Van der Waals colloquium by dr. Jan Klärs from the University of Twente (Netherlands) on "The thermalization, condensation and flickering of photons". Jan’s lab investigates physical phenomena at the interface between thermodynamics and quantum mechanics in optical and optomechanical systems.

Abstract: Does a photon gas condense? Its bosonic and interaction-free nature should make a photon gas an obvious candidate for a Bose–Einstein condensation. However, the thermodynamic behavior of photon gases usually does not include a condensation process. For blackbody radiation, the most omnipresent Bose gas, the number of photons follows the available thermal energy as is quantitatively described by the Stefan–Boltzmann law. At low temperatures, the photon number simply decreases and no macroscopic occupation of the cavity ground state occurs. The situation changes, however, when one considers photons in a quantum-confined structure, such as an optical microcavity, whose length is comparable to the wavelength of the light itself. In such systems, photons effectively behave like two-dimensional gases of massive particles where temperature and chemical potential can be tuned independently. This creates the conditions for the occurrence of a Bose-Einstein condensation of photons. The study of such condensation effects in the last years has revealed a rich phenomenology of equilibrium and non-equilibrium (critical) behavior, which I will discuss in more detail in this talk. This includes, for example, the occurrence of anomalously large particle number fluctuations, the so-called grand canonical fluctuations catastrophe, and non-equilibrium condensation phenomena with controlled feedback. Finally, I will discuss how systems of coupled condensates can be used to solve mathematical optimization problems.

The Joan van der Waals colloquium is an ongoing bi-weekly lecture series, organized by LION. As all speakers are instructed to give an accessible lecture for everyone, these lectures aid in getting a broader view of physics. Each colloquium is preceded by an opening act: a PhD student or postdoc from LION and a different field than the invited speaker, to give a 10-min presentation. After the colloquium, there is a chance to meet your colleagues and participate in discussions at the borrel. More information can be found here.