## Fachbereich Physik

### Filtern

#### Erscheinungsjahr

#### Dokumenttyp

- Preprint (157)
- Wissenschaftlicher Artikel (42)
- Teil eines Periodikums (21)
- Dissertation (5)
- Arbeitspapier (5)
- Diplomarbeit (1)

#### Sprache

- Englisch (231) (entfernen)

#### Schlagworte

- resonances (10)
- Quantum mechanics (8)
- Wannier-Stark systems (8)
- lifetimes (8)
- quantum mechanics (6)
- lifetime statistics (5)
- Lasererzeugtes Plasma (3)
- entropy (3)
- localization (3)
- Brillouin light scattering spectroscopy (2)

Based on the Lindblad master equation approach we obtain a detailed microscopic model of photons in a dye-filled cavity, which features condensation of light. To this end we generalise a recent non-equilibrium approach of Kirton and Keeling such that the dye-mediated contribution to the photon-photon interaction in the light condensate is accessible due to an interplay of coherent and dissipative dynamics. We describe the steady-state properties of the system by analysing the resulting equations of motion of both photonic and matter degrees of freedom. In particular, we discuss the existence of two limiting cases for steady states: photon Bose-Einstein condensate and laser-like. In the former case, we determine the corresponding dimensionless photon-photon interaction strength by relying on realistic experimental data and find a good agreement with previous theoretical estimates. Furthermore, we investigate how the dimensionless interaction strength depends on the respective system parameters.

Annual Report 2017
(2017)

Bulk-boundary correspondence in non-equilibrium dynamics of one-dimensional topological insulators
(2017)

Dynamical phase transitions (DPT) are receiving a rising interest. They are known to behave analogously to
equilibrium phase transitions (EPT) to a large extend. However, it is easy to see that DPT can occur in finite
systems, while EPT are only possible in the thermodynamic limit. So far it is not clear how far the analogy of
DPT and EPT goes. It was suggested, that there is a relation between topological phase transitions (TPT)
and DPT, but many open questions remain.
Typically, to study DPT, the Loschmidt echo (LE) after a quench is investigated, where DPT are visible as
singularities. For one-dimensional systems, each singularity is connected to a certain critical time scale, which
is given by the dispersion in the chain.
In topological free-fermion models with winding numbers 0 or 1, only the LE in periodic boundary conditions
(PBC) has been investigated. In open boundary conditions (OBC), these models are characterized by symmetry
protected edge modes in the topologically non-trivial phase. It is completely unclear how these modes affect
DPT. We investigate systems with PBC governed by multiple time scales with a Z topological invariant. In
OBC, we provide numerical evidence for the presence of bulk-boundary correspondence in DPT in quenches
across a TPT.

Annual Report 2016
(2017)

Combining ultracold atomic gases with the peculiar properties of Rydberg excited atoms gained a lot of theoretical and experimental attention in recent years. Embedded in the ultracold gas, an interaction between the Rydberg atom and the surrounding ground state atoms arises through the scattering of the Rydberg electron from an intruding perturber atom. This peculiar interaction gives rise to a plenitude of previously unobserved effects. Within the framework of the present thesis, this interaction is studied in detail for Rydberg \(P\)-states in rubidium.
Due to their long lifetime, atoms in Rydberg states are subject to scattering with the surrounding ground state atoms in the ultracold cloud. By measuring their lifetime as a function of the ground state atom flux, we are able to obtain the total inelastic scattering cross section as well as the partial cross section for associative ionisation. The fact that the latter is three orders of magnitude larger than the size of the formed molecular
ion indicates the presence of an efficient mass transport mechanism that is mediated by the Rydberg–ground state interaction. The immense acceleration of the collisional process shows a close analogy to a catalytic process. The increase of the scattering cross section renders associative ionisation an important process that has to be considered for experiments in dense ultracold systems.
The interaction of the Rydberg atom with a ground state perturber gives rise to a highly oscillatory potential that supports molecular bound states. These so-called ultralong-range Rydberg molecules are studied with high resolution time-of-flight spectroscopy, where we are able to determine the binding energies and lifetimes of the molecular states between the two fine structure split \(25P\)-states. Inside an electric field, we observe a broadening of the
molecular lines that indicates the presence of a permanent electric dipole moment, induced by the mixing with high angular momentum states. Due to the mixing of the ground state atom’s hyperfine states by the molecular interaction, we are able to observe a spin-flip of the perturber upon creation of a Rydberg molecule. Furthermore, an incidental near-degeneracy in the underlying level scheme of the \(25P\)-state gives rise to highly entangled states between the Rydberg fine structure state and the perturber’s hyperfine structure. These mechanisms can be used to manipulate the quantum state of a remote particle over distances that exceed by far the typical contact interaction range.
Apart from the ultralong-range Rydberg molecules that predominantly consist of only one low angular momentum state, a class of Rydberg molecules is predicted to exist that strongly mixes the high angular momentum states of the degenerate hydrogenic manifolds. These states, the so-called trilobite- and butterfly Rydberg molecules, show very peculiar properties that cannot be observed for conventional molecules. Here we present the first experimental observation of butterfly Rydberg molecules. In addition to an extensive spectroscopy that reveals the binding energy, we are also able to observe the rotational structure of these exotic molecules. The arising pendular states inside an electric field allow us, in comparison to the model of a dipolar rotor, to extract the precise bond
length and dipole moment of the molecule. With the information obtained in the present study, it is possible to photoassociate butterfly molecules with a selectable bond length, vibrational state, rotational state, and orientation inside an electric field.
By shedding light on various previously unrevealed aspects, the experiments presented in this thesis significantly deepen our knowledge on the Rydberg–ground state interaction and the peculiar effects arising from it. The obtained spectroscopic information on Rydberg molecules and the changed reaction dynamics for molecular ion creation will surely provide valuable data for quantum chemical simulations and provide necessary data to plan future experiments. Beyond that, our study reveals that the hyperfine interaction in Rydberg molecules and the peculiar properties of butterfly states provide very promising new ways to alter the short- and long-range interactions in ultracold many-body systems. In this sense the investigated Rydberg–ground state interaction not only lies right at
the interface between quantum chemistry, quantum many-body systems, and Rydberg physics, but also creates many new and fascinating possibilities by combining these fields.

Annual Report 2015
(2016)

Annual Report, Jahrbuch AG Magnetismus

Annual Report 2014
(2015)

Annual Report, Jahrbuch AG Magnetismus

Annual Report 2013
(2014)

Annual Report, Jahrbuch AG Magnetismus

Annual Report 2012
(2013)

Annual Report, Jahrbuch AG Magnetismus

Annual Report 2011
(2012)

Annual Report, Jahrbuch AG Magnetismus