What is a BEC and how to make one ?
A Bose-Einstein condensate (BEC) is a non-classical state of matter, that means we can investigate properties, which are not accessible under common environmental conditions. We are interested in the quantum mechanical characteristics, who exhibit in such a regime and in many other systems the same behavior. This allows us to observe phenomenons, which are not visible in other systems.
To produce a BEC, we need to cool a gas down to 100nK above absolute zero. If a gas is cooled under normal conditions to such a low temperature, it forms a liquid or a solid. To prevent this, the experiments are pervormend in an ultra high vacuum chamber with very low pressure, and the atosm can not form a solid. Experimentally we produce a BEC with 87Rb with up to 150 thousand atoms. Using the so-called laser cooling, the atoms can be cooled from room temperature down to 100nK within 10 seconds. In addition there is an scanning electron microcope integrated into the setup. With that the atoms can be ionized, which does two things for us. First the ions are no longer trapped and so are kicked out of the ultracold cloud, on the other hand we can measure these ions, to compute a picture of the could.
What can we investigate with that ?
Key point of the experiment in our lab is the behavior of diven quantum systems. That means the atoms are either manipulated with externnal potentials, which can be spatially and temporally varied, or with an local dissipative process caused by the scanning electron microscope. With that we can investigate the transport in driven quantum systems and their stationary states fundamentally. Such states can differ dramatically from the steady states in closed systems. Another topic is the research on atom transport and wqunatum mechanical tunneling in periodic potentials. By removing atoms from on place in the cloud, an atomic current is created, which can be depending on the ratio of losses and tunneling strength either classical or superfluid, without friction. In that system also a stady state can be observed, in which the number of atoms in the well are jumping between a low and a high occupation number.
Selected publications
Jian Jiang, Erik Bernhart, Marvin Röhrle, Jens Benary, Marvin Beck, Chistian Baals, Herwig Ott
"Kapitza Trap for Ultracold Atoms"
We report on the experimental realization of a Kapitza trap for ultracold atoms. Using time-periodic attractive and repulsive Gaussian potentials, we create an effective trap for ultracold neutral atoms in a regime where the time average of the potential is equal to zero. We analyze the role of experimental imperfections, the stability of the trapped atomic cloud, and the magnitude of the effective potential. We find good agreement with the high-frequency expansion of the underlying system dynamics. Our experimental approach opens up new possibilities to study Floquet systems of neutral atoms.
J. Benary, C. Baals, E. Bernhart, J. Jiang, M. Röhrle, H Ott
„Experimental observation of a dissipative phase transition in a multi-mode many-body quantum system“
New Journal of Physics: 24, 103034 (2022)
Dissipative phase transitions are a characteristic feature of open systems. One of the paradigmatic
examples for a first order dissipative phase transition is the driven nonlinear single-mode optical
resonator. In this work, we study a realization with an ultracold bosonic quantum gas, which
generalizes the single-mode system to many modes and stronger interactions. We measure the
effective Liouvillian gap of the system and find evidence for a first order dissipative phase
transition. Due to the multi-mode nature of the system, the microscopic dynamics is much richer
and allows us to identify a non-equilibrium condensation process.
C. Baals, H. Ott, J. Brand and A.M. Mateo
„Nonlinear standing waves in an array of coherently coupled Bose-Einstein condensates“
Stationary solitary waves are studied in an array of M linearly coupled one-dimensional Bose-Einstein condensates (BECs) by means of the Gross-Pitaevskii equation. Solitary wave solutions with the character of overlapping dark solitons, Josephson vortex-antivortex arrays, and arrays of half-dark solitons are constructed for M>2 from known solutions for two coupled BECs. Additional solutions resembling vortex dipoles and rarefaction pulses are found numerically. Stability analysis of the solitary waves reveals that overlapping dark solitons can become unstable and susceptible to decay into arrays of Josephson vortices. The Josephson vortex arrays have mixed stability but for all parameters we find at least one stationary solitary wave configuration that is dynamically stable. The different families of nonlinear standing waves bifurcate from one another. In particular we demonstrate that Josephson-vortex arrays bifurcate from dark soliton solutions at instability thresholds. The stability thresholds for dark soliton and Josephson-vortex type solutions are provided, suggesting the feasibility of realization with optical lattice experiments.
Ralf Labouvie, Bodhaditya Santra, Simon Heun, and Herwig Ott
"Bistability in a Driven-Dissipative Superfluid"
We experimentally study a driven-dissipative Josephson junction array, realized with a weakly interacting Bose-Einstein condensate residing in a one-dimensional optical lattice. Engineered losses on one site act as a local dissipative process, while tunneling from the neighboring sites constitutes the driving force. We characterize the emerging steady states of this atomtronic device. With increasing dissipation strength γ the system crosses from a superfluid state, characterized by a coherent Josephson current into the lossy site, to a resistive state, characterized by an incoherent hopping transport. For intermediate values of γ, the system exhibits bistability, where a superfluid and an incoherent branch coexist. We also study the relaxation dynamics towards the steady state, where we find a critical slowing down, indicating the presence of a nonequilibrium phase transition.
T. Gericke, P. Würtz, D. Reitz, T. Langen, and H. Ott
"High resolution scanning electron microscopy of an ultracold quantum gas"
Nature Physics 4, 949-953 (2008)
Our knowledge of ultracold quantum gases is strongly influenced by our ability to probe these objects. In situ imaging combined with single-atom sensitivity is an especially appealing scenario, as it can provide direct information on the structure and the correlations of such systems. For a precise characterization a high spatial resolution is mandatory. In particular, the perspective to study quantum gases in optical lattices makes a resolution well below one micrometre highly desirable. Here, we report on a novel microscopy technique, which is based on scanning electron microscopy and allows for the detection of single atoms inside a quantum gas with a spatial resolution of better than 150 nm. We document the great functionality of this technique by precise density measurements of a trapped Bose–Einstein condensate and the first experimental demonstration of single-site addressability in a submicrometre optical lattice.