atoms in double-well traps. These systems are perfectly isolated, yet the experiments manifest an intriguing divergence: While some exhibit dissipation-free Josephson oscillations, others show strong damping. Some of us have demonstrated before how inelastic collisions of incoherent excitations can lead to damping and eventual thermalization [PRL 116, 225304 (2016)]. Here we scrutinize the generation of such excitations in realistic traps and their effect on damping. This cannot be achieved on the usual Gross-Pitaevskii (GP) level. Using a Keldysh path-integral formalism, we develop a time-dependent, multi-mode description beyond the GP equation, including quadratic fluctuations. We find an excess of fluctuations when their renormalized excitation energy, ε, is near the renormalized Josephson frequency, ω_J. Both ε and ω_J are strongly renormalized by interactions. Calculating the system parameters and coupling constants quantitatively, we show that these renormalizations can explain the apparently contradictory damping behavior of two well-known experiments. This sheds light on the unresolved origin of damping observed in these isolated systems.