Impurity dynamics in tunable 1D quantum gases

Univ. Prof. Dr. Hanns-Christoph Nägerl1, Dr. Mikhail Zvonarev2 1. Institute for Experimental Physics, University of Innsbruck, Austria 2. LPTMS Orsay, France The phenomenon of Bose-Einstein condensation occurs for atomic gases at temperatures close to absolute zero temperature. When bosonic atoms are sufficiently well cooled, they can all unite in a common matter wave and behave like a macroscopic quantum object. Confining such a wave tightly in space strongly amplifies quantum effects and the quantum nature of the wave becomes clearly visible. In the case of one-dimensional geometry then one speaks of an atomic quantum wire. Within this collaborative project, which involves two groups from Austria and France, we investigate, jointly in experiment and theory, the dynamics of impurities that are introduced deliberately into atomic quantum wires. Impurity dynamics is generally a key theme in solid state physics. Future nano-electronic conductors are expected to have lateral dimensions of a single atom in view of the ongoing miniaturization occurring over the next 15 to 20 years. Our project is aimed at identifying already now the quantum effects that might play a role in the quantum wires of the future. We use atoms in different spin states, each of which, roughly speaking, simulate the role of electrons (impurities) and the role of the background atoms in the wire. We load atoms from a Bose-Einstein condensate into artificial optical lattice potentials to produce strong confinement in one dimension. With this system we aim at investigating, in a very fundamental manner, the conduction properties of the impurity atoms. For example, oscillations have been predicted to occur that are caused by the background atoms and indicate their quantum state. Or there may be non-classical diffusion processes or even the phenomenon of "pinning", i.e. the transition to an insulator and the localization of the impurity atoms due to very strong interactions. A great advantage of using ultracold atoms to simulate the properties of quantum wires lies in the fact that the strength and the sign of the interaction, i.e. whether it acts repulsively or attractively, can be freely adjusted. We are thus able, in a controlled way, to switch between the different regimes of quantum transport, e.g. conductor and insulator, and to identify effects that are the result of interactions.

Near temperatures of absolute zero atomic gases may exhibit the phenomenon of Bose-Einstein condensation. When bosonic atoms are cooled to sufficiently low temperatures, they can all join into a common matter wave and behave like a macroscopic quantum object. Such a wave, for example, behaves like a superfluid. Confining this wave spatially enhances the quantum effects, which then become clearly detectable in the experiment. One speaks of a "quantum wire" when the confinement is essentially one-dimensional and the particles can only move along one specific di-rection in space. Our joint French-Austrian ANR-FWF project was aimed at identifying quantum effects that might become relevant in future quantum wires in view of the ongoing miniaturization of electronic circuitry in the context of nano-electronics. For this we use atoms confined to potential structures made of laser light to effectively simulate the properties of charge carriers (such as electrons) confined to the potential of a crystal lattice and possibly strong lateral barriers. We were able to obtain one central result from our proposal already at the very beginning of this project in view of our signifi-cant precursor work: The observation of Bloch oscillations of impurity atoms without the presence of a lattice potential, published in "Science". Previous experiments, in particular from our group carried out in strict 1D geometry, had detected Bloch oscillations only in the presence of an externally imprinted lattice. In our case, the oscillating impurity atoms experience a homogeneous background gas. However, they "sense" the strong two-body correlations that the background gas is subject to in view of the strong interactions that we induce my means of interaction tuning. We hence interpret our data as coherent scattering off the two-body correlation function. This effect appears for the case of accelerated impurities. More strikingly, our French collaboration partner had proposed oscillatory behavior of mobile impurities that are not subject to accelera-tion. We have thus turned to detecting this "quantum flutter" effect and have put great efforts into the experimental machine to improve the detection sensitivity. While we have not yet been successful, we hope to find evidence for flutter within our follow-up project.