Quantum gases provide a test bed for many-body physics under both in- and out-of-equilibrium settings. Experimental control over dimensionality, potential energy landscapes, or the coupling to reservoirs offers wide possibilities to explore different phases of matter.
In our lab, we study 2D quantum gases of photons in box potentials. In the finite-size homogeneous system, we observed BEC, as evidenced from the measured position and momentum distributions. By exerting a mechanical force onto the photon gas prepared in a regime around the phase transition and studying the density response, we measured both the bulk isothermal compressibility and the EOS of the optical quantum gas. In the future, uniform optical gases may enable thermodynamic machines with light as a work medium. An additional perspective is the exploration of sound. The required dynamic manipulation of optical quantum gases can become feasible through, for example, electro-optic trap modulation or spatiotemporally resolved pumping of the dye reservoir. Beyond ideal gas theory, a nonvanishing healing length can be achieved by either adding Kerr media or exploiting the weakly dissipative nature of photon condensates.
Topological states are considered as a key resource in quantum technology. Microcavity photon condensates offer a conceptually new approach to engineer such topological states: Inducing topological phases and protected edge states solely by controlling reservoirs. This recipe will allow us to perform fundamental tests of the yet unexplored interplay between topology and reservoir-induced fluctuations. To date, understanding topology at finite temperature is one of the outstanding problems in physics.
In the experiments, we work on a versatile room-temperature microcavity platform hosting one-dimensional arrays of photon condensates with tuneable tunnelling, coherence and single-site-resolved control of dissipation and gain. Non-Hermitian topological states will then be realised by controlling the pump laser masks. A technological appeal of this approach is the possibility to synthesise topological phases in a dynamical way. Reconfigurable topological phases, conveniently imprinted on topologically trivial photonic crystal structures inside dye-microcavities, can be envisaged for robust routing of light on photonic chips with on-demand updating.
K. Takata and M. Notomi, Phys. Rev. Lett. 121, 213902 (2018)
H. Wetter et al., Phys. Rev. Lett. 131, 083801 (2023)
The combination of nanostructuring methods and optical quantum gases opens up studies of novel quantum states of light, ranging from fundamental research to applications in technology.
To realize custom-shaped nanostructures for high-finesse optical resonators, the group is working on the development of a surface structuring method for high-reflectivity dielectric Bragg mirrors. For this purpose, a focussed laser beam at 532 nm is scanned on the rear side through a SiO2 glass substrate of a dielectric plane mirror via an absorbing amorphous silicon thin film. The generated heat input ensures an expansion of the λ/4 layers of a few %, which results in a permanent surface enhancement of the coating. The spatial resolution of the direct laser writing (DLW) is 3µm (lateral) and 1Å (height), so that variable potential landscapes for light can be generated in the microresonator. The figure shows an exemplary structured mirror surface with different printed structures. In addition, we also investigate alternative structuring methods for optical cavities using additive 3D DLW methods with a 100-nm spatial resolution, which has recently enabled the realisation of 1D quantum gases of light.
Conventional computing structures are based on physically separate processing and memory units (‘von Neumann architecture’), which are fundamentally limited by the time it takes to exchange information between these units. This motivates a paradigm shift towards new types of computing machines: ‘physical computers’.
Building on our previous work on thermodynamics in optical systems, our goal is the experimental realization of networks of coupled optical condensates, which represent a candidate system for ultrafast and energy-efficient physical computation. Specifically, the photons solve a given 'computational' problem that is 'pogrammed' in the network structure according to physical principles, e.g. energy minimisation or gain maximisation. The approach is based on the analogy of coupled condensates with adjustable couplings and spin systems. In our group we work on the development of such analogue simulators as a new class of computing devices. Due to the fast light-matter interaction and coupling rates between the lattice sites, this novel computer structure promises the solution of complex optimisation problems in picoseconds, which means a significant speed-up compared to today's computer architectures.
See, e.g. theory work by Berloff group: K.P. Kalinin et al., Sci. Rep. 8, 17791 (2018)
