**Quantum aggregates are assemblies of monomers (molecules, atoms, quantum dots...), where the monomers largely keep their individuality. However, interactions between the monomers can lead to collective phenomena, like superradiance or efficient excitation transfer. We study different kinds of aggregates (e.g. light harvesting systems, arrays of Rydberg atoms, self-assembled organic dyes...)
Using various methods (ranging from Green-operator methods over stochastic Schroedinger equations to semicalssical surface hopping) we study optical and excitation transport in these systems. Of particular interest is the coupling of the excitation to nuclear degrees of freedom.
**

## Photosynthesis

- Efficiency of energy transfer
- Quantum effects and classical description
- Molecular Dynamics simulations
- Non-Markovian Quantum State Diffusion approach

## Molecular aggregates

- J- and H-aggregates of organic dyes
- Superradiance
- Near field spectroscopy

## Rydberg aggregates

- Adiabatic transport of excitation and entanglement
- Conical Intersections
- Interplay between Dipole blockade, atomic motion and dressing

## Simulating open quantum systems

- Using supercunducting circuits
- Using coupled classical oscillators
- Theoretical description with stochastic Schrödinger equations

## Various Topics

- Molecular electronics
- Heavy tailed disorder and localization
- PT symmetry & symmetry breaking
- Machine learning

## Method development

- Mixed quantum-classical approaches (surface hopping, MM/QM)
- Green operator techniques
- Large scale numerical propagations and diagonalization
- Stochastic Schrödinger equations

# Photosynthesis

The ability of photosynthetic plants, algae and bacteria to efficiently harvest sunlight has attracted researchers for decades and a fairly clear picture of photosynthesis has emerged: Sunlight is absorbed by assemblies of chromophores, e.g. chlorophylls. These assemblies, termed light harvesting complexes, transfer the excitation energy with high efficiency to so-called reaction centers, where the excitation energy is converted into a trans-membrane chemical potential.

# Organic Dye Aggregates

Certain molecular aggregates consisting
of organic dyes are remarkable in exhibiting an intense
and very narrow absorption peak, known as a J-band, which is red-shifted away from the region of monomer absorption. Apart from those dyes showing the J-band on aggregation, there are also dyes where the absorption maximum is shifted to higher energies. The width of the resulting absorption band (called an H-band) is comparable to that of the monomeric dyes and shows a complicated vibrational structure.

# Rydberg Aggregates

While in moleculare aggregates there is a strong coupling to vibrational degrees ultracold Rydberg atoms are ideally suited to study the above described phenomena in an clean and controllable environment.

In particular we have found that the coupling of electronic and nuclear degrees of freedoms allows to transport entanglement along a chain of atoms in effecive way. Furthermore we have investigate the effect of conical intersection in ultracold Rydberg gases on the excitation dynamics. We found that it is possible to directly observe the effect of Berry's phase.

# Superconducting circuits

Open quantum system approaches are widely used in the description of physical, chemical and biological systems to describe the coupling of electronic degrees of freedom to vibrations. This structured vibrational environment makes simulations on a classical computer very demanding. We propose an analogue quantum simulator of complex open system dynamics which is based on superconducting circuits.

# Stochastic Schrödinger equations

We have recently extended the non-Markovian Quantum State Diffusion approach to treat the energy transfer of coupled molecules.
The picture shows Transport on a ring, when initially the excitation is
localised on monomer 8. (a) No coupling to the
environment. (b) coupling to a structured
environment. This plot is an average of single
trajectories (examples are shown in (c)-(e)).

Publications:

[1] A short description how to calculate absorption and energy
transfer