Robert Weis

The Kirchhoff Institute for Physics (KIP) is named after a prominent physicist of the 19th Century: **Gustav Robert Kirchhoff**, who worked in Heidelberg for 21 years. His well-known lectures on experimental and theoretical physics attracted many students. Kirchhoff's ground-breaking research was extraordinarily diverse, spanning electrical, magnetic, optical, elastic, hydrodynamic and thermal processes. His laws for electrical circuits are well-known. At the time he was in Heidelberg, in conjunction with Robert Wilhelm Bunsen, he discovered spectral analysis and its application to solar radiation. In this way, Kirchhoff laid the foundation for modern astrophysics, as well as formulating the laws of thermal radiation, which played a key role in the discovery of quantum physics. The KIP aims to continue in this tradition of diverse scientific research and education.

Free spirit. Pioneer. Visionary: Gustav Kirchhoff's scientific findings are still of great importance today for many current research topics. As the founder of spectral analysis in the 19th century, the outstanding physicist (1824 to 1887) not only paved the way for modern astrophysics, but also environmental physics, modern atomic and molecular physics, chemistry and quantum physics still use spectroscopy today. And without Kirchhoff's rules for electrical networks, chip development and the analysis of electrical circuits would be inconceivable.

The Ruperto Carola lecture series in the summer semester 2024 on the occasion of the 200th birthday of Gustav Kirchhoff, who researched and taught as a professor at Heidelberg University for more than 20 years, provides - in addition to a historical introduction to Kirchhoff's life and work - insights into areas of modern research on which Kirchhoff's work has had an influence to this day.

* Prof. Dr. Johannes Quaas, Theoretische Meteorologie, Universität Leipzig,
*
Particles in the atmosphere - aerosols - may serve as cloud condensation nuclei.
Increases in aerosol concentrations thus change cloud droplet concentrations and
thus enhance the brightness of clouds. Such aerosol-cloud interactions exert a cooling
effect on climate.
more...

Next CQD Colloquium (funded by Structures) will be given by Prof. Herwig Ott.

**Please note the special place and time: Wednesday, 22.05. at 4:30 p.m., KIP, INF 227, HS 1**

The main talk will be given by Prof. Herwig Ott, Department of Physics and Research Center OPTIMAS University of Kaiserslautern-Landau about:

**Dissipation in Bose-Einstein Condensates**

I will present two experiments with atomic Bose-Einstein condensates, where dissipation plays a central role.The first is the observation of a dissipative phase transition. We realize an atomic superfluid, which is subject to drive and dissipation at the same time. The steady-states of the system depend on the competition between the two processes. Therefore, upon parameter change, the system can undergo a dissipative phase transition between different types of steady-states. One of the paradigmatic examples for a first order dissipative phase transition is the driven nonlinear single-mode optical resonator. I will report on the corresponding realization within an ultracold bosonic gas, which generalizes the single-mode system to many modes and stronger interactions [1]. 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 allows us to identify a non-equilibrium condensation process.

#In the second experiment we have realized Shapiro steps in a Bose-Einstein condensate. Shapiro steps occur in the reverse AC Josephson effect, which is one of the three fundamental effects in superconducting Josephson junctions. When a DC and an AC current are applied simultaneously to a Josephson junction, finite voltage steps are generated across the junction. The voltage is linked to the applied frequency via V= h/e x f, where f is the frequency of the alternating current. The series connection of several such junctions in one device corresponds to the current voltage standard. Following the protocol proposed by Singh et al [2], we move a narrow barrier through the superfluid at a constant velocity, which corresponds to a DC particle current through the barrier. At the same time, we perform a sinusoidal modulation of the barrier velocity with frequency f, which corresponds to an additional AC current through the barrier. When the instantaneous velocity of the barrier exceeds the critical velocity of the superfluid, a finite particle imbalance occurs between the two sides of the barrier. We find that the corresponding chemical potential difference takes on discrete values corresponding to Shapiro steps. We characterize the Shapiro steps and the parameter range in which they can be observed in our experiment.

Our experiment continues a long story of research with atomic Josephson physics, which had is origin in Heidelberg almost two decades ago.

[1] J. Benary et al., New J. Phys. 24, 103034 (2022)

[2] V. P. Singh, J. Polo, L. Mathey, and L. Amico. arXiv:2307.08743 (2023)

[3] M. Albiez et al. Phys. Rev. Lett. **95**, 010402 (2005)

The pretalk will be given by Yannick Deller.

For information about the CQD Colloquium, please see: https://cqd.uni-heidelberg.de/events/cqdcolloquium