tag:www.physik.uni-hamburg.de,2005:/en/iqp/sengstock/research/femto-bec/femto-newsFemto BEC News2024-05-16T12:34:24ZNAGR-fakmin-38053041-production2024-04-04T22:00:00ZCooling dynamics of a free ion in a Bose-Einstein condensate<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/38053525/femto-ion-cooling-733x414-6be7d2a2a8aa048abc72e47893d2514c11e8742b.jpg" /><p>We investigate the dynamics of an ion moving through a homogeneous Bose-Einstein condensate (BEC) after an initial momentum is imparted. For this, we derive a master equation in the weak-coupling limit and Lamb-Dicke approximation for the reduced density matrix of the ion. We study the time evolution of the ion's kinetic energy and observe that its expectation value, identified as the ion temperature Tion, is reduced by several orders of magnitude in a time on the order of microseconds for a condensate density in the experimentally relevant range between 1013 cm−3 and 1014 cm−3. We characterize this behavior by defining the duration at half maximum as the time required by Tion to reach half of its initial value, and study its dependence on the system parameters. Similarly, we find that the expectation value of the ion's momentum operator is reduced by nine orders of magnitude on the same timescale, making the ion's position converge to a final value. Based on these results, we conclude that the interaction with the bosonic bath allows for cooling and pinning of the ion by decreasing the expectation value of its kinetic energy and velocity, which constitutes a result of direct relevance for current atom-ion experiments.</p>
<p>L. Oghittu, J. Simonet, P. Wessels-Staarmann, M. Drescher, K. Sengstock, L. Mathey, and A. Negretti, Phys. Rev. Research 6, 023024 (2024).</p>
<p>CUI: Advanced Imaging of Matter News Outlet:<br>Catching an ion with a quantum gas</p><p>Photo: L. Oghittu / Ph. Wessels-Staarmann</p>NAGR-fakmin-22639833-production2021-01-25T23:00:00ZUltrafast Electron Cooling in an Expanding Ultracold Plasma<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/22639803/femto-microplasma-733x414-c508105767ed64a22e29ec6344eb83d6e25df2db.jpg" /><p>Plasma dynamics critically depends on density and temperature, thus well-controlled experimental realizations are essential benchmarks for theoretical models. The formation of an ultracold plasma can be triggered by ionizing a tunable number of atoms in a micrometer-sized volume of a 87Rb Bose-Einstein condensate (BEC) by a single femtosecond laser pulse. The large density combined with the low temperature of the BEC give rise to an initially strongly coupled plasma in a so far unexplored regime bridging ultracold neutral plasma and ionized nanoclusters. Here, we report on ultrafast cooling of electrons, trapped on orbital trajectories in the long-range Coulomb potential of the dense ionic core, with a cooling rate of 400 K ps−1. Furthermore, our experimental setup grants direct access to the electron temperature that relaxes from 5250 K to below 10 K in less than 500 ns.</p>
<p>T. Kroker, M. Großmann, K. Sengstock, M. Drescher, Ph. Wessels-Staarmann*, and J. Simonet*, Nat. Commun. 12, 596 (2021).<br>* Equal contribution.</p>
<p>CUI: Advanced Imaging of Matter News Outlet:<br>Electron refrigerator: Ultrafast cooling mechanism discovered in novel plasma</p><p>Photo: UHH/Großmann</p>NAGR-fakmin-20753441-production2018-07-03T22:00:00ZAbsolute strong-field ionization probabilities of ultracold rubidium atoms<img width="293" height="165" style="float:left" src="https://assets.rrz.uni-hamburg.de/instance_assets/fakmin/18368372/absolute-strong-field733x414-8138b2ecf44af95866f9b1dafc4fee5ec945832f.png" /><p>Understanding strong-field ionization requires a quantitative comparison between experimental data and theoretical models which is notoriously difficult to achieve. Optically trapped ultracold atoms allow to extract absolute nonlinear ionization probabilities by imaging the atomic density after exposure to the field of an ultrashort laser pulse. We report on such precise measurements for rubidium in the intensity range of 1 × 1011 – 4 × 1013 W cm−2. The experimental data are in perfect agreement with ab-initio theory, based on solving the time-dependent Schrödinger equation without any free parameters. We investigate the strong-field response of 87Rb atoms at two different wavelengths representing non-resonant and resonant processes in the demanding regime where the Keldysh parameter is close to unity.</p>
<p>Ph. Wessels, B. Ruff, T. Kroker, A. K. Kazansky, N. M. Kabachnik, K. Sengstock, M. Drescher, and J. Simonet, Commun. Phys. 1, 32 (2018).</p>
<p>Press release University of Hamburg:<br>Ultracold atoms and ultrafast lasers: Hamburg scientists combine experimental expertise</p><p>Photo: UHH/Wessels-Staarmann, "Non-resonant strong-field ionization yield of ultracold <sup>87</sup>Rb", <a href="https://creativecommons.org/licenses/by/4.0/">CC BY 4.0</a>; cropped and remixed</p>